ALBERT

Description: Similarity estimators for irregular and age uncertain time series Climate of the Past Discussions, 9, 5299-5346, 2013 Author(s): K. Rehfeld and J. Kurths Paleoclimate time series are often irregularly sampled and age uncertain, which is an important technical challenge to overcome for successful reconstruction of past climate variability and dynamics. Visual comparison and interpolation-based linear correlation approaches have been used to infer dependencies from such proxy time series. While the first is subjective, not measurable and not suitable for the comparison of many datasets at a time, the latter introduces interpolation bias, and both face difficulties if the underlying dependencies are nonlinear. In this paper we investigate similarity estimators that could be suitable for the quantitative investigation of dependencies in irregular and age uncertain time series. We compare the Gaussian-kernel based cross correlation (gXCF, Rehfeld et al., 2011) and mutual information (gMI, Rehfeld et al., 2013) against their interpolation-based counterparts and the new event synchronization function (ESF). We test the efficiency of the methods in estimating coupling strength and coupling lag numerically, using ensembles of synthetic stalagmites with short, autocorrelated, linear and nonlinearly coupled proxy time series, and in the application to real stalagmite time series. In the linear test case coupling strength increases are identified consistently for all estimators, while in the nonlinear test case the correlation-based approaches fail. The lag at which the time series are coupled is identified correctly as the maximum of the similarity functions in around 60–55% (in the linear case) to 53–42% (for the nonlinear processes) of the cases when the dating of the synthetic stalagmite is perfectly precise. If the age uncertainty increases beyond 5% of the time series length, however, the true coupling lag is not identified more often than the others for which the similarity function was estimated. Age uncertainty contributes up to half of the uncertainty in the similarity estimation process. Time series irregularity contributes less, particularly for the adapted Gaussian-kernel based estimators and the event synchronization function. The introduced link strength concept summarizes the hypothesis test results and balances the individual strengths of the estimators: while gXCF is particularly suitable for short and irregular time series, gMI and the ESF can identify nonlinear dependencies. ESF could, in particular, be suitable to study extreme event dynamics in paleoclimate records. Programs to analyze paleoclimatic time series for significant dependencies are included in a freely available software toolbox.

Description: Impact of precession on the climate, vegetation and fire activity in southern Africa during MIS4 Climate of the Past Discussions, 9, 5391-5438, 2013 Author(s): M.-N. Woillez, G. Levavasseur, A.-L. Daniau, M. Kageyama, D. H. Urrego, and M.-F. Sánchez-Goñi The relationships between climate, vegetation and fires are a major subject of investigation in the context of climate change. In southern Africa, fire is known to play a crucial role in the existence of grasslands and Mediterranean-like biomes. Microcharcoal-based reconstructions of past fire activity in that region have shown a tight correlation between grass-fueled fires and the precessional cycle, with maximum fire activity during maxima of the climatic precession index. These changes have been interpreted as the result of changes in fuel load in response to precipitation changes in eastern southern Africa. Here we use the general circulation model IPSL_CM5A and the dynamical vegetation model LPJ-LMfire to investigate the response of climate, vegetation and fire activity to precession changes in southern Africa during Marine Isotopic Stage 4. We perform two climatic simulations, for a maximum and minimum of the precession index, and use a statistical downscaling method to increase the spatial resolution of the IPSL_CM5A outputs over southern Africa and perform high-resolution simulations of the vegetation and fire activity. Our results show an anti-correlation between the North and South African monsoons in response to precession changes. A decrease of the precession climatic index leads to a precipitation decrease in the summer rainfall area of southern Africa. The drying of climate leads to a decrease of vegetation cover and fire activity. Our results are in qualitative agreement with data and confirm that fire activity in southern Africa is strongly dependent on the vegetation type.

Description: Ensemble meteorological reconstruction using circulation analogues of 1781–1785 Climate of the Past Discussions, 9, 5157-5182, 2013 Author(s): P. Yiou, M. Boichu, R. Vautard, M. Vrac, S. Jourdain, E. Garnier, F. Fluteau, and L. Menut This paper uses a method of atmospheric flow analogues to reconstruct an ensemble of atmospheric variables (namely sea-level pressure, surface temperature and wind speed) between 1781 and 1785. The properties of this ensemble are investigated and tested against observations of temperature. The goal of the paper is to assess whether the atmospheric circulation during the Laki volcanic eruption (in 1783) and the subsequent winter were similar to the conditions that prevailed in the winter 2009/2010 and during spring 2010. We find that the three months following the Laki eruption in June 1783 barely have analogues in 2010. The cold winter of 1783/1784 yields circulation analogues in 2009/2010. Our analysis suggests that it is unlikely that the Laki eruption was responsible for the cold winter of 1783/1784, of the relatively short memory of the atmospheric circulation.

Description: Seasonal changes in glacial polynya activity inferred from Weddell Sea varves Climate of the Past Discussions, 9, 5123-5156, 2013 Author(s): D. Sprenk, M. E. Weber, G. Kuhn, V. Wennrich, T. Hartmann, and K. Seelos The Weddell Sea and the associated Filchner-Rønne Ice Shelf constitute key regions for global bottom-water production today. However, little is known about bottom-water production under different climate and ice-sheet conditions. Therefore, we studied core PS1795, which consists primarily of fine-grained siliciclastic varves that were deposited on contourite ridges in the southeastern Weddell Sea during the Last Glacial Maximum (LGM). We conducted high-resolution X-ray fluorescence (XRF) analysis and grain-size measurements with the RADIUS tool (Seelos and Sirocko, 2005) using thin sections to characterize the two seasonal components of the varves at sub-mm resolution to distinguish the seasonal components of the varves. Bright layers contain coarser grains that can mainly be identified as quartz in the medium to coarse silt grain size. They also contain higher amounts of Si, Zr, Ca, and Sr, as well as more ice-rafted debris (IRD). Dark layers, on the other hand, contain finer particles such as mica and clay minerals from the chlorite and illite groups. In addition, chemical elements, Fe, Ti, Rb, and K are elevated as well. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of glacially enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity. Accordingly, katabatic (i.e. offshore blowing) winds removed sea ice from the ice edge, leading to coastal polynya formation. We suggest that glacial processes were similar to today with stronger katabatic winds and enhanced coastal polynya activity during the winter season. If this is correct, silty layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom water formation and increased sediment transport. Vice versa, finer-grained clayey layers were then deposited during summer, when coastal polynya activity was likely reduced.

Description: Is blue intensity ready to replace maximum latewood density as a strong temperature proxy? A tree-ring case study on Scots pine from northern Sweden Climate of the Past Discussions, 9, 5227-5261, 2013 Author(s): J. A. Björklund, B. E. Gunnarson, K. Seftigen, J. Esper, and H. W. Linderholm At high latitudes, where low temperatures mainly limit tree-growth, measurements of wood density (e.g. Maximum Latewood Density, MXD) using the X-Ray methodology provide a temperature proxy that is superior to that of TRW. Density measurements are however costly and time consuming and have lead to experimentation with optical flatbed scanners to produce Maximum Blue Intensity (BI max ). BI max is an excellent proxy for density on annual scale but very limited in skill on centennial scale. Discolouration between samples is limiting BI max where specific brightnesses can have different densities. To overcome this, the new un-exploited parameter Δ blue intensity (ΔBI) was constructed by using the brightness in the earlywood (BI EW ) as background, (BI max − BI EW = ΔBI). This parameter was tested on X-Ray material (MXD − earlywood density = ΔMXD) and showed great potential both as a quality control and as a booster of climate signals. Unfortunately since the relationship between grey scale and density is not linear, and between-sample brightness can differ tremendously for similar densities, ΔBI cannot fully match ΔMXD in skill as climate proxy on centennial scale. For ΔBI to stand alone, the range of brightness/density offset must be reduced. Further studies are needed to evaluate this possibility, and solutions might include heavier sample treatment (reflux with chemicals) or image-data treatment (digitally manipulating base-line levels of brightness).

Description: Migrating subtropical front and Agulhas Return Current affect the southwestern Indian Ocean during the late Quaternary Climate of the Past Discussions, 9, 5521-5551, 2013 Author(s): D. K. Naik, R. Saraswat, N. Khare, A. C. Pandey, and R. Nigam The position of sub-tropical front (STF), Agulhas Current (AC) and Agulhas Return Current (ARC) controls the hydrography of southwestern Indian Ocean. Although, equator-ward migration of STF and reduction in Agulhas leakage has been reported during the last glacial period, the fate of ARC during the last glacial–interglacial cycle is not clear. Therefore, in order to understand changes in the position and strength of ARC during the last glacial–interglacial cycle, here we reconstruct hydrographic changes in the southwestern Indian Ocean from temporal variation in planktic foraminiferal abundance, stable isotopic ratio (δ 18 O) and trace metal ratio (Mg / Ca) of planktic foraminifera Globigerina bulloides in a core collected from the Agulhas Retroflection Region (ARR) in the southwestern Indian Ocean. Increased abundance of G. bulloides suggests that the productivity in the southwestern Indian Ocean increased during glacial period which confirms previous reports of high glacial productivity in the Southern Ocean. The increased productivity was likely driven by a combination of equator-ward migration of subtropical front and westerlies. Increase in relative abundance of Neogloboquadrina pachyderma Dextral suggests warming of ARR leading to strong thermocline in the southwestern Indian Ocean during the last glacial period. We suggest that the warming of Agulhas Retroflection Region was driven by strengthened ARC which shifted to the east of its present location, thus bringing warmer and saltier water to the southwestern Indian Ocean. Therefore, it is inferred that over the last glacial–interglacial cycle, the hydrography of southwestern Indian Ocean was driven by an eastward shift of retroflection region as well as migrating subtropical front.

Description: The response of the Peruvian Upwelling Ecosystem to centennial-scale global change during the last two millennia Climate of the Past Discussions, 9, 5479-5519, 2013 Author(s): R. Salvatteci, D. Gutiérrez, D. Field, A. Sifeddine, L. Ortlieb, I. Bouloubassi, M. Boussafir, H. Boucher, and F. Cetin The Tropical Pacific ocean-atmosphere system influences global climate on interannual, decadal, as well as at longer timescales. Given the uncertainties in the response of the Tropical Pacific to the ongoing greenhouse effect, it is important to assess the natural range of the Tropical Pacific climate variability in response to global natural changes, and to understand the underlying mechanisms. The Peruvian Upwelling Ecosystem (PUE) represents an ideal area to reconstruct past changes in ocean-atmosphere systems because productivity and subsurface oxygenation are strongly linked to changes in the strength of the Walker circulation. Throughout the last 2000 yr, warmer (the Roman Warm Period [RWP], the Medieval Climate Anomaly [MCA] and the Current Warm Period [CWP]), and colder (the Dark Ages Cold Period [DACP] and Little Ice Age [LIA]) intervals occurred with considerable changes around the globe. In order to reconstruct the PUE response to these climatic periods and reveal the underlying mechanisms, we use a multi-proxy approach including organic and inorganic proxies in finely laminated sediments retrieved off Pisco (~ 14° S), Peru. Our results indicate that the PUE exhibited a La Niña-like mean state during the warm periods, characterized by an intense OMZ and high marine productivity. During cold periods the PUE exhibited an El Niño-like mean state, characterized by a weak OMZ and low marine productivity. Comparing our results with other relevant paleoclimatic reconstructions revealed that changes in the strength of the Walker circulation and the expansion/contraction of the South Pacific Sub-tropical High controlled productivity and subsurface oxygenation in the PUE during the last two millennia. This indicate that large scale circulation changes are the driving forces in maintaining productivity and subsurface oxygenation off Peru at centennial time scales during the past two millennia.

Description: Cumulated insolation: a simple explanation of Milankovitch's forcing on climate changes Climate of the Past Discussions, 9, 5553-5568, 2013 Author(s): F. Marra The occurrence of the sudden melting of the ice sheets during the glacial terminations is explained in this paper as the consequence of the combined role of the minima and the maxima of mean summer insolation on the Northern Hemisphere, providing a new contribution to understand the mechanisms ruling glacial forcing. Indeed, no satisfactory answer has been provided so far to the question why one specific maximum, after a series of consecutive maxima of insolation, has the potentiality to trigger a deglaciation. The explanation proposed in this paper accounts for a pre-conditioning factor, represented by "mild" (warmer) minimum, followed by a sufficiently warm maximum as the conditions that cause the end of a glacial cycle. These conditions are realized whenever the sum of the values of each consecutive minima and maxima ("cumulated insolation") on the curve of mean summer insolation at 65° N exceeds 742 Watt m −2 . The comparison of the succession of these cumulated insolation values with the astronomically tuned Oxygen isotopes record provides a satisfactory match with the occurrence of all the glacial terminations in the last 800 ka.

Description: Chronology of Lake El'gygytgyn sediments Climate of the Past Discussions, 9, 3061-3102, 2013 Author(s): N. R. Nowaczyk, E. M. Haltia, D. Ulbricht, V. Wennrich, M. A. Sauerbrey, P. Rosén, H. Vogel, A. Francke, C. Meyer-Jacob, A. A. Andreev, and A. V. Lozhkin A 318 m long sedimentary profile drilled by the International Continental Scientific Drilling Program (ICDP) at Site 5011-1 in Lake El'gygytgyn, Far East Russian Arctic, has been analysed for its sedimentologic response to global climate modes by chrono-stratigraphic methods. The 12 km wide lake is sited in an 18 km large crater that was created by the impact of a meteorite 3.58 Ma ago. Since then sediments have been continuously deposited. For establishing their chronology, major reversals of the Earth's magnetic field provided initial tie points for the age model, confirming that the impact occurred in the earliest Gauss chron. Various stratigraphic parameters, reflecting redox conditions at the lake floor and climatic conditions in the catchment were tuned synchronously to Northern Hemisphere insolation variations and the marine oxygen isotope stack, respectively. Thus, a robust age model comprising more than 600 tie points could be defined. It could be shown that deposition of sediments in Lake El'gygytgyn occurred in concert with global climatic cycles. The upper ~160 m of sediments represent the past 3.3 Ma, equivalent to sedimentation rates of 4 to 5 cm ka −1 , whereas the lower 160 m represent just the first 0.3 Ma after the impact, equivalent to sedimentation rates in the order of 45 cm ka −1 .

Description: Late Cenozoic continuous aridification in the western Qaidam Basin: evidence from sporopollen records Climate of the Past Discussions, 9, 1485-1508, 2013 Author(s): Y. F. Miao, X. M. Fang, F. L. Wu, M. T. Cai, C. H. Song, Q. Q. Meng, and L. Xu Cenozoic climate changes in inner Asia provide a basis for understanding linkages between global cooling, the Tibetan Plateau uplift, and possibly the development of the East Asian monsoon. Based on the compiled palynological results from the western Qaidam Basin, this study reconstructed an 18 Ma record of changing vegetation and paleoclimates since the middle Miocene. Thermophilic taxa percentages were highest between 18 and 14 Ma and decreased after 14 Ma, corresponding closely with the Middle Miocene Climatic Optimum (MMCO) between 18 and 14 Ma and the following global climatic cooling. After 3.6 Ma, the thermophilic taxa percentages further decreased, showing the inevitable relations with the ice-sheets enlargement in the North Hemisphere. During the same period of time, the increase in xerophytic taxa percentages and decrease in conifers percentages imply aridification in both the basin and surrounding mountains since 18 Ma. These results indicate that global cooling mainly controlled the climate change from a relative warm-wet stage to a cold-dry stage during the late Cenozoic at the western Qaidam Basin, and that the Tibetan Plateau uplift also contributed in contrast to the East Asian summer monsoon.

Description: Post-Pliocene establishment of the present monsoonal climate in SW China: evidence from the late Pliocene Longmen megaflora Climate of the Past Discussions, 9, 1675-1701, 2013 Author(s): T. Su, F. M. B. Jacques, R. A. Spicer, Y.-S. Liu, Y.-J. Huang, Y.-W. Xing, and Z.-K. Zhou The paleoclimate of the late Pliocene Longmen flora from Yongping County located at the southeastern boundary of the Qinghai-Tibet Plateau was reconstructed using two leaf physiognomy based methods, i.e. Leaf Margin Analysis (LMA) and Climate Leaf Analysis Multivariate Program (CLAMP), to understand the paleoclimate condition and geographical pattern of monsoonal climate in southwestern China during the late Pliocene. The mean annual temperatures (MATs) estimated by LMA and CLAMP are 17.4 ± 3.3 °C and 17.4 ± 1.3 °C, respectively, compared with 15.9 °C at present. Meanwhile, the growing season precipitation (GSP) estimated by CLAMP is 1735.5 ± 217.7 mm in the Longmen flora, compared with 986.9 mm nowadays. The calculated monsoon index (MSI) of the Longmen flora is significantly lower than that of today. These results appear consistent with previous studies based on the coexistence approach (CA), and further suggest that there was a slightly warmer and much wetter climate during the late Pliocene than the present climate in western Yunnan. We conclude that the significant change of the monsoonal climate might have been resulted from the continuous uplift of mountains in western Yunnan, as well as the intensification of eastern Asian winter monsoon, both occurring concurrently in the post-Pliocene period.

Description: Holocene vegetation and climate changes in central Mediterranean inferred from a high-resolution marine pollen record (Adriatic Sea) Climate of the Past Discussions, 9, 1969-2014, 2013 Author(s): N. Combourieu-Nebout, O. Peyron, V. Bout-Roumazeilles, S. Goring, I. Dormoy, S. Joannin, L. Sadori, G. Siani, and M. Magny To understand the effects of future climate change on the ecology of the central Mediterranean we can look to the impacts of long-term, millennial to centennial-scale climatic variability on vegetation in the basin. Pollen data from the Adriatic Marine core MD 90-917 allows us to reconstruct vegetation and regional climate changes over the south central Mediterranean during the Holocene. Clay mineral ratios from the same core reflect the relative contributions of riverine (illite and smectite) and eolian (kaolinite) contributions to the site, and thus act as an additional proxy with which to test precipitation changes in the Holocene. Vegetation reconstruction shows vegetation responses to the late-Glacial Preboreal oscillation, most likely driven by changes in seasonal precipitation. Pollen-inferred temperature declines during the early-mid Holocene, but increases during the mid-late Holocene, similar to southern-western Mediterranean climatic patterns during the Holocene. Several short climatic events appear in the record, indicating the sensitivity of vegetation in the region to millennial-scale variability. Reconstructed summer precipitation shows a regional maximum between 8000 and 7000 cal yr BP similar to the general pattern across southern Europe. Two important shifts in vegetation occur at 7700 and between 7500 and 7000 yr. These vegetation shifts are linked to changes in seasonal precipitation and are correlated to increased river inputs respectively from the north (7700 event) and from the central Adriatic borderlands (7500–7000 event). These results reinforce the strengths of multi-proxy analysis and provide a deeper understanding of the role of precipitation and particularly the seasonality of precipitation in mediating vegetation change in the central Mediterranean during the Holocene.

Description: Southern Hemisphere orbital forcing and its effects on CO 2 and tropical Pacific climate Climate of the Past Discussions, 9, 1869-1900, 2013 Author(s): K. Tachikawa, A. Timmermann, L. Vidal, C. Sonzogni, and O. E. Timm The western Pacific warm pool (WPWP) is an important heat source for the atmospheric circulation and influences climate conditions worldwide. Understanding its sensitivity to past radiative perturbations may help better contextualize the magnitudes and patterns of current and projected tropical climate change. Here we present a new Mg/Ca-based sea surface temperature (SST) reconstruction over the past 400 kyr from the Bismarck Sea, off Papua New Guinea, along with results from a transient earth system model simulation. Our results document the primary influence of CO 2 forcing on glacial/interglacial WPWP SSTs and secondary effects due to changes in wind-driven tropical boundary currents. In addition to the SST, deep ocean temperature reconstructions from this core are linked with Southern Ocean temperature and sea-ice variations on timescales of ~ 23 kyr. It is proposed that Southern Hemisphere insolation changes serve as pacemaker for sea-ice variations in the Southern Ocean, which in turn modulate windstress curl-driven upwelling of carbon-rich waters, hence controlling atmospheric CO 2 and tropical WPWP temperatures.

Description: A regional climate palaeosimulation for Europe in the period 1500–1990 – Part 1: Model validation Climate of the Past Discussions, 9, 1803-1839, 2013 Author(s): J. J. Gómez-Navarro, J. P. Montávez, S. Wagner, and E. Zorita We present and analyse a high-resolution regional climate palaeosimulation encompassing the European region for the period 1500–1990. We use the regional model MM5 coupled to the global model ECHO-G. Both models were driven by reconstructions of three external factors: greenhouse gas concentrations, Total Solar Irradiance and volcanic activity. The simulation has been assessed in a recent period by comparing the model results with the Climate Research Unit (CRU) database. The results show that although the regional model is tightly driven by the boundary conditions, it is able to improve the reliability of the simulations, narrowing the differences to the observations, especially in areas of complex topography. Additionally, the evolution of the spatial distributions of temperature and precipitation through the last five centuries has been analysed. The mean values of temperature reflects the influence of the external forcings but, contrary to the results obtained under climate change scenario conditions, we found that higher-order momenta of the probability distribution of seasonal temperature and precipitation are hardly affected by changes in the external forcings

Description: North–south palaeohydrological contrasts in the central Mediterranean during the Holocene: tentative synthesis and working hypotheses Climate of the Past Discussions, 9, 1901-1967, 2013 Author(s): M. Magny, N. Combourieu Nebout, J. L. de Beaulieu, V. Bout-Roumazeilles, D. Colombaroli, S. Desprat, A. Francke, S. Joannin, O. Peyron, M. Revel, L. Sadori, G. Siani, M. A. Sicre, S. Samartin, A. Simonneau, W. Tinner, B. Vannière, B. Wagner, G. Zanchetta, F. Anselmetti, E. Brugiapaglia, E. Chapron, M. Debret, M. Desmet, J. Didier, L. Essallami, D. Galop, A. Gilli, J. N. Haas, N. Kallel, L. Millet, A. Stock, J. L. Turon, and S. Wirth On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north–south transect, data collected in the Central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and temporal coherency. Contrasting patterns of palaeohydrological changes have been evidenced in the Central Mediterranean: south (north) of around 40° N of latitude, the middle part of the Holocene was characterised by lake-level maxima (minima), during an interval dated to ca. 10 300–4500 cal BP to the south and 9000–4500 cal BP to the north. Available data suggest that these contrasting palaeohydrological patterns operated throughout the Holocene, both on millennial and centennial scales. Regarding precipitation seasonality, maximum humidity in the Central Mediterranean during the middle part of the Holocene was characterised by humid winters and dry summers north of ca. 40° N, and humid winters and summers south of ca. 40° N. This may explain an apparent conflict between palaeoclimatic records depending on the proxies used for reconstruction as well as the synchronous expansion of tree species taxa with contrasting climatic requirements. In addition, south of ca. 40° N, the first millennium of the Holocene was characterised by very dry climatic conditions not only in the Eastern, but also in the Central and the Western Mediterranean zones as reflected by low lake levels and delayed reforestation. These results suggest that, in addition to the influence of the Nile discharge reinforced by the African monsoon, the deposition of Sapropel 1 has been favoured (1) by an increase in winter precipitation in the northern Mediterranean borderlands, and (2) by an increase in winter and summer precipitation in the southern Mediterranean area. The climate reversal following the Holocene climate optimum appears to have been punctuated by two major climate changes around 7500 and 4500 cal BP. In the Central Mediterranean, the Holocene palaeohydrological changes developed in response to a combination of orbital, ice-sheet and solar forcing factors. The maximum humidity interval in the south-central Mediterranean started at ca. 10 300 cal BP, in correlation with the decline (1) of the possible blocking effects of the North Atlantic anticyclone linked to maximum insolation, and/or (2) of the influence of the remnant ice sheets and fresh water forcing in the North Atlantic Ocean. In the north-central Mediterranean, the lake-level minimum interval began only around 9000 cal BP when the Fennoscandian ice-sheet disappeared and a prevailing positive NAO-type circulation developed in the North Atlantic area. The major palaeohydrological oscillation around 4500–4000 cal BP may be a non-linear response to the gradual decrease, with additional key seasonal and interhemispherical changes, in insolation. On a centennial scale, the successive climatic events which punctuated the entire Holocene in the central Mediterranean coincided with cooling events associated with deglacial outbursts in the North Atlantic area and decreases in solar activity during the interval 11 700–7000 cal BP, and to a possible combination of NAO-type circulation and solar forcing since ca. 7000 cal BP onwards. Thus, regarding the centennial-scale climatic oscillations, the Mediterranean Basin appears to have been strongly linked to the North Atlantic area and affected by solar activity over the entire Holocene. In addition to model experiments, a better understanding of forcing factors and past atmospheric circulation patterns behind the Holocene palaeohydrological changes in the Mediterranean area will require further investigation to establish additional high-resolution and well-dated records in selected locations around the Mediterranean Basin and in adjacent regions. Special attention should be paid to greater precision in the reconstruction, on millennial and centennial time scales, of changes in the latitudinal location of the limit between the northern and southern palaeohydrological Mediterranean sectors, depending on (1) the intensity and/or characteristics of climatic periods/oscillations (e.g. Holocene thermal maximum versus Neoglacial, as well as, for instance, the 8.2 ka event versus the 4 ka event or the Little Ice Age), and (2) on varying geographical conditions from the western to the eastern Mediterranean areas (longitudinal gradients).

Description: The last 7 millennia of vegetation and climate changes at Lago di Pergusa (central Sicily, Italy) Climate of the Past Discussions, 9, 2059-2094, 2013 Author(s): L. Sadori, E. Ortu, O. Peyron, G. Zanchetta, B. Vannière, M. Desmet, and M. Magny The aim of this study is to investigate climate changes and human activities under the lens of palynology. Based on a new high-resolution pollen sequence (PG2) from Lago di Pergusa (667 m a.s.l., central Sicily, Italy) covering the last 6700 yr, we propose a reconstruction of climate and landscape changes over the recent past in central Sicily. Compared to former studies from Lago di Pergusa (Sadori and Narcisi, 2001), this work provides a reconstruction of the evolution of vegetation and climate over the last millennia in central Sicily, indeed completing previous results with new data which is particularly detailed on the last 3000 yr. Joint actions of increasing dryness, climate oscillations, and human impact shaped the landscape of this privileged site. Lago di Pergusa, in fact, besides being the main inland lake of Sicily, is very sensitive to climate change and its territory was inhabited and exploited continuously since the prehistory. The lake sediments turned out to be a good observatory for the natural phenomena occurred in the last thousands of years. Results of the pollen-based study are integrated with changes in magnetic susceptibility and a tephra layer characterization. The tephra layer was shown to be related to the Sicanians' event, radiocarbon dated at 3055 ± 75 yr BP (Sadori and Narcisi, 2001). We performed palaeoclimate reconstructions by MAT and WA-PLS. Palaeoclimate reconstructions based on the core show important climate fluctuations throughout the Holocene. Climate reconstruction points out four phases of cooling and enhanced wetness in the last three millennia (2600–2000, 1650–1100, 850–550, 400–200 cal BP). This appears to be the evidence of local responses to global climate oscillations during the recent past.

Description: Dust and associated trace element fluxes in a firn core from the coastal East Antarctica and its linkages with the Southern Hemisphere climate variability over the last ~ 50 yr Climate of the Past Discussions, 9, 1841-1867, 2013 Author(s): C. M. Laluraj, M. Thamban, and K. Satheesan High-resolution records of dust and trace element fluxes were studied in a firn core from the coastal Dronning Maud Land (cDML) in East Antarctica to identify the influence of climate variability on accumulation of these components over the past ~ 50 yr. A doubling of dust deposition was observed since 1985, coinciding with a shift in the Southern Annular Mode (SAM) index to positive values and associated increase in the wind speed. Back-trajectories showed that an increase in dust deposition is associated with the air parcels originating from north-west of the site, possibly indicating its origin from the Patagonian region. Our results suggest that while multiple processes could have influenced the increased dust formation, shift in SAM had a dominant influence on its transport. It is observed that since the 1985s the strength of easterlies increased significantly over the cDML region, which could sink air and dust material to the region that were brought by the westerlies through mass compensation. The correlation between the dust flux and δ 18 O records further suggest that enhanced dust flux in the firn core occurred during periods of colder atmospheric temperature, which reduced the moisture content and increased dust fall. Interestingly, the timing and amplitude of the insoluble dust peaks matched remarkably well with the fluxes of Ba, Cr, Cu, and Zn confirming that dust was the main carrier/source of atmospheric trace elements to East Antarctica during the recent past.

Description: An inter-laboratory investigation of the Arctic sea ice biomarker proxy IP 25 in marine sediments: key outcomes and recommendations Climate of the Past Discussions, 9, 5263-5298, 2013 Author(s): S. T. Belt, T. A. Brown, L. Ampel, P. Cabedo-Sanz, K. Fahl, J. J. Kocis, G. Massé, A. Navarro-Rodriguez, J. Ruan, and Y. Xu We describe the results of an inter-laboratory investigation into the identification and quantification of the Arctic sea ice biomarker proxy IP 25 in marine sediments. 7 laboratories took part in the study, which consisted of the analysis of IP 25 in a series of sediment samples from different regions of the Arctic, sub-Arctic and Antarctic, additional sediment extracts and purified standards. The results obtained allowed 4 key outcomes to be determined. First, IP 25 was identified by all laboratories in sediments from the Canadian Arctic with inter-laboratory variation in IP 25 concentration being substantially larger than within individual laboratories. This greater variation between laboratories was attributed to the difficulty in accurately determining instrumental response factors for IP 25 , despite provision of appropriate standards. Second, the identification of IP 25 by 3 laboratories in sediment from SW Iceland that was believed to represent a blank, was interpreted as representing a better limit of detection or quantification for such laboratories, contamination or mis-identification. These alternatives could not be distinguished conclusively with the data available, although it is noted that the precision of these data was significantly poorer compared with the other IP 25 concentration measurements. Third, 3 laboratories reported the occurrence of IP 25 in a sediment sample from the Antarctic Peninsula even though this biomarker is believed to be absent from the Southern Ocean. This anomaly is attributed to a combined chromatographic and mass spectrometric interference that results from the presence of a di-unsaturated highly branched isoprenoid (HBI) pseudo-homologue of IP 25 that occurs in Antarctic sediments. Finally, data are presented that suggest that extraction of IP 25 is consistent between Automated Solvent Extraction (ASE) and sonication methods and that IP 25 concentrations based on 7-hexylnonadecane as an internal standard are comparable using these methods. Recoveries of some more unsaturated HBIs and the internal standard 9-octylheptadecene, however, were lower with the ASE procedure, possibly due to partial degradation of these more reactive chemicals as a result of higher temperatures employed with this method. For future measurements, we recommend the use of reference sediment material with known concentration(s) of IP 25 for determining and routinely monitoring instrumental response factors. Given the significance placed on the presence (or otherwise) of IP 25 in marine sediments, some further recommendations pertaining to quality control are made that should also enable the two main anomalies identified here to be addressed.

Description: Expressions of climate perturbations in western Ugandan crater lake sediment records during the last 1000 yr Climate of the Past Discussions, 9, 5183-5226, 2013 Author(s): K. Mills, D. B. Ryves, N. J. Anderson, C. L. Bryant, and J. J. Tyler Equatorial East Africa has a complex, regional patchwork of climate regimes, with multiple interacting drivers. Recent studies have focussed on large lakes and reveal signals that are smoothed in both space and time, and, whilst useful at a continental scale, are of less relevance when understanding short-term, abrupt or immediate impacts of climate and environmental changes. Smaller-scale studies have highlighted spatial complexity and regional heterogeneity of tropical palaeoenvironments in terms of responses to climatic forcing (e.g. the Little Ice Age [LIA]) and questions remain over the spatial extent and synchroneity of climatic changes seen in East African records. Sediment cores from paired crater lakes in western Uganda were examined to assess ecosystem response to long-term climate and environmental change as well as testing responses to multiple drivers using redundancy analysis. These archives provide annual to sub-decadal records of environmental change. The records from the two lakes demonstrate an individualistic response to external (e.g. climatic) drivers, however, some of the broader patterns observed across East Africa suggest that the lakes are indeed sensitive to climatic perturbations such as a dry Mediaeval Climate Anomaly (MCA; 1000–1200 AD) and a relatively drier climate during the main phase of the LIA (1500–1800 AD); though lake levels in western Uganda do fluctuate. The relationship of Ugandan lakes to regional climate drivers breaks down c. 1800 AD, when major changes in the ecosystems appear to be a response to sediment and nutrient influxes as a result of increasing cultural impacts within the lake catchments. The data highlight the complexity of individual lake response to climate forcing, indicating shifting drivers through time. This research also highlights the importance of using multi-lake studies within a landscape to allow for rigorous testing of climate reconstructions, forcing and ecosystem response.

Description: Evaluation of seasonal climates of the Mediterranean and nothern Africa in the CMIP5 simulations Climate of the Past Discussions, 9, 5347-5389, 2013 Author(s): A. Perez-Sanz, G. Li, P. González-Sampériz, and S. P. Harrison We analyze the spatial expression of seasonal climates of the Mediterranean and northern Africa in pre-Industrial ( piControl ) and mid-Holocene ( midHolocene , 6 ka) simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Modern observations show four distinct precipitation regimes characterized by differences in the seasonal distribution and total amount of precipitation: an equatorial band characterized by a double peak in rainfall, the monsoon zone characterized by summer rainfall, the desert characterized by low seasonality and total precipitation, and the Mediterranean zone characterized by summer drought. Most models correctly simulate the position of the Mediterranean and the equatorial climates in the piControl simulations, but over-estimate the extent of monsoon influence and underestimate the extent of desert. However, most models fail to reproduce the amount of precipitation in each zone. Model biases in the simulated magnitude of precipitation are unrelated to whether the models reproduce the correct spatial patterns of each regime. In the midHolocene , the models simulate a reduction in winter rainfall in the equatorial zone, and a northward expansion of the monsoon with a significant increase in summer and autumn rainfall. Precipitation is slightly increased in the desert, mainly in summer and autumn, with northward expansion of the monsoon. Changes in the Mediterranean are small, although there is an increase in spring precipitation consistent with palaeo-observations of increased growing-season rainfall. Comparison with reconstructions shows that most models under-estimate the mid-Holocene changes in annual precipitation, except in the equatorial zone. Biases in the piControl have only a limited influence on midHolocene anomalies in ocean-atmosphere models; carbon-cycle models show no relationship between piControl bias and midHolocene anomalies. Biases in the prediction of the midHolocene monsoon expansion are unrelated to how well the models simulate changes in Mediterranean climate.

Description: Orbital and millennial-scale environmental changes between 64 and 25 ka BP recorded in Black Sea sediments Climate of the Past Discussions, 9, 5439-5477, 2013 Author(s): L. S. Shumilovskikh, D. Fleitmann, N. R. Nowaczyk, H. Behling, F. Marret, A. Wegwerth, and H. W. Arz High-resolution pollen and dinoflagellate cyst records from marine sediment core 25-GC1 were used to reconstruct vegetation dynamics in Northern Anatolia and surface conditions of the Black Sea between 64 and 25 ka BP. During this period, the dominance of Artemisia in the pollen record indicates a steppe landscape and arid climate conditions. However, the presence of temperate and warm-temperate arboreal pollen suggests the existence of glacial refugia in Northern Anatolia. A general cooling trend towards 25 ka BP is evidenced by the decrease of Quercus and increase of Pinus . There is evidence of orbital-driven vegetation dynamics in Northern Anatolia during 64–25 ka BP with spread of steppe during precession minima (insolation maxima) and development of forests during precession maxima (insolation minima). Dansgaard–Oeschger (D–O) events are characterized by a marked increase in temperate tree pollen, indicating a spread of forests due to warm and wet conditions in Northern Anatolia. The dominance of Pyxidinopsis psilata and Spiniferites cruciformis in the dinocyst record indicates a rather brackish Black Sea during the last glacial period. The decrease of marine indicators (marine dinocysts, acritachs) at ~ 54 ka BP and increase of freshwater algae ( Pediastrum, Botryococcus ) from 32 to 25 ka BP reveals freshening of the Black Sea surface water, related to orbital-driven arid/humid phases in the region, influencing hydrology and level changes of the Black Sea. D–O interstadials are characterized by high dinocyst concentrations and calcium carbonate content, as a result of an increase in primary productivity in the Black Sea. Heinrich events show a similar impact on the environment in Northern Anatolia/Black Sea region as D–O stadials.

Description: Tropical climate and vegetation changes during Heinrich Event 1: comparing climate model output to pollen-based vegetation reconstructions with emphasis on the region around the tropical Atlantic Ocean Climate of the Past Discussions, 7, 1973-2019, 2011 Author(s): D. Handiani, A. Paul, and L. Dupont Abrupt climate changes associated with Heinrich Event 1 (HE1) about 18 to 15 thousand years before present (ka BP) strongly affected climate and vegetation patterns not only in the Northern Hemisphere, but also in tropical regions in the South Atlantic Ocean. We used the University of Victoria (UVic) Earth System-Climate Model (ESCM) with dynamical vegetation and land surface components to simulate four scenarios of climate-vegetation interaction: the pre-industrial era (PI), the Last Glacial Maximum (LGM), and a Heinrich-like event with two different climate backgrounds (interglacial and glacial). The HE1-like simulation with a glacial climate background produced sea surface temperature patterns and enhanced interhemispheric thermal gradients in accordance with the "bipolar seesaw" hypothesis. It allowed us to investigate the vegetation changes that result from a transition to a drier climate as predicted for northern tropical Africa due to a southward shift of the Intertropical Convergence Zone (ITCZ). We found that a cooling of the Northern Hemisphere caused a southward shift of those plant-functional types (PFTs) in Northern Tropical Africa that are indicative of an increased desertification, and a retreat of broadleaf forests in Western Africa and Northern South America. We used the PFTs generated by the model to calculate mega-biomes to allow for a direct comparison between paleodata and palynological vegetation reconstructions. Our calculated mega-biomes for the pre-industrial period and the LGM corresponded well to the modern and LGM sites of the BIOME6000 (v.4.2) reconstruction, except that our present-day simulation predicted the dominance of grassland in Southern Europe and our LGM simulation simulated more forest cover in tropical and sub-tropical South America. The mega-biomes from the HE1 simulation with glacial background climate were in agreement with paleovegetation data from land and ocean proxies in West, Central, and Northern Tropical Africa as well as Northeast South America. However, our model did not agree well with predicted biome distributions in Eastern South America.

Description: Bridging the Faraoni and Selli oceanic anoxic events: short and repetitive dys- and anaerobic episodes during the late Hauterivian to early Aptian in the central Tethys Climate of the Past Discussions, 7, 2021-2059, 2011 Author(s): K. B. Föllmi, M. Bôle, N. Jammet, P. Froidevaux, A. Godet, S. Bodin, T. Adatte, V. Matera, D. Fleitmann, and J. E. Spangenberg A detailed stratigraphical and geochemical analysis was performed on the upper part of the Maiolica Formation outcropping in the Breggia (southern Switzerland) and Capriolo sections (northern Italy). In these localities, the Maiolica Formation consists of well-bedded, partly siliceous, pelagic, micritic carbonate, which lodges numerous thin, dark and organic-rich layers. Stable-isotope, phosphorus, organic-carbon and a suite of redox-sensitive trace-metal contents (RSTE: Mo, U, Co, V and As) were measured. Higher densities of organic-rich layers were identified in the uppermost Hauterivian, lower Barremian and the Barremian-Aptian boundary intervals, whereas the upper Barremian interval and the interval immediately following the Barremian-Aptian boundary interval are characterized by lower densities of organic-rich layers. TOC contents, RSTE pattern and C org :P tot ratios indicate that most layers were deposited under dysaerobic rather than anaerobic conditions and that latter conditions were likely restricted to short intervals in the latest Hauterivian, the early Barremian and the pre-Selli early Aptian. Correlations are possible with organic-rich intervals in central Italy (the Gorgo a Cerbara section) and the Boreal northwest German Basin, and with the facies and drowning pattern in the evolution of the Helvetic segment of the northern Tethyan carbonate platform. Our data and correlations suggest that the latest Hauterivian witnessed the progressive installation of dysaerobic conditions in the Tethys, which went along with the onset in sediment condensation, phosphogenesis and platform drowning on the northern Tethyan margin, and which culminated in the Faraoni anoxic episode. This brief episode is followed by further episodes of dysaerobic conditions in the Tethys and the northwest German Basin, which became more frequent and progressively stronger in the late early Barremian. Platform drowning persisted and did not halt before the latest early Barremian. The late Barremian witnessed diminishing frequencies and intensities in dysaerobic conditions, which went along with the progressive installation of the Urgonian carbonate platform. Near the Barremian-Aptian boundary, the increasing density in dysaerobic episodes in the Tethyan and northwest German Basins is paralleled by a change towards heterozoan carbonate production on the northern Tethyan shelf. The following return to more oxygenated conditions is correlated with the second phase of Urgonian platform growth and the period immediately preceding and corresponding to the Selli anoxic episode is characterized by renewed platform drowning and the change to heterozoan carbonate production. Changes towards more humid climate conditions were likely the cause for the repetitive installation of dys- to anaerobic conditions in the Tethyan and Boreal basins and the accompanying changes in the evolution of the carbonate platform towards heterozoan carbonate-producing ecosystems and platform drowning.

Description: Degree-day melt models for paleoclimate reconstruction from tropical glaciers: calibration from mass balance and meteorological data of the Zongo glacier (Bolivia, 16 ° S) Climate of the Past Discussions, 7, 2119-2158, 2011 Author(s): P.-H. Blard, P. Wagnon, J. Lavé, A. Soruco, J.-E. Sicart, and B. Francou This paper describes several simple positive degree-day models (hereafter referred as "PDD models") designed to provide past climatic reconstruction from tropical glacier paleo-equilibrium altitude lines (paleo-ELA). Several ablation laws were tested and calibrated using the monthly ablation and meteorological data recorded from 1997 to 2006 on the Zongo glacier (Cordillera Real, Bolivia, 16° S). The performed inversion analyses indicate that the model provides a better reconstruction of the mass balance if the ablation is modeled with different melting factors for snow and ice. The inclusion of short-wave solar radiations does not induce a substantial improvement. However, this type of model may be very useful to quantify the effects of local topographic (orientation, shading) and to take into account incoming solar radiation changes at geological timescale. The performed sensitivity test indicates that, in spite of the uncertainty in the calibrated snow-ice ablation factors, all models are able to provide paleotemperatures with ~1 °C uncertainty for a given paleoprecipitation. This error includes a 50 m uncertainty in the estimate of the paleoELA. Finally, the models are characterized by different precipitation-temperature sensitivities: if a similar warming is applied, model including different ablation factors for snow and ice requires a lower precipitation increase (by ∼15 %) than others to maintain the ELA.

Description: Deglaciation records of 17 O- excess in East Antarctica: reliable reconstruction of oceanic relative humidity from coastal sites Climate of the Past Discussions, 7, 1845-1886, 2011 Author(s): R. Winkler, A. Landais, H. Sodemann, L. Dümbgen, F. Prié, V. Masson-Delmotte, B. Stenni, and J. Jouzel We measured δ 17 O and δ 18 O in two Antarctic ice cores at EPICA Dome C (EDC) and TALDICE (TD), respectively and computed 17 O- excess with respect to VSMOW. The comparison of our 17 O- excess data with the previous record obtained at Vostok (Landais et al., 2008) revealed differences up to 35 ppm in 17 O- excess mean level and evolution for the three sites. Our data showed that the large increase depicted at Vostok (20 ppm) during the last deglaciation, is a regional and not a general pattern in the temporal distribution of 17 O- excess in East Antarctica. The EDC data display an increase of 13 ppm, whereas the TD data show no significant variation from the Last Glacial Maximum (LGM) to the Early Holocene (EH). Lagrangian moisture source diagnostic revealed very different source regions for Vostok and EDC compared to TD. These findings combined with the results of a sensitivity analysis, using a Rayleigh-type isotopic model, suggest that relative humidity (RH) at the oceanic source region (OSR) are a determining factor for the spatial differences of 17 O- excess in East Antarctica. However, 17 O- excess in remote sites of continental Antarctica (e.g. Vostok) may be highly sensitive to local effects. Hence, we consider 17 O- excess in coastal East Antarctic ice cores (TD) to be more reliable as a proxy for RH at the OSR.

Description: Impact of oceanic processes on the carbon cycle during the last termination Climate of the Past Discussions, 7, 1887-1934, 2011 Author(s): N. Bouttes, D. Paillard, D. M. Roche, C. Waelbroeck, M. Kageyama, A. Lourantou, E. Michel, and L. Bopp During the last termination (from ~18 000 yr ago to ~9000 yr ago) the climate significantly warmed and the ice sheets melted. Simultaneously, atmospheric CO 2 increased from ~190 ppm to ~260 ppm. Although this CO 2 rise plays an important role in the deglacial warming, the reasons for its evolution are difficult to explain. Only box models have been used to run transient simulations of this carbon cycle transition, but by forcing the model with data constrained scenarios of the evolution of temperature, sea level, sea ice, NADW formation, Southern Ocean vertical mixing and biological carbon pump. More complex models (including GCMs) have investigated some of these mechanisms but they have only been used to try and explain LGM versus present day steady-state climates. In this study we use a climate-carbon coupled model of intermediate complexity to explore the role of three oceanic processes in transient simulations: the sinking of brines, stratification-dependant diffusion and iron fertilization. Carbonate compensation is accounted for in these simulations. We show that neither iron fertilization nor the sinking of brines alone can account for the evolution of CO 2 , and that only the combination of the sinking of brines and interactive diffusion can simultaneously simulate the increase in deep Southern Ocean δ 13 C. The scenario that agrees best with the data takes into account all mechanisms and favours a rapid cessation of the sinking of brines around 18 000 yr ago, when the Antarctic ice sheet extent was at its maximum. Sea ice formation was then shifted to the open ocean where the salty water is quickly mixed with fresher water, which prevents deep sinking of salty water and therefore breaks down the deep stratification and releases carbon from the abyss. Based on this scenario it is possible to simulate both the amplitude and timing of the CO 2 increase during the last termination in agreement with data. The atmospheric δ 13 C appears to be highly sensitive to changes in the terrestrial biosphere, underlining the need to better constrain the vegetation evolution during the termination.

Description: Productivity response of calcareous nannoplankton in the South Atlantic to the Eocene Thermal Maximum 2 (ETM2) Climate of the Past Discussions, 7, 2089-2118, 2011 Author(s): M. Dedert, H. M. Stoll, D. Kroon, N. Shimizu, and P. Ziveri The Early Eocene Thermal Maximum 2 (ETM2) at ~53.7 Ma is one of multiple hyperthermal events that followed the Paleocene-Eocene Thermal Maximum (PETM, ~55 Ma). In order to reconstruct the primary productivity response to the ETM2 in the South Atlantic, we have analyzed Sr/Ca ratios in various size fractions of bulk sediments and in picked monogeneric populations of calcareous nannofossils. The latter technique of measuring selected nannofossil populations using the ion probe circumvents possible contamination with secondary calcite. Avoiding such contamination is important for interpretation of the nannoplankton productivity record, since diagenetic processes can bias the productivity signal, as we demonstrate for Sr/Ca measurements in the fine (

Description: Reconstruction of southeast Tibetan Plateau summer cloud cover over the past two centuries using tree ring δ 18 O Climate of the Past Discussions, 7, 1825-1844, 2011 Author(s): C. Shi, V. Daux, C. Risi, S.-G. Hou, M. Stievenard, M. Pierre, Z. Li, and V. Masson-Delmotte A tree-ring δ 18 O chronology of Linzhi spruce, spanning from AD 1781 to 2005, was developed in Bomi, Southeast Tibetan Plateau (TP). During the period with instrumental data (1961–2005), this record is strongly correlated with regional summer cloud cover, which is supported by a precipitation δ 18 O simulation conducted with the isotope-enabled atmospheric general circulation model LMDZiso. A 225-yr regional cloud cover reconstruction was therefore achieved. The observed cloud cover increased in the 1980s and this increase is not unprecedented in the entire reconstruction. The reconstructed cloud cover appears smaller and more stable in the 20th century than previously. A late 19th century decrease in our reconstructed cloud cover is consistent with a decrease in the TP glacier accumulation recorded in ice cores. Our data reveal a strong anomaly in the 1810s, which coincides with volcanic eruption in 1809 and the 1815 Tambora volcanic eruption.

Description: Volcanic and ENSO effects in China in simulations and reconstructions: Tambora eruption 1815 Climate of the Past Discussions, 7, 2061-2088, 2011 Author(s): D. Zhang, R. Blender, and K. Fraedrich The co-operative effects of volcanic eruptions and ENSO (El Niño/Southern Oscillation) on the climate in China are analyzed in a millennium simulation for 800–2005 AD using the earth system model (ESM) ECHAM5/MPIOM/JSBACH subject to anthropogenic and natural forcings. The experiment includes two ensembles with weak (5 members) and strong (3 members) total solar irradiance variability. In the absence of El Niño and La Niña events, volcanoes, which are the dominant forcing in both ensembles, cause a dramatic cooling in West China (−2 °C) and a drought in East China during the year after the eruption. The recovery times for the volcano induced cooling vary globally between one and 12 yr; in China these values are mostly within 1–4 yr, but reach 10 yr in the Northeast. Without volcanoes, after El Niño events the summer precipitation is reduced in the North, while South China becomes wetter (indicated by the Standardized Precipitation Index, SPI, for summers, JJA); La Niña events cause opposite effects. El Niño events in the winters after eruptions compensate the cooling in most regions of China, while La Niña events intensify the cooling (up to −2.5 °C). The simulated impact of the eruption of the Tambora in 1815, which caused the "year without summer" 1816 in Europe and North America and coldness and famines for several years in the Chinese province Yunnan, depends crucially on the ENSO state of the coupled model. A comparison with reconstructed El Niño events shows a moderate cool climate with wet (in the South) and extreme dry anomalies (in the North) persisting for several years.

Description: The Middle Miocene climate as modelled in an atmosphere-ocean-biosphere model Climate of the Past Discussions, 7, 1935-1972, 2011 Author(s): M. Krapp and J. H. Jungclaus We present simulations with a coupled ocean-atmosphere-biosphere model for the Middle Miocene 15 million years ago. The Middle Miocene topography, which alters both large-scale ocean and atmospheric circulations, causes a global warming of 0.7 K compared to present-day. Higher than present-day CO 2 levels of 480 and 720 ppm cause a global warming of 2.8 and 4.9 K, thereby matching proxy-based Middle Miocene global temperature estimates of 3–6 K warming. Higher CO 2 levels and the associated water vapour feedback enhance the greenhouse effect and lead to a polar amplification of the warming. Although oceanic and atmospheric poleward heat transport are individually altered by 10–30 % in the mid and high latitudes, changes of the total heat transport account only for 4–8 %, pointing toward a compensation between oceanic and atmospheric heat transport. Our model reproduces a denser vegetation in agreement with fossil records. These results suggest that higher than present-day CO 2 levels are essential to drive the warm Middle Miocene climate.

Description: Synchronicity of the East Asian Summer Monsoon variability and Northern Hemisphere climate change since the last deglaciation Climate of the Past Discussions, 7, 2159-2192, 2011 Author(s): T. Shinozaki, M. Uchida, K. Minoura, M. Kondo, S. F. Rella, and Y. Shibata Understanding of the mechanism of the East Asian Summer Monsoon (EASM) is required for the prediction of climate change in East Asia in a scenario of modern global warming. In this study, we present high-resolution climate records from peat sediments in Northeast Japan to reconstruct the EASM variability based on peat bulk cellulose δ 13 C since the last deglaciation. We used a 8.8 m long peat sediment core collected from the Tashiro Bog, Northeast Japan. Based on 42 14 C measurements, the core bottom reaches ~15.5 ka. δ 13 C, accumulation rate and accumulation flux time-series correlate well to Greenland ice core δ 18 O variability, suggesting that the climate record in Northeast Japan is linked to global climate changes. The δ 13 C record at Tashiro Bog and other paleo-EASM records at Northeast and Southern China consistently demonstrate that hydrological environments were spatially different in mid-high and mid-low latitude regions over the last 15.5 kyr. During global cooling (warming) periods, mid-high and mid-low latitude regions were characterized by wet (dry) and dry (wet) environments, respectively. We suggest that these climatic patterns are related to the migration of the EASM-related rain belt during global climate changes, as a consequence of variations in intensity and location of both the Intertropical Convergence Zone (ITCZ) and the Western Pacific Subtropical High (STH). The location of the rain belt largely influences the East Asian hydrological environment. Our δ 13 C time-series are characterized by a 1230 yr throughout the Holocene and a 680 yr periodicity during the early Holocene. The 1230 yr periodicity is in agreement with North Atlantic ice-rafted debris (IRD) events, suggesting a teleconnection between the Northeast Japan and the North Atlantic during the Holocene. In addition, it is the first evidence that the Bond events were recorded in terrestrial sediment in Japan. On the other hand, the 680 yr periodicity between 10.0 and 8.0 kyr is consistent with a prominent 649 yr solar activity cycle, suggesting that solar activity affected EASM precipitation during the Hypsithermal, when orbital-scale solar insolation was at a maximum in the Northern Hemisphere.

Description: Sensitivity of interglacial Greenland temperature and δ 18 O to orbital and CO 2 forcing: climate simulations and ice core data Climate of the Past Discussions, 7, 1585-1630, 2011 Author(s): V. Masson-Delmotte, P. Braconnot, G. Hoffmann, J. Jouzel, M. Kageyama, A. Landais, Q. Lejeune, C. Risi, L. Sime, J. Sjolte, D. Swingedouw, and B. Vinther The sensitivity of interglacial Greenland temperature to orbital and CO 2 forcing is investigated using the NorthGRIP ice core data and coupled ocean-atmosphere IPSL-CM4 model simulations. These simulations were conducted in response to different interglacial orbital configurations, and to increased CO 2 concentrations. These different forcings cause very distinct simulated seasonal and latitudinal temperature and water cycle changes, limiting the analogies between the last interglacial and future climate. However, the IPSL-CM4 model shows similar magnitudes of Arctic summer warming and climate feedbacks in response to 2 × CO 2 and orbital forcing of the last interglacial period (126 000 yr ago). The IPSL model produces a remarkably linear relationship between top of atmosphere incoming summer solar radiation and simulated changes in summer and annual mean central Greenland temperature. This contrasts with the stable isotope record from the Greenland ice cores, showing a multi-millennial lagged response to summer insolation. During the early part of interglacials, the observed lags may be explained by ice sheet-ocean feedbacks linked with changes in ice sheet elevation and the impact of meltwater on ocean circulation, as investigated with sensitivity studies. A quantitative comparison between ice core data and climate simulations requires to explore the stability of the stable isotope – temperature relationship. Atmospheric simulations including water stable isotopes have been conducted with the LMDZiso model under different boundary conditions. This set of simulations allows to calculate a temporal Greenland isotope-temperature slope (0.3–0.4 ‰ per °C) during warmer than present Arctic climates, in response to increased CO 2 , increased ocean temperature and orbital forcing. This temporal slope appears twice as small as the modern spatial gradient and is consistent with other ice core estimates. A preliminary comparison with other model results implies that other mechanisms could also play a role. This suggests that further simulations and detailed inter-model comparisons are also likely to be of benefit. Comparisons with Greenland ice core stable isotope data reveals that IPSL/LMDZiso simulations strongly underestimate the amplitude of the ice core signal during the last interglacial, which could reach +8–10 °C at fixed-elevation. While the model-data mismatch may result from missing positive feedbacks (e.g. vegetation), it could also be explained by a reduced elevation of the central Greenland ice sheet surface by 300–400 m.

Description: The sharp decline of East Asian summer monsoon at mid-Holocene indicated by the lake-wetland transition in the Sanjiang Plain, northeastern China Climate of the Past Discussions, 10, 4595-4622, 2014 Author(s): Z. Q. Zhang, G. P. Wang, X. G. Lv, H. J. Jia, and Q. H. Xu The timing of the waxing and wining of the East Asian summer monsoon during the Holocene is still under debate. In present study, we present the high-resolution grain-size and LOI records from a well-dated mud/peat profile to reveal the lake-wetland transition in the Sanjiang Plain and discuss its significance to Holocene monsoon evolutions. The results show that the shallow-water lakes have developed in low-lying areas of the plain before 4600 yr BP, corresponding to the Holocene monsoon maximum. Thereafter, the wetlands began to initiate with the extinction of the paleolakes, marking a lake-shrinking stage with the relative dry climate. Considering the prevalent monsoon climate in the Sanjiang Plain, we suggest the lake-wetland transition at 4600 yr BP indicate a sharp decline of the summer monsoon rather than the basin infilling process. Such a remarkable monsoon weakening event has been widely documented in northern China, and we associated it with the ocean–atmosphere interacting processes in low-latitude regions.

Description: A two thousand year annual record of snow accumulation rates for Law Dome, East Antarctica Climate of the Past Discussions, 10, 4469-4497, 2014 Author(s): J. Roberts, C. Plummer, T. Vance, T. van Ommen, A. Moy, S. Poynter, A. Treverrow, M. Curran, and S. George Accurate high resolution records of snow accumulation rates in Antarctica are crucial for estimating ice sheet mass balance and subsequent sea level change. Snowfall rates at Law Dome, East Antarctica, have been linked with regional atmospheric circulation to mid-latitudes as well as regional Antarctic snowfall. Here, we extend the Law Dome accumulation record from 750 to 2035 years, using recent annual layer dating that extends to AD −22. Accumulation rates were calculated as the ratio of measured to modelled layer thicknesses, multiplied by the long term mean accumulation rate. The modelled layer thicknesses were based on a power law vertical strain rate profile fitted to observed annual layer thickness. The periods AD 380–442, AD 727–783 and AD 1970–2009 have above average snow accumulation rates, while AD 663–704, AD 933–975 and AD 1429–1468 were below average. The calculated snow accumulation rates show good correlation with atmospheric reanalysis estimates, and significant spatial correlation over a wide expanse of East Antarctica, demonstrating that the Law Dome record captures larger scale variability across a large region of East Antarctica well beyond the immediate vicinity of the Law Dome summit. Spectral analysis reveals periodicities in the snow accumulation record which may be related to ENSO and Interdecadal Pacific Oscillation frequencies.

Description: Technical Note: Probabilistically constraining proxy age–depth models within a Bayesian hierarchical reconstruction model Climate of the Past Discussions, 10, 4499-4533, 2014 Author(s): J. P. Werner and M. P. Tingley Reconstructions of late-Holocene climate rely heavily upon proxies that are assumed to be accurately dated by layer counting, such as measurement on tree rings, ice cores, and varved lake sediments. Considerable advances may be achievable if time uncertain proxies could be included within these multiproxy reconstructions, and if time uncertainties were recognized and correctly modeled for proxies commonly treated as free of age model errors. Current approaches to accounting for time uncertainty are generally limited to repeating the reconstruction using each of an ensemble of age models, thereby inflating the final estimated uncertainty – in effect, each possible age model is given equal weighting. Uncertainties can be reduced by exploiting the inferred space–time covariance structure of the climate to re-weight the possible age models. Here we demonstrate how Bayesian Hierarchical climate reconstruction models can be augmented to account for time uncertain proxies. Critically, while a priori all age models are given equal probability of being correct, the probabilities associated with the age models are formally updated within the Bayesian framework, thereby reducing uncertainties. Numerical experiments show that updating the age-model probabilities decreases uncertainty in the climate reconstruction, as compared with the current de-facto standard of sampling over all age models, provided there is sufficient information from other data sources in the region of the time-uncertain proxy. This approach can readily be generalized to non-layer counted proxies, such as those derived from marine sediments.

Description: High-resolution records of the beryllium-10 solar activity proxy in ice from Law Dome, East Antarctica: measurement, reproducibility and principal trends Climate of the Past Discussions, 7, 677-713, 2011 Author(s): J. B. Pedro, A. M. Smith, K. J. Simon, T. D. van Ommen, and M. A. J. Curran Three near-monthly resolution 10 Be records are presented from the Dome Summit South (DSS) ice core site, Law Dome, East Antarctica. The chemical preparation and Accelerator Mass Spectrometer (AMS) measurement of these records is described. The reproducibility of 10 Be records at DSS is assessed through intercomparison of the ice core data with data from two previously published and contemporaneous snow pits. We find generally good agreement between the five records, comparable to that observed between other trace chemical records from the site. This result allays concerns raised by a previous Antarctic study (Moraal et al., 2005) about poor reproducibility of ice core 10 Be records. A single composite series is constructed from the three ice cores providing a monthly-resolved record of 10 Be concentrations at DSS over the past decade (1999 to 2009). To our knowledge, this is the first published ice core data spanning the recent exceptional solar minimum of solar cycle 23. 10 Be concentrations are significantly correlated to the cosmic ray flux recorded by the McMurdo neutron monitor ( r xy = 0.64, with 95% CI of 0.53 to 0.71), suggesting that solar modulation of the atmospheric production rate may explain up to ~40% of the variance in 10 Be concentrations at DSS. Sharp concentration peaks occur in most years during the summer-to-autumn, possibly caused by stratospheric incursions. Our results underscore the presence of both production and meteorological signals in ice core 10 Be data.

Description: Tracking climate variability in the western Mediterranean during the Late Holocene: a multiproxy approach Climate of the Past Discussions, 7, 635-675, 2011 Author(s): V. Nieto-Moreno, F. Martínez-Ruiz, S. Giralt, F. Jiménez-Espejo, D. Gallego-Torres, M. Rodrigo-Gámiz, J. García-Orellana, M. Ortega-Huertas, and G. J. de Lange Climate variability in the western Mediterranean is reconstructed for the last 4000 yr using marine sediments recovered in the west Algerian-Balearic basin, near the Alboran basin. Fluctuations in chemical and mineralogical sediment composition as well as grain size distribution are linked to fluvial-eolian oscillations, changes in redox conditions and paleocurrent intensity. Multivariate analyses allowed us to characterize three main groups of geochemical and mineralogical proxies determining the sedimentary record of this region. These three statistical groups were applied to reconstruct paleoclimate conditions at high resolution during the Late Holocene. An increase in fluvial-derived elements (Rb/Al, Ba/Al, REE/Al, Si/Al, Ti/Al, Mg/Al and K/Al ratios), finer grain size, slower flows and oxygen-poor bottom waters – as suggested by sortable silt (10–63 μm), clays ( 〈 2 μm) and redox-sensitive elements (V/Al, Cr/Al, Ni/Al and Zn/Al ratios) – depict the Roman Humid Period (RHP) and the Little Ice Age (LIA), while drier environmental conditions are recognized during the Late Bronze Age-Iron Age (LBA-IA) and the Medieval Warm Period (MWP). Although no Ba excess was registered, other paleoproductivity indicators (total organic carbon content, Br/Al ratio, and organometallic ligands such as U and Cu) display the highest values during the RHP, this period exhibiting by far the most intense productivity of the last 4000 yr. These marine sediments evidence oscillations that support the link of the westernmost Mediterranean climate with the North Atlantic coupled ocean-atmosphere climatic system, pointing to solar irradiance and the North Atlantic Oscillation (NAO) variability as the main driving mechanisms behind natural climate variability over decadal to centennial time-scales for the last 4000 yr.

Description: Impact of maximum borehole depths on inverted temperature histories in borehole paleoclimatology Climate of the Past Discussions, 7, 715-748, 2011 Author(s): H. Beltrami, J. E. Smerdon, G. S. Matharoo, and N. Nickerson A quantitative assessment is presented for the impact of the maximum depth of a temperature-depth profile on the estimate of the climatic transient and the resultant ground surface temperature (GST) reconstruction used in borehole paleoclimatology. The depth of the profile is important because the downwelling climatic signal must be separated from the quasi-steady state thermal regime established by the energy in the Earth's interior. This component of the signal is estimated as a linear increase in temperature with depth from the lower section of a borehole temperature profile, which is assumed to be unperturbed by recent changes in climate at the surface. The validity of this assumption is dependent on both the subsurface thermophysical properties and the character of the downwelling climatic signal. Such uncertainties can significantly impact the determination of the quasi-steady state thermal regime, and consequently the magnitude of the temperature anomaly interpreted as a climatic signal. The quantitative effects and uncertainties that arise from the analysis of temperature-depth profiles of different depths are presented. Results demonstrate that widely different GST histories can be derived from a single temperature profile truncated at different depths. Borehole temperature measurements approaching 500–600 m depths are shown to provide the most robust GST reconstructions spanning 500 to 1000 ybp. It is further shown that the bias introduced by a temperature profile of depths shallower than 500–600 m remains even if the time span of the reconstruction target is shortened.

Description: Upper ocean climate of the Eastern Mediterranean Sea during the Holocene Insolation Maximum – a model study Climate of the Past Discussions, 7, 1457-1509, 2011 Author(s): F. Adloff, U. Mikolajewicz, M. Kucera, R. Grimm, E. Maier-Reimer, G. Schmiedl, and K. Emeis Nine thousand years ago, the Northern Hemisphere experienced enhanced seasonality caused by an orbital configuration with a minimum of the precession index. To assess the impact of the "Holocene Insolation Maximum" (HIM) on the Mediterranean Sea, we use a regional ocean general circulation model forced by atmospheric input derived from global simulations. A stronger seasonal cycle is simulated in the model, which shows a relatively homogeneous winter cooling and a summer warming with well-defined spatial patterns, in particular a subsurface warming in the Cretan and Western Levantine areas. The comparison between the SST simulated for the HIM and the reconstructions from planktonic foraminifera transfer functions shows a poor agreement, especially for summer, when the vertical temperature gradient is strong. However, a reinterpretation of the reconstructions is proposed, to consider the conditions throughout the upper water column. Such a depth-integrated approach accounts for the vertical range of preferred habitat depths of the foraminifera used for the reconstructions and strongly improves the agreement between modelled and reconstructed temperature signal. The subsurface warming is recorded by both model and proxies, with a light shift to the south in the model results. The mechanisms responsible for the peculiar subsurface pattern are found to be a combination of enhanced downwelling and wind mixing due to strengthened Etesian winds, and enhanced thermal forcing due to the stronger summer insolation in the Northern Hemisphere. Together, these processes induce a stronger heat transfer from the surface to the subsurface during late summer in the Western Levantine; this leads to an enhanced heat piracy in this region.

Description: NALPS: a precisely dated European climate record 120–60 ka Climate of the Past Discussions, 7, 1049-1072, 2011 Author(s): R. Boch, H. Cheng, C. Spötl, R. L. Edwards, X. Wang, and Ph. Häuselmann Accurate and precise chronologies are essential in understanding the rapid and recurrent climate variations of the Last Glacial – known as Dansgaard-Oeschger (D-O) events – found in the Greenland ice cores and other climate archives. The existing chronological uncertainties during the Last Glacial, however, are still large. Radiometric age data and stable isotopic signals from speleothems are promising to improve the absolute chronology. We present a record of several precisely dated stalagmites from caves located at the northern rim of the Alps (NALPS), a region that favours comparison with the climate in Greenland. The record covers most of the interval from 120 to 60 ka at an average temporal resolution of 2 to 22 a and 2 σ-age uncertainties of ca. 200 to 500 a. The rapid and large oxygen isotope shifts of 1 to 4.5‰ occurred within decades to centuries and strongly mimic the Greenland D-O pattern. Compared to the current Greenland ice-core timescale the NALPS record suggests overall younger ages of rapid warming and cooling transitions between 120 to 60 ka. In particular, there is a discrepancy in the duration of stadial 22 between the ice-core and the stalagmite chronology (ca. 3000 vs. 3650 a). The short-lived D-O events 18 and 18.1 are not recorded in NALPS, provoking questions with regard to the nature and the regional expression of these events. NALPS resolves recurrent short-lived climate changes within the cold Greenland stadial (GS) and warm interstadial (GI) successions, i.e. abrupt warming events preceding GI 21 and 23 (precursor-type events) and at the end of GI 21 and 25 (rebound-type events), as well as intermittent cooling events during GI 22 and 24. Such superimposed Last Glacial events have not been documented in Europe before.

Description: Holocene vegetation and biomass changes on the Tibetan Plateau – a model-pollen data comparison Climate of the Past Discussions, 7, 1073-1111, 2011 Author(s): A. Dallmeyer, M. Claussen, U. Herzschuh, and N. Fischer Results of a transient numerical experiment, performed in a coupled atmosphere-ocean-vegetation model with orbital forcing alone, are compared to pollen-based vegetation reconstructions from four representative sites on the Tibetan Plateau, covering the last 6000 years. Causes of the vegetation change and consequences for the biomass storage are analysed. In general, simulated and reconstructed vegetation trends at each site are in good agreement. Both methods reveal a general retreat of the biomass-rich vegetation that is particularly manifested in a strong decrease of forests. However, model and reconstructions differ with regard to the climatic factors causing this vegetation change. The reconstructions primarily identify decreasing summer monsoon precipitation as the responsible mechanism for the vegetation shift. In the model, the land cover change originates from differences in near-surface air temperature arising out of orbitally-induced insolation changes. According to the model results, the averaged forest fraction on the Plateau is shrinking by almost one-third from mid-Holocene (41.4%) to present-day (28.3%). Shrubs, whose fraction is quadrupled at present-day (12.3%), replace most of this forest. Gras fraction increases from 38.9% during the mid-Holocene to 42.3% at present-day. This land cover change results in a decrease of living biomass by 0.62 kgC m −2 . Total biomass on the Tibetan Plateau decreases by 1.9 kgC m −2 , i.e. approx. 6.64 PgC are released due to the natural land cover change.

Description: Hydrological variability in northern Levant over the past 250 ka Climate of the Past Discussions, 7, 1511-1566, 2011 Author(s): F. Gasse, L. Vidal, A.-L. Develle, and E. Van Campo The Levant features sharp climatic gradients from North to South and from West to East resulting in a large environmental diversity. The lack of long-term record from the northern Levant limits our understanding of the regional response to glacial-interglacial boundary conditions in this key area. The 250 ka paleoenvironmental reconstruction presented here is a first step to fill this geographical gap. The record comes from a 36 m lacustrine-palustrine sequence cored in the small intra-mountainous karstic basin of Yammoûneh (northern Lebanon). The paper combines times series of sediment properties, paleovegetation, and carbonate oxygen isotopes, to yield a comprehensive view of paleohydrologic-paleoclimatic fluctuations in the basin over the two last glacial-interglacial cycles. Efficient moisture was higher than today during interglacial peaks around 240, 215–220, ~130–120 ka and 11–9 ka (although under different Precipitation minus Evaporation balance). Moderate wetting events took place around 170, 150, 105–100, 85–75, 60–55 and 35 ka. The penultimate glacial period was generally wetter than the last glacial stage. Local aridity culminated from the LGM to 15 ka, possibly linked to water storage as ice in the surrounding highlands. An overall decrease in local water availability is observed from the profile base to top. Fluctuations in available water seem to be primarily governed by changes in local summer insolation controlled by the orbital eccentricity modulated by the precession cycle, and by changes in precipitation and temperature seasonality. Our record is roughly consistent with long-term climatic fluctuations in northeastern Mediterranean lands, except during the penultimate glacial phase. It shares some features with speleothem records of western Israel. Conversely, after 130 ka, it is clearly out of phase with hydrological changes in the Dead Sea basin. Potential causes of these spatial heterogeneities, e.g., changes in atmospheric circulation, regional topographic patterns, site-specific climatic and hydrological factors, are discussed.

Description: Exploring errors in paleoclimate proxy reconstructions using Monte Carlo simulations: paleotemperature from mollusk and coral geochemistry Climate of the Past Discussions, 7, 2477-2510, 2011 Author(s): M. Carré, J. P. Sachs, J. M. Wallace, and C. Favier Reconstructions of the past climate from proxy records involve a wide range of uncertainties at every step of the process. These uncertainties and the subsequent error bar in the reconstruction of a paleoclimatic variable need to be understood and quantified in order to properly interpret the reconstructed variability and to perform meaningful comparisons with climate model outputs. Classic proxy calibration-validation techniques are not well-suited for identifying the causes of reconstruction errors, estimating their relative contribution, or understanding how errors accumulate from a multitude of sources. In this study, we focus on high resolution proxy records based on calcium carbonate geochemistry of sessile organisms such as mollusks, corals, or sclerosponges, and propose an approach based on Monte Carlo simulations with simple numerical surrogate proxies. A freely available algorithm (MoCo, http://www.isem.cnrs.fr/spip.php?rubrique472 ) is provided for estimating systematic and standard errors of mean temperature, seasonality and variance reconstructed from marine accretionary archive geochemistry. This algorithm is then used for sensitivity experiments in a case study to characterize and quantitatively evaluate the sensitivity of systematic and standard errors to sampling randomness, stochastic uncertainty sources and systematic proxy limitations. The results of the experiments yield an illustrative example of the range of variations that climate reconstruction errors may undergo, and bring to light their complexity. One of the main improvements of this method is the identification and estimation of systematic bias that would not otherwise be detected. It thus offers the possibility of correcting the proxy-based climate from these biases for a more accurate reconstruction. Beyond the findings of error sources for coral and mollusk-based reconstructions, our study demonstrates that numerical simulations based on Monte Carlo analyses are a simple and powerful approach to improve the proxy calibration process. A thourough understanding of the proxy record errors is essential for the interpretation of paleoclimate records from proxies derived from accretionary skeleton geochemistry. The error estimates provided by MoCo are much more comprehensive and therefore closer to reality than error estimates provided by typical calibration studies.

Description: Internal and external variability in regional simulations of the Iberian Peninsula climate over the last millennium Climate of the Past Discussions, 7, 2579-2607, 2011 Author(s): J. J. Gómez-Navarro, J. P. Montávez, P. Jiménez-Guerrero, S. Jerez, R. Lorente-Plazas, J. F. González-Rouco, and E. Zorita In this study we analyse the role of internal variability in regional climate simulations through a comparison of two regional paleoclimate simulations for the last millennium. They share the same external forcings and model configuration, differing only in the initial condition used to run the driving global model simulation. A comparison of these simulations allows us to study the role of internal variability in climate models at regional scales, and how it affects the long-term evolution of climate variables such as temperature and precipitation. The results indicate that, although temperature is homogeneously sensitive to the effect of external forcings, the evolution of precipitation is more strongly governed by random and unpredictable internal dynamics. There are, however, some areas where the role of internal variability is lower than expected, allowing precipitation to respond to the external forcings, and we explore the underlying physical mechanisms responsible. We find that special attention should be paid when comparing the evolution of simulated precipitation with proxy reconstructions at regional scales. In particular, this study identifies areas, depending on the season, in which this comparison would be meaningful, but also other areas where good agreement between model simulations and reconstructions should not be expected even if both are perfect.

Description: Spring-summer temperatures reconstructed for northern Switzerland and south-western Germany from winter rye harvest dates, 1454–1970 Climate of the Past Discussions, 7, 2609-2654, 2011 Author(s): O. Wetter and C. Pfister This paper presents a unique 517 yr long documentary data – based reconstruction of spring-summer (MAMJJ) temperatures for northern Switzerland and south western Germany from 1454 to 1970. It is composed of 25 partial series of winter grain (secale cereale) harvest starting dates (WGHD) that are in one part based on harvest related bookkeeping of institutions (hospitals, municipalities), in the other part to (early) phenological observations. The resulting main Basel WGHD series was homogenised with regard to dating style, data type and altitude. The calibration and verification approach was applied using the homogenous HISTALP temperature series from 1774–1824 for calibration ( r = 0,78) and from 1920–1970 for verification ( r = 0.75). The latter result even suffer from the weak data basis available for 1870–1950. Temperature reconstructions based on WGHD are more influenced by spring temperatures than those based on grape harvest dates (GHD), because rye in contrast to vines already begins to grow as soon as sunlight brings the plant to above freezing. The earliest and latest harvest dates were checked for consistency with narrative documentary weather reports. Comparisons with other European documentary-based GHD and WGHD temperature reconstructions generally reveal significant correlations decreasing with the distance from Switzerland. The new Basel WGHD series shows better skills in representing highly climate change sensitive variations of Swiss Alpine glaciers than available GHD series.

Description: Holocene evolution of the Southern Hemisphere westerly winds in transient simulations with global climate models Climate of the Past Discussions, 7, 1797-1824, 2011 Author(s): V. Varma, M. Prange, U. Merkel, T. Kleinen, G. Lohmann, M. Pfeiffer, H. Renssen, A. Wagner, S. Wagner, and M. Schulz The Southern Hemisphere Westerly Winds (SWW) have been suggested to exert a critical influence on global climate through wind-driven upwelling of deep water in the Southern Ocean and the potentially resulting atmospheric CO 2 variations. The investigation of the temporal and spatial evolution of the SWW along with forcings and feedbacks remains a significant challenge in climate research. In this study, the evolution of the SWW under orbital forcing from the mid-Holocene (7 kyr BP) to pre-industrial modern times (250 yr BP) is examined with transient experiments using the comprehensive coupled global climate model CCSM3. In addition, a model inter-comparison is carried out using orbitally forced Holocene transient simulations from four other coupled global climate models. Analyses and comparison of the model results suggest that the annual and seasonal mean SWW were subject to an overall strengthening and poleward shifting trend during the course of the mid-to-late Holocene under the influence of orbital forcing, except for the austral spring season, where the SWW exhibited an opposite trend of shifting towards the equator.

Description: A critical humidity threshold for monsoon transitions Climate of the Past Discussions, 7, 1737-1765, 2011 Author(s): J. Schewe, A. Levermann, and H. Cheng Monsoon systems around the world are governed by the so-called moisture-advection feedback. Here we show that, in a minimal conceptual model, this feedback implies a critical threshold with respect to the atmospheric specific humidity q o over the ocean adjacent to the monsoon region. If q o falls short of this critical value q o c , monsoon rainfall over land cannot be sustained. Such a case could occur if evaporation from the ocean was reduced, e.g. due to low sea surface temperatures. Within the restrictions of the conceptual model, we estimate q o c from present-day reanalysis data for four major monsoon systems, and demonstrate how this concept can help understand abrupt variations in monsoon strength on orbital timescales as found in proxy records.

Description: Glacial CO 2 cycle as a succession of key physical and biogeochemical processes Climate of the Past Discussions, 7, 1767-1795, 2011 Author(s): V. Brovkin, A. Ganopolski, D. Archer, and G. Munhoven During glacial-interglacial cycles, atmospheric CO 2 concentration varied by about 100 ppmv in amplitude. While testing mechanisms that had led to the low glacial CO 2 level could be done in equilibrium model experiments, an ultimate goal is to explain CO 2 changes in transient simulations through the complete glacial-interglacial cycle. A computationally efficient Earth System model of intermediate complexity CLIMBER-2 is used to simulate global biogeochemistry over the last glacial cycle (126 kyr). The physical core of the model (atmosphere, ocean, land and ice sheets) is driven by orbital changes and reconstructed radiative forcing from greenhouses gases, ice, and aeolian dust. The carbon cycle model is able to reproduce the main features of the CO 2 changes: a 50 ppmv CO 2 drop during glacial inception, a minimum concentration at the last glacial maximum by 80 ppmv lower than the Holocene value, and an abrupt 60 ppmv CO 2 rise during the deglaciation. The model deep ocean δ 13 C also resembles reconstructions from deep-sea cores. The main drivers of atmospheric CO 2 evolve with time: changes in sea surface temperatures and in the volume of bottom water of southern origin controls atmospheric CO 2 during the glacial inception and deglaciation, while changes in carbonate chemistry and marine biology are dominant during the first and second parts of the glacial cycle, respectively. These feedback mechanisms could also significantly impact the ultimate climate response to the anthropogenic perturbation.

Description: Hydroclimate variability in the low-elevation Atacama Desert over the last 2500 years Climate of the Past Discussions, 7, 3165-3202, 2011 Author(s): E. M. Gayo, C. Latorre, C. M. Santoro, A. Maldonado, and R. De Pol-Holz Paleoclimate reconstructions reveal that Earth system has experienced sub-millennial scale climate changes over the past two millennia in response to internal/external forcing. Although sub-millennial hydroclimate fluctuations have been detected in the central Andes during this interval, the timing, magnitude, extent and direction of change of these events remain poorly defined. Here, we present a reconstruction of hydroclimate variations on the Pacific slope of the central Andes based on exceptionally well-preserved plant macrofossils and associated archaeological remains from a hyperarid drainage (Quebrada Maní, ~21° S, 1000 m a.s.l.) in the Atacama Desert. During the late Holocene, riparian ecosystems and farming social groups flourished in the hyperarid Atacama core as surface water availability increased throughout this presently sterile landscape. Twenty-six radiocarbon dates indicate that these events occurred between 1050–680, 1615–1350 and 2500–2040 cal yr BP. Regional comparisons with rodent middens and other records suggest that these events were synchronous with pluvial stages detected at higher-elevations in the central Andes over the last 2500 years. These hydroclimate changes also coincide with periods of pronounced SST gradients in the Tropical Pacific (La Niña-like mode), conditions that are conducive to significantly increased rainfall in the central Andean highlands and flood events in the low-elevation watersheds at inter-annual timescales. Our findings indicate that the positive anomalies in the hyperarid Atacama over the past 2500 years represent a regional response of the central Andean climate system to changes in the global hydrological cycle at centennial timescales. Furthermore, our results provide support for the role of tropical Pacific sea surface temperature gradient changes as the primary mechanism responsible for climate fluctuations in the central Andes. Finally, our results constitute independent evidence for comprehending the major trends in cultural evolution of prehistoric peoples that inhabited the region.

Description: Little ice age advance and retreat of Glaciar Jorge Montt, Chilean Patagonia, recorded in maps, air photographs and dendrochronology Climate of the Past Discussions, 7, 3131-3164, 2011 Author(s): A. Rivera, M. Koppes, C. Bravo, and J. C. Aravena Glaciar Jorge Montt (48°20' S/73°30' W), one of the main tidewater glaciers of the Southern Patagonian Icefield (SPI), has experienced the fastest frontal retreat observed in Patagonia during the past century, with a recession of 19.5 km between 1898 and 2011. This record retreat uncovered trees overridden during the Little Ice Age (LIA) advance of the glacier. Samples of these trees were dated using radiocarbon methods, yielding burial ages between 460 and 250 cal yr BP. The dendrochronology and maps indicate that Glaciar Jorge Montt was at its present position before the beginning of the LIA, in concert with several other glaciers in Southern Patagonia, and reached its maximum advance position between 1650 and 1750 AD. The post-LIA retreat is most likely triggered by climatically induced changes during the 20th century, however, Glaciar Jorge Montt has responded more dramatically than its neighbours. The retreat of Jorge Montt opened a new fjord 19.5 km long, and up to 391 m deep, with a varied bathymetry well correlated with glacier retreat rates, suggesting that dynamic responses of the glacier are at least partially connected to near buoyancy conditions at the ice front, resulting in high calving fluxes, accelerating thinning rates and rapid ice velocities.

Description: A multi-proxy perspective on millennium-long climate variability in the Southern Pyrenees Climate of the Past Discussions, 7, 3049-3089, 2011 Author(s): M. Morellón, A. Pérez-Sanz, J. P. Corella, U. Büntgen, J. Catalán, P. González-Sampériz, J. J. González-Trueba, J. A. López-Sáez, A. Moreno, S. Pla, M. Á. Saz-Sánchez, P. Scussolini, E. Serrano, F. Steinhilber, V. Stefanova, T. Vegas-Vilarrúbia, and B. Valero-Garcés A review of selected, well-dated, multiproxy paleoclimatic records (lacustrine, dendrochronological, geomorphological) characterizes the main environmental changes occurred in the Southern Pyrenees during the last millennium. Warmer and relatively arid conditions prevailed during the Medieval Climate Anomaly (MCA,

Description: Climate variability of the mid- and high-latitudes of the Southern Hemisphere in ensemble simulations from 1500 to 2000 AD Climate of the Past Discussions, 7, 3091-3129, 2011 Author(s): S. B. Wilmes, C. C. Raible, and T. F. Stocker To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability we assess an ensemble of 4 transient simulations over the last 500 yr performed with a state-of-the-art atmosphere ocean general circulation model. The model is forced with reconstructions of solar irradiance, greenhouse gas (GHG) and volcanic aerosol concentrations. A 1990 control simulation shows that the model is able to represent the Southern Annular Mode (SAM), and to some extent the South Pacific Dipole (SPD) and the Zonal Wave 3 (ZW3). During the past 500 yr we find that SPD and ZW3 variability remain stable, whereas SAM shows a significant shift towards its positive state during the 20th century. Regional temperatures over South America are strongly influenced by changing both GHG concentrations and volcanic eruptions whereas precipitation shows no significant response to the varying external forcing. For temperature this stands in contrast to proxy records suggesting that SH climate is dominated by internal variability rather than external forcing. The underlying dynamics of the temperature changes generally point to a combination of several modes, thus, hampering the possibilities regional reconstructions of the modes from proxy records. The linear imprint of the external forcing is as expected, i.e. a warming for increase in the combined solar and GHG forcing and a cooling after volcanic eruptions. Dynamically only the increase in SAM with increased combined forcing is simulated.

Description: Persistent influence of ice sheet melting on high northern latitude climate during the early Last Interglacial Climate of the Past Discussions, 7, 3239-3286, 2011 Author(s): A. Govin, P. Braconnot, E. Capron, E. Cortijo, J.-C. Duplessy, E. Jansen, L. Labeyrie, A. Landais, O. Marti, E. Michel, E. Mosquet, B. Risebrobakken, D. Swingedouw, and C. Waelbroeck Although the Last Interglacial (LIG) is often considered as a possible analogue for future climate in high latitudes, its precise climate evolution and associated causes remain uncertain. Here we compile high-resolution marine sediment records from the North Atlantic, Labrador Sea, Norwegian Sea and the Southern Ocean. We document a delay in the establishment of peak interglacial conditions in the North Atlantic, Labrador and Norwegian Seas as compared to the Southern Ocean. In particular, we observe a persistent iceberg melting at high northern latitudes at the beginning of the LIG. It suggests that the input of meltwater has maintained (1) colder and fresher surface-water conditions in the North Atlantic, Labrador and Norwegian Seas and (2) weaker ventilation of North Atlantic deep waters during the early LIG (129–125.5 ka) compared to the late LIG. Results from an ocean-atmosphere coupled model with insolation as a sole forcing for three key periods of the LIG show that insolation variations alone lead to warmer North Atlantic surface waters and stronger Atlantic overturning during the early LIG (126 ka) than the late LIG (122 ka). Hence insolation variations alone do not explain the delay in peak interglacial conditions observed at high northern latitudes. When freshwater input is interactively computed at 126 ka in response to the high boreal summer insolation, the model simulates colder, fresher North Atlantic surface waters and weaker Atlantic overturning during the early LIG (126 ka) compared to the late LIG (122 ka). This result indicates that both insolation variations and ice sheet melting have to be considered to reproduce the LIG climate evolution and supports our hypothesis that optimal thermal and deep ocean circulation conditions at high northern latitudes develop during the late LIG only, when the freshwater supply has already ceased.

Description: Element/Calcium ratios in middle Eocene samples of Oridorsalis umbonatus from Ocean Drilling Program Site 1209 Climate of the Past Discussions, 7, 3795-3821, 2011 Author(s): C. F. Dawber and A. K. Tripati Culturing studies and empirical-based calibrations suggest that elemental ratios in benthic foraminifera can be used as proxies to reconstruct past variations in bottom water temperature and saturation state (Δ[CO 3 2− ]). However the mechanism(s) linking elemental ratios to Δ[CO 3 2− ] are poorly constrained. We present middle Eocene records of Oridorsalis umbonatus Li/Ca, B/Ca, Mg/Ca and Sr/Ca from Ocean Drilling Program Site 1209. We apply calibrations developed from core top samples to estimate middle Eocene variations in intermediate water Δ[CO 3 2− ]. The fidelity of bottom water Δ[CO 3 2− ] reconstructions based on single element ratios are assessed by comparing the X /Ca-based reconstructions to each other and to carbon cycle proxy records (benthic foraminifera δ 13 C, organic carbon content, foraminifera dissolution indices), and a seawater δ 18 O reconstruction for Site 1209. Discrepancies in the reconstructed Δ[CO 3 2− ] values for the middle Eocene based on these different metal ratios suggests that there are still gaps in our understanding of the parameters influencing X /Ca. The downcore record of O. umbonatus Mg/Ca does not exhibit any similarities with the Li/Ca, B/Ca and Sr/Ca records, suggesting that bottom water Δ[CO 3 2− ] is not the dominant influence on Mg/Ca ratios for this species. This hypothesis is supported by the coefficients of multiple linear regression models on new and published Mg/Ca data.

Description: Changing climatic and anthropogenic influences on the Bermejo wetland, through archival documents – Mendoza, Argentina, 16th–20th centuries Climate of the Past Discussions, 7, 3775-3794, 2011 Author(s): M. R. Prieto and F. Rojas The wrong management of watering in the highest zones of the Mendoza northern oasis, the topography of the terrain and the deficient drainage, together with neotectonics phenomena, but mostly a dramatic and progressive increase of the Rio Mendoza flow volume originated the expansion of the wetlands area at the NE of the city of Mendoza at the turn of the 18th century, while in previous centuries it had retracted to a minimum. The area grew until reaching the dimension of large wetlands in the lowest oasis zones, resulting from a larger runoff and soil saturation by the rise of the phreatic layers. This situation remained throughout the 19th century, affecting the extension and use of the available land for human activity. The purpose of this study was to research this process that culminated in 1930 with the partial desiccation of the area. We have given particular importance to the influence of the climatic fluctuations in the Cordillera de los Andes and to the consequent variations of the Rio Mendoza flow volume in this process. For the analysis we used snowfall series at the cordillera and flow volume of the Rio Mendoza, built by Prieto (2009) with documental data. We analyzed which were the mediate and immediate consequences of the growth and later desiccation of the wetlands over the environment and its present repercussion on the ecosystem (salinization, poor soil drainage, soil alkalinization, sedimentation). In addition, we have also worked over georeferenced historic charts that partially reflect the behavior of the Cienaga del Bermejo during the 18th, 19th and 20th centuries. This behavior characterized by "growth pulses" and retraction moments is reflected in the analyzed charts, where those moments of major growth coincide with cycles of bigger snowstorms and larger flow volume in the Rio Mendoza.

Description: Snow and weather climatic control on snow avalanche occurrence fluctuations over 50 yr in the French Alps Climate of the Past Discussions, 7, 4173-4221, 2011 Author(s): H. Castebrunet, N. Eckert, and G. Giraud Snow avalanche activity is controlled to a large extent by snow and weather patterns. However, its response to climate fluctuations remains poorly documented. Previous studies have focused on direct extraction of trends in avalanche and winter climate data, and this study employs a time-implicit method to model annual avalanche activity in the French Alps during the 1958–2009 period from its most representative climatic drivers. Modeled snow and weather data for different elevations and aspects are considered as covariates that explain actual observed avalanche counts, modeled instability indexes, and a combination of both avalanche activity indicators. These three series present relatively similar fluctuations over the period and good consistency with historically harsh winters. A stepwise procedure is used to obtain regression models that accurately represent trends as well as high and low peaks with a small number of physically meaningful covariates, showing their climatic relevance. The activity indicators and their regression models seen as time series show, within a high interannual variability, a predominant bell-shaped pattern presumably related to a short period of colder and snowier winters around 1980, as well as a very slight but continuous increase between 1975 and 2000 concomitant with warming. Furthermore, the regression models quantify the respective weight of the different covariates, mostly temperature anomalies and south-facing snowpack characteristics to explain the trends and most of the exceptional winters. Regional differences are discussed as well as seasonal variations between winter and spring activity and confirm rather different snow and weather regimes influencing avalanche activity over the Northern and Southern Alps, depending on the season.

Description: Hydrological evidence for a North Atlantic oscillation during the Little Ice Age outside its range observed since 1850 Climate of the Past Discussions, 7, 4149-4171, 2011 Author(s): C. Martín-Puertas, I. Dorado-Liñán, A. Brauer, E. Zorita, B. L. Valero-Garcés, and E. Gutierrez An annual-resolved precipitation reconstruction for the last 800 yr in Southern Spain has been performed using stable carbon isotope (δ 13 C) of Pinus nigra tree rings. The reconstruction exhibits high- to low-frequency variability and distinguishes a Little Ice Age (LIA, AD 1350–1850) characterized by lower averaged rainfall than both in the transition from the Medieval Climate Anomaly to the LIA and in the 20th century. The driest conditions are recorded during the Maunder solar Minimum (mid 17th–early 18th centuries), in good agreement with the Spanish documentary archive. Similar linkage between solar activity (maximum/minimum) and precipitation (increase/decrease) is observed throughout the entire LIA. Additionally, the relationship between the hydrological pattern in the Iberian Peninsula and Morocco during the LIA suggests different spatial distribution of precipitation in the south-eastern sector of the North Atlantic region such as it is known currently. Whereas in the instrumental record the precipitation evolves similarly in both regions and opposite to the North Atlantic oscillation (NAO) index, the coldest periods of the LIA shows a contrasting pattern with drier conditions in the South of Spain and wetter in Northern Africa. We suggest an extreme negative NAO conditions, accompanied by a southward excursion of the winter rainfall band beyond that observed in the last century, can explain this contrast. The sustained NAO conditions could have been triggered by solar minima and higher volcanic activity during the LIA.

Description: The relative roles of CO 2 and palaeogeography in determining Late Miocene climate: results from a terrestrial model-data comparison Climate of the Past Discussions, 8, 715-786, 2012 Author(s): C. D. Bradshaw, D. J. Lunt, R. Flecker, U. Salzmann, M. J. Pound, A. M. Haywood, and J. T. Eronen The Late Miocene (∼11.6–5.3 Ma) palaeorecord provides evidence for a warmer and wetter climate than that of today and there is uncertainty in the palaeo-CO 2 record of at least 150 ppmv. We present results from fully coupled atmosphere-ocean-vegetation simulations for the Late Miocene that examine the relative roles of palaeogeography (topography and ice sheet geometry) and CO 2 concentration in the determination of Late Miocene climate through comprehensive terrestrial model-data comparisons. Assuming that the data accurately reflects the Late Miocene climate, and that the Late Miocene palaeogeographic reconstruction used in the model is robust, then results indicate that the proxy-derived precipitation differences between the Late Miocene and modern can be largely accounted for by the palaeogeographic changes alone. However, the proxy-derived temperatures differences between the Late Miocene and modern can only begin to be accounted for if we assume a palaeo-CO 2 concentration towards the higher end of the range of estimates.

Description: Changes in the strength and width of the Hadley circulation since 1871 Climate of the Past Discussions, 8, 695-713, 2012 Author(s): J. Liu, M. Song, Y. Hu, and X. Ren Recent studies demonstrate that the Hadley Circulation has intensified and expanded for the past three decades, which has important implications for subtropical societies and may lead to profound changes in global climate. However, the robustness of this intensification and expansion that should be considered when interpreting long-term changes of the Hadley Circulation is still matters of debate. It also remains largely unknown how the Hadley Circulation has evolved over longer periods. Here we present long-term variability of the Hadley Circulation using the 20th Century Reanalysis. It shows a slight strengthening and widening of the Hadley Circulation since the late 1970s, which is not inconsistent with recent assessments. However, over centennial timescales (1871–2008), the Hadley Circulation shows a tendency towards more intense and narrower state. More importantly, the width of the Hadley Circulation has not yet completed a life-cycle since 1871. The strength and width of the Hadley Circulation during the late 19th and early 20th century show strong natural variability, exceeding variability that coincides with global warming in recent decades. These findings raise the question that the recent change of the Hadley Circulation is primarily attributed to greenhouse warming or a long-period oscillation of the Hadley Circulation substantially longer than that observed in previous studies.

Description: Millennial-length forward models and pseudoproxies of stalagmite δ 18 O: an example from NW Scotland Climate of the Past Discussions, 8, 869-907, 2012 Author(s): A. Baker, C. Bradley, S. J. Phipps, M. Fischer, I. J. Fairchild, L. Fuller, C. Spötl, and C. Azcurra The stable oxygen isotope parameter δ 18 O remains the most widely utilised speleothem proxy for past climate reconstructions. Uncertainty can be introduced into stalagmite δ 18 O from a number of factors, one of which is the heterogeneity of groundwater flow in karstified aquifers. Here, we present a lumped parameter hydrological model, KarstFor, which is capable of generating monthly simulations of surface water – ground water – stalagmite δ 18 O for more than thousand year time periods. Using a variety of climate input series, we use this model for the first time to compare observational with modelled (pseudoproxy) stalagmite δ 18 O series for a site at Assynt, NW Scotland, where our knowledge of δ 18 O systematics is relatively well understood. The use of forward modelling allows us to quantify the relative contributions of climate, peat and karst hydrology, and disequilibrium effects in stalagmite δ 18 O, from which we can identify potential stalagmite δ 18 O responses to climate variability. Comparison of the modelled and actual stalagmite δ 18 O for two stalagmites from the site demonstrates that for the period of overlapping growth, the two series do not correlate with one another, but forward modelling demonstrates that this falls within the range explicable by differences in flow routing to the stalagmites. Pseudoproxy δ 18 O stalagmite series highlight the potential significance of peat hydrology in controlling stalagmite δ 18 O over the last 1000 years at this site.

Description: Reconstruction of high resolution atmospheric fields for Northern Europe using analog-upscaling Climate of the Past Discussions, 8, 819-868, 2012 Author(s): F. Schenk and E. Zorita The analog method (AM) has found application to reconstruct gridded climate fields from the information provided by proxy data and climate model simulations. Here, we test the skill of different set-ups of the AM, in a controlled but realistic situation, by analysing several statistical properties of reconstructed daily high-resolution atmospheric fields for Northern Europe for a 50-year period. In this application, station observations of sea-level pressure and air temperature are combined with atmospheric fields from a 50-year high-resolution regional climate simulation. This reconstruction aims at providing homogeneous and physically consistent atmospheric fields with daily resolution suitable to drive high resolution ocean and ecosystem models. Different settings of the AM are evaluated in this study for the period 1958-2007 to estimate the robustness of the reconstruction and its ability to replicate high and low-frequent variability, realistic probability distributions and extremes of different meteorological variables. It is shown that the AM can realistically reconstruct variables with a strong physical link to daily sea-level pressure on daily and monthly scale. However, to reconstruct low-frequency decadal and longer temperature variations, additional monthly mean station temperature as predictor is required. Our results suggest that the AM is a suitable upscaling tool to predict daily fields taken from regional climate simulations based on sparse historical station data. After this testing and characterization of the different set-ups the method will be applied to reconstruct the high-resolution atmospheric fields for the last 160 years.

Description: Madagascar corals reveal Pacific multidecadal modulation of rainfall since 1708 Climate of the Past Discussions, 8, 787-817, 2012 Author(s): C. A. Grove, J. Zinke, F. Peeters, W. Park, T. Scheufen, S. Kasper, B. Randriamanantsoa, M. T. McCulloch, and G.-J. A. Brummer The Pacific Ocean modulates Australian and North American rainfall variability on multidecadal timescales, in concert with the Pacific Decadal Oscillation (PDO). It has been suggested that Pacific decadal variability may also influence Indian Ocean surface temperature and rainfall in a far-field response, similar to the El Niño Southern Oscillation (ENSO) on interannual timescales. However, instrumental records of rainfall are too short and too sparse to confidently assess such multidecadal climatic teleconnections. Here, we present four climate archives spanning the past 300 yr from giant Madagascar corals. We decouple 20th century human deforestation effects from rainfall induced soil erosion using spectral luminescence scanning and geochemistry. The corals provide the first evidence for Pacific decadal modulation of rainfall over the Western Indian Ocean. We find that positive PDO phases are associated with increased Indian Ocean temperatures and rainfall in Eastern Madagascar, while precipitation in Southern Africa and Eastern Australia declines. Consequently, the negative PDO phase that started in 1998 should lead to reduced rainfall over Eastern Madagascar and increased precipitation in Southern Africa and Eastern Australia. We conclude that the PDO has important implications for future multidecadal variability of African rainfall, where water resource management is increasingly important under the warming climate.

Description: Detailed insight into Arctic climatic variability during MIS 11 at Lake El'gygytgyn, NE Russia Climate of the Past Discussions, 8, 6309-6339, 2012 Author(s): H. Vogel, C. Meyer-Jacob, M. Melles, J. Brigham-Grette, A. A. Andreev, V. Wennrich, and P. Rosén Here we present a detailed multiproxy record of the climate and environmental evolution at Lake El'gygytgyn/Far East Russian Arctic during the period 430–395 ka covering the Marine Isotope Stage (MIS) 12/11 transition and the thermal maximum of super interglacial MIS 11. The MIS 12/11 transition at Lake El'gygytgyn is characterized by initial warming followed by a cold reversal implying similarities to the Bølling/Allerød (B/A) to Younger Dryas (YD) pattern of the last deglaciation. Full and remarkably stable interglacial conditions with mean temperatures of warmest month (MTWM) ranging between ca. 10–15 °C, annual precipitation (PANN) ranging between ca. 300–600 mm, strong in-lake productivity, coincide with dark coniferous forests in the catchment, annual disintegration of the lake ice cover and full mixis of the water column. Such conditions persisted for ca. 27 kyrs between ca. 425–398 ka. The Lake El'gygytgyn record closely resembles the climate pattern recorded in Lake Baikal (SE Siberia) sediments and Antarctic ice cores implying strong teleconnections between Northern and Southern Hemispheres during MIS 11. A peak warm period between ca. 418–415.5 ka and a precipitation anomaly at ca. 401 ka at Lake El'gygytgyn, in contrast, appear to be an expression of more regionally confined climate variations.

Description: Vegetation responses to interglacial warming in the Arctic, examples from Lake El'gygytgyn, northeast Siberia Climate of the Past Discussions, 9, 245-267, 2013 Author(s): A. V. Lozhkin and P. M. Anderson Palynological data from Lake El'gygytgyn reveal responses of plant communities to a range of climatic conditions that can help assess the possible impact of global warming on arctoboreal ecosystems. Vegetation associated with climatic optima suggests two types of interglacial responses: one is dominated by deciduous taxa (the postglacial thermal maximum (PGTM) and marine isotope stage (MIS5)) and the second by evergreen conifers (MIS11, MIS31). The MIS11 forests show a similarity to Picea-Larix-Betula-Alnus forests of Siberia. While dark coniferous forest also characterizes MIS31, the pollen taxa show an affinity to the modern boreal forest of the lower Amur valley in the Russian Far East. Despite vegetation differences during the thermal maxima, all four glacial-interglacial transitions are alike, being dominated by deciduous woody taxa. Initially Betula shrub tundra established and was replaced by tundra with tree-sized shrubs (PGTM), Betula woodland (MIS5), or Betula-Larix (MIS11, MIS31) forest. The consistent occurrence of deciduous forest and/or high shrub tundra in all interglaciations as they approach or achieve maximum warmth underscores the significance of this biome for modeling efforts. The El'gygytgyn data also suggest the possible elimination or massive reduction of arctic plant communities under extreme warm-earth scenarios.

Description: The effect of precipitation seasonality on Eemian ice core isotope records from Greenland Climate of the Past Discussions, 9, 269-296, 2013 Author(s): W. J. van de Berg, M. R. van den Broeke, E. van Meijgaard, and F. Kaspar The previous interglacial (Eemian, 130–114 kyr BP) had a mean sea level highstand 4 to 7 m above the current level, and, according to climate proxies, a 2 to 6 K warmer Arctic summer climate. Greenland ice cores extending back into the Eemian show a reduced depletion in δ 18 O of about 3‰ for this period, which suggests a significant warming of several degrees over the Greenland ice sheet. Since the depletion in δ 18 O depends, among other factors, on the condensation temperature of the precipitation, we analyze climatological processes other than mean temperature changes that influence condensation temperature, using output of the regional climate model RACMO2. This model is driven by ERA-40 reanalysis and ECHO-G GCM boundaries for present-day, preindustrial and Eemian climate. The processes that affect the condensation temperature of the precipitation are analyzed using 6-hourly model output. Our results show that changes in precipitation seasonality can cause significant changes of up to 2 K in the condensation temperature that are unrelated to changes in mean temperature.

Description: Why could ice ages be unpredictable? Climate of the Past Discussions, 9, 1053-1098, 2013 Author(s): M. Crucifix It is commonly accepted that the variations of Earth's orbit and obliquity control the timing of Pleistocene glacial-interglacial cycles. Evidence comes from power spectrum analysis of palaeoclimate records and from inspection of the timing of glacial and deglacial transitions. However, we do not know how tight this control is. Is it, for example, conceivable that random climatic fluctuations could cause a delay in deglaciation, bad enough to skip a full precession or obliquity cycle and subsequently modify the sequence of ice ages? To address this question, seven previously published conceptual models of ice ages are analysed by reference to the notion of generalised synchronisation. Insight is being gained by comparing the effects of the astronomical forcing with idealised forcings composed of only one or two periodic components. In general, the richness of the astronomical forcing allows for synchronisation over a wider range of parameters, compared to periodic forcing. Hence, glacial cycles may conceivably have remained paced by the astronomical forcing throughout the Pleistocene. However, all the models examined here also show a range of parameters for which the structural stability of the ice age dynamics is weak. This means that small variations in parameters or random fluctuations may cause significant shifts in the succession of ice ages if the system were effectively in that parameter range. Whether or not the system has strong structural stability depends on the amplitude of the effects associated with the astronomical forcing, which significantly differ across the different models studied here. The possibility of synchronisation on eccentricity is also discussed and it is shown that a high Rayleigh number on eccentricity, as recently found in observations, is no guarantee of reliable synchronisation.

Description: What controls the spatio-temporal distribution of D-excess and 17 O-excess in precipitation? A general circulation model study Climate of the Past Discussions, 8, 5493-5543, 2012 Author(s): C. Risi, A. Landais, R. Winkler, and F. Vimeux Combined measurements of the H 2 18 O and HDO isotopic ratios in precipitation, leading to second-order parameter D-excess, have provided additional constraints on past climates compared to the H 2 18 O isotopic ratio alone. More recently, measurements of H 2 17 O have led to another second-order parameter: 17 O-excess. Recent studies suggest that 17 O-excess in polar ice may provide information on evaporative conditions at the moisture source. However, the processes controlling the spatio-temporal distribution of 17 O-excess are still far from being fully understood. We use the isotopic general circulation model LMDZ to better understand what controls D-excess and 17 O-excess in precipitation at present-day (PD) and during the last glacial maximum (LGM). The simulation of D-excess and 17 O-excess is evaluated against measurements in meteoric water, water vapor and polar ice cores. A set of sensitivity tests and diagnostics are used to quantify the relative effects of evaporative conditions (sea surface temperature and relative humidity), Rayleigh distillation, precipitation re-evaporation and supersaturation during condensation at low temperature. Simulations suggest that in the tropics convective processes and rain re-evaporation are important controls on D-excess and 17 O-excess. In higher latitudes, the effect of distillation, transport and mixing balance the effect of supersaturation. Since these terms are very large and near cancellation, results for both PD and LGM are very sensitive to the supersaturation function. The lower D-excess and 17 O-excess at LGM simulated at LGM are dominated by the supersaturation effect. Evaporative conditions had previously been suggested to be key controling factors of D-excess and 17 O-excess. In LMDZ, evaporative conditions are key in explaining the PD latitudinal and seasonal distributions, but play little role for 17 O-excess and for LGM variations. However, the LMDZ may underestimate this role. More generally, some shortcomings in the simulation of 17 O-excess by LMDZ suggest that GCMs are not yet the perfect tool to quantify with confidence all processes controlling 17 O-excess.

Description: Simulating the temperature and precipitation signal in an Alpine ice core Climate of the Past Discussions, 8, 6111-6134, 2012 Author(s): S. Brönnimann, I. Mariani, M. Schwikowski, R. Auchmann, and A. Eichler Accumulation and δ 18 O data from Alpine ice cores provide information on past temperature and precipitation. However, their correlation with seasonal or annual mean temperature and precipitation at nearby sites is often low. Based on an example we argue that, to some extent, this is due to the irregular sampling of the atmosphere by the ice core (i.e. ice cores only record precipitation events and not dry periods) and the possible incongruity between annual layers and calendar year due to dating uncertainty. Using daily meteorological data from nearby stations and reanalyses we replicate the ice core from the Grenzgletscher (Switzerland, 4200 m a.s.l.) on a sample-by-sample basis. Over the last 15 yr of the ice core record, accumulation and δ 18 O variations can be well reproduced on a sub-seasonal scale. This allows a wiggle-matching approach for defining quasi-annual layers. For this period, correlations between measured and replicated quasi-annual δ 18 O values approach 0.8. Further back in time, the quality of the agreement deteriorates rapidly. Nevertheless, we find significant correlations for accumulation and precipitation over the entire length of the record (1938–1993), which is not the case when comparing ice core δ 18 O with annual mean temperature. A Monte Carlo resampling approach of long meteorological time series is used to further explore the relation, in a replicated ice core, between δ 18 O and annual mean temperature. Results show that meteorologically very different years can lead to quasi-identical values for δ 18 O. This poses limitations to the use of δ 18 O from Alpine ice cores for temperature reconstructions in regions with a variable seasonality in precipitation.

Description: East Asian monsoon climate simulated in the PlioMIP Climate of the Past Discussions, 9, 1135-1164, 2013 Author(s): R. Zhang, Q. Yan, Z. S. Zhang, D. Jiang, B. L. Otto-Bliesner, A. M. Haywood, D. J. Hill, A. M. Dolan, C. Stepanek, G. Lohmann, C. Contoux, F. Bragg, W.-L. Chan, M. A. Chandler, A. Jost, Y. Kamae, A. Abe-Ouchi, G. Ramstein, N. A. Rosenbloom, L. Sohl, and H. Ueda Based on the simulations with fifteen climate models in the Pliocene Model Intercomparison Project (PlioMIP), the regional climate of East Asia (focusing on China) during the mid-Pliocene is investigated in this study. Compared to the pre-industrial, the multi-model ensemble mean (MMM) of all models shows the East Asian summer wind (EASW) largely strengthens in monsoon China, and the East Asian winter wind (EAWW) strengthens in south monsoon China but slightly weakens in north monsoon China in mid-Pliocene. The MMM of all models also illustrates a warmer and wetter mid-Pliocene climate in China. The simulated weakened mid-Pliocene EAWW in north monsoon China and intensified EASW in monsoon China agree well with geological reconstructions. However, the model-model discrepancy in simulating mid-Pliocene East Asian monsoon climate, in particular EAWW, should be further addressed in the future work of PlioMIP.

Description: A reconstruction of radiocarbon production and total solar irradiance from the Holocene 14 C and CO 2 records: implications of data and model uncertainties Climate of the Past Discussions, 9, 1165-1235, 2013 Author(s): R. Roth and F. Joos Past atmospheric CO 2 concentrations reconstructed from polar ice cores combined with its Δ 14 C signature as conserved in tree-rings provide important information both on the cycling of carbon as well as the production of radiocarbon ( Q ) in the atmosphere. The latter is modulated by changes in the strength of the magnetic field enclosed in the solar wind and is a proxy for past changes in solar activity. We perform transient carbon-cycle simulations spanning the past 21 kyr using Bern3D-LPX, a fully featured Earth System Model of Intermediate Complexity (EMIC) with a 3-D ocean, sediment and a dynamic vegetation model. Using the latest atmospheric IntCal09/SHCal04 radiocarbon records, we reconstruct the Holocene radiocarbon fluxes and the total production rate. Our carbon-cycle based modern estimate of Q ≈ 1.7 atoms cm −2 s −1 is lower than previously reported by Masarik and Beer (2009) and more in line with Kovaltsov et al. (2012). Q is then translated into the solar modulation potential (Φ) using the latest geomagnetic field reconstruction and linked to a recent reanalysis of early instrumental data. In contrast to earlier reconstructions, our record suggests that periods of high solar activity (〉600 MeV) were quite common not only in recent millennia but throughout the Holocene. Solar activity in our decadally-smoothed record is during 28% of the time higher than the modern average of 650 MeV during the past 9 ka. But due to considerable uncertainties in the normalization of Φ to instrumental data, the absolute value of Φ remains weakly constrained. Further, our simulations with a spatially resolved model (taking the interhemispheric Δ 14 C gradient into account) show that reconstructions that rely on the Northern Hemisphere 14 C record only are biased towards low values during the Holocene. Notable deviations on decadal-to-centennial time scales are also found in comparison with earlier reconstructions. In a last step, past total solar irradiance (TSI) is quantified using a recently published Φ-TSI relationship yielding small changes in Holocene TSI of order 1 W m −2 with a Maunder Minimum irradiance reduction of 0.85 ± 0.17 W m −2 . Future extension of TSI using autoregressive modeling suggest a declining solar activity in the next decades towards average Holocene conditions. Past TSI changes are finally translated into changes in surfaces atmospheric temperature (SAT) by forcing the Bern3D-LPX model with our new TSI record, yielding SAT anomalies of less than 0.1 K.

Description: Northward advection of Atlantic water in the eastern Nordic Seas over the last 3000 yr: a coccolith investigation of volume transport and surface water changes Climate of the Past Discussions, 9, 1259-1295, 2013 Author(s): C. V. Dylmer, J. Giraudeau, F. Eynaud, K. Husum, and A. De Vernal Three marine sediment cores distributed along the Norwegian (MD95-2011), Barents Sea (JM09-KA11-GC), and Svalbard (HH11-134-BC) continental margins have been investigated in order to reconstruct changes in the poleward flow of Atlantic Waters (AW) and in the nature of upper surface water masses within the eastern Nordic Seas over the last 3000 yr. These reconstructions are based on a limited set of coccolith proxies: the abundance ratio between Emiliania huxleyi and Coccolithus pelagicus , an index of Atlantic vs. Polar-Arctic surface water masses; and Gephyrocapsa muellerae , a drifted coccolith species from the temperate North Atlantic, whose abundance changes are related to variations in the volume transport of the North Atlantic Current and its northernmost extension (the West Spitsbergen Current – WSC) off western Svalbard. The entire investigated area, from 66 to 77° N, was affected by an overall increase in volume flow of AW from 3000 cal yr BP to Present. The long-term modulation of westerlies strength and location which are essentially driven by the dominant mode of the North Atlantic Oscillation (NAO), is thought to explain the observed dynamics of poleward AW flow. The same mechanism also reconciles the recorded opposite zonal shifts in the location of the Arctic Front between the area off western Norway and the Barents Sea-eastern Fram Strait region. The Little Ice Age was governed by deteriorating conditions, with Arctic/Polar waters dominating in the surface off western Svalbard and western Barents Sea, possibly associated with both severe sea-ice conditions and a strongly reduced AW volume flow. A sudden short pulse of resumed high WSC flow interrupted this cold spell in eastern Fram Strait from 330 to 410 cal yr BP, with a a magnitude only surpassed by the one which characterizes the Modern Period. Our dataset not only confirms the high amplitude warming of surface waters at the turn of the 19th century off western Svalbard, it also shows that such a warming was primarily induced by an excess volume flow of AW which stands as unprecedented over the last 3000 yr.

Description: On the Milankovitch sensitivity of the Quaternary deep-sea record Climate of the Past Discussions, 9, 1237-1257, 2013 Author(s): W. H. Berger The response of the climate system to external forcing has become an item of prime interest in the context of global warming, especially with respect to the rate of melting land-based ice masses. The deep-sea record of ice-age climate change has been useful in assessing the sensitivity of the climate system to such forcing, notably to orbital forcing, which is well-known for the last several million years. When comparing response and forcing, one finds that sensitivity varies greatly through time, apparently in dependence on the state of the system. The changing stability of ice masses presumably is the underlying cause for the changing state of the system. A buildup of vulnerable ice masses within the latest Tertiary, when going into the ice ages, is conjectured to cause a stepwise increase of climate variability since the early Pliocene.

Description: Testing the potential of OSL, TT-OSL, IRSL and post-IR IRSL luminescence dating on a Middle Pleistocene sediment record of Lake El'gygytgyn, Russia Climate of the Past Discussions, 8, 4779-4815, 2012 Author(s): A. Zander and A. Hilgers Lake El'gygytgyn is a 12 km wide crater lake located in remote Chukotka in the far East Russian Arctic about 100 km to the north of the Arctic Circle. It was formed by a meteorite impact about 3.58 Ma ago. This study tests the paleomagnetic and proxy data-based Mid- to Late-Pleistocene sediment deposition history using novel luminescence dating techniques of sediment cores taken from the centre of the 175 m deep lake. For dating polymineral and quartz fine grains (4–11 μm grain size range) were extracted from nine different levels from the upper 28 m of sediment cores 5011-1A and 5011-1B. Polymineral sub-samples were analysed by infra-red stimulated luminescence (IRSL) and post-IR infra-red stimulated luminescence (post-IR IRSL) using single aliquot regenerative dose (SAR) sequences. SAR protocols were further applied to measure the blue light optically stimulated luminescence (OSL) and thermally-transferred OSL (TT-OSL) of fine-grained quartz supplemented by a multiple aliquot TT-OSL approach. According to an independent age model, the lowest sample from 27.8–27.9 m below lake bottom level correlates to the Brunhes-Matuyama (B/M) reversal. Finally, the SAR post-IR-IRSL protocol applied to polymineral fine grains was the only luminescence technique able to provide dating results of acceptable accuracy up to ca. 700 ka. Major factors limiting precision and accuracy of the luminescence chronology are, for some samples, natural signals already approaching saturation level, and overall the uncertainty related to the sediment water content and its variations over geological times.

Description: Estimate of climate sensitivity from carbonate microfossils dated near the Eocene-Oligocene global cooling Climate of the Past Discussions, 8, 4923-4939, 2012 Author(s): M. W. Asten Climate sensitivity is a crucial parameter in global temperature modelling. An estimate is made at the time 33.4 Ma using published high-resolution deep-sea temperature proxy obtained from foraminiferal δ 18 O records from DSDP site 744, combined with published data for atmospheric partial pressure of CO 2 ( p CO 2 ) from carbonate microfossils, where δ 11 B provides a proxy for p CO 2 . The p CO 2 data shows a p CO 2 decrease accompanying the major cooling event of about 4 °C from greenhouse conditions to icecap conditions following the Eocene-Oligocene boundary (33.7 My). During the cooling p CO 2 fell from 1150 to 770 ppmv. The cooling event was followed by a rapid and huge increase in p CO 2 back to 1130 ppmv in the space of 50 000 yr. The large p CO 2 increase was accompanied by a small deep-ocean temperature increase estimated as 0.59 ± 0.063 °C. Climate sensitivity estimated from the latter is 1.1 ± 0.4 °C (66% confidence) compared with the IPCC central value of 3 °C. The post Eocene-Oligocene transition (33.4 Ma) value of 1.1 °C obtained here is lower than those published from Holocene and Pleistocene glaciation-related temperature data (800 Kya to present) but is of similar order to sensitivity estimates published from satellite observations of tropospheric and sea-surface temperature variations. The value of 1.1 °C is grossly different from estimates up to 9 °C published from paleo-temperature studies of Pliocene (3 to 4 Mya) age sediments. The range of apparent climate sensitivity values available from paleo-temperature data suggests that either feedback mechanisms vary widely for the different measurement conditions, or additional factors beyond currently used feedbacks are affecting global temperature-CO 2 relationships.

Description: Thermomagnetic properties of vivianite nodules, Lake El'gygytgyn, Northeast Russia Climate of the Past Discussions, 8, 4989-5027, 2012 Author(s): P. S. Minyuk, T. V. Subbotnikova, L. L. Brown, and K. J. Murdock Vivianite, a hydrated iron phosphate, is abundant in sediments of El'gygytgyn Lake, located in the Anadyr Mountains of Central Chukotka, Northeastern Russia (67° 30' N; 172° 05' E). Magnetic measurements, including weight low-field AC magnetic susceptibility, field dependent magnetic susceptibility, hysteresis parameters, temperature dependence of the saturation magnetization, as well as susceptibility in different heating media provide ample information on vivianite. Electron-microprobe analyses, electron microscopy and energy dispersive spectroscopy were used to identify diagnostic minerals. Vivianite nodules are abundant in both sediments of cold (anoxic) and warm (oxic) stages. Magnetic susceptibility of the nodules varies from 0.78 × 10 −6 m 3 kg −1 to 1.72 × 10 −6 m 3 kg −1 (average = 1.05 × 10 −6 m 3 kg −1 ) and is higher than the susceptibility of sediments from the cold intervals. Magnetic properties of vivianite are due to product of oxidation as well as sediment and mineral inclusions. Three types of curves of high temperature dependence susceptibility of vivianite indicate different degree of oxidation and inclusions in the nodules. Vivianite acts as a reductant and reduces hematite to magnetite and suppresses the goethite-hematite transition during heating. Heating vivianite and sulfur mixture stimulate the formation of monoclinic pyrrhotite. An additive of arsenic inhibits the formation of magnetite prior to its Curie temperature. Heating selective vivianite and pyrite mixtures produces formation of several different minerals – magnetite, monoclinic pyrrhotite, and hexagonal pyrrhotite, and make it difficult to interpret the thermomagnetic curves.

Description: Orbital changes, variation in solar activity and increased anthropogenic activities: controls on the Holocene flood frequency in the Lake Ledro area, Northern Italy Climate of the Past Discussions, 8, 4701-4744, 2012 Author(s): B. Vannière, M. Magny, S. Joannin, A. Simonneau, S. B. Wirth, Y. Hamann, E. Chapron, A. Gilli, M. Desmet, and F. S. Anselmetti Two lacustrine sediment cores from Lake Ledro in Northern Italy were studied to produce chronologies of flood events for the past 10 000 yr. For this purpose, we have developed an automatic method that objectively identifies the sedimentary imprint of river floods in the downstream lake basin. The automatic counting of flood deposits was based on colour data extracted from processed core photographs, and the count data were processed to capture the flood signal. Automatic quantification was compared with naked-eye counting. Counts were performed twice on the proximal and distal cores to provide an objective and reproducible record of flood frequency. Geophysical and geochemical analyses made it possible to distinguish event deposits from background sedimentation. Flood frequency and reconstructed sedimentary dynamics were compared with lake-level changes and pollen dynamics inferred from vegetation data. The data suggest a record marked by low flood frequency during the early and middle Holocene (10 000–4500 cal BP). Only modest increases during short intervals are recorded at ca. 8000, 7500, and 7100 cal BP. The last third of the Holocene is characterised by a shift toward increased flood frequency at ca. 4500–4000 cal BP. With the exception of two short intervals around 2900–2500 and 1800–1400 cal BP, which show a slightly reduced number of floods, the trend of increasing flood frequency prevailed until the 20th century, reaching a maximum between the 16th and the 19th centuries. Brief-flood frequency increases recorded during the early and middle Holocene can be attributed to cold climatic oscillations. On a centennial time scale, major changes in flood frequency, such as those observed at ca. 4500 and 500 cal BP, can be attributed to large-scale climatic changes such as the Neo-glacial and Little Ice Age, which are under orbital and possibly solar control. The role of climate as the main forcing factor in flood activity is supported by the lake-level records: the major lake-level rises are synchronous with flood frequency increases. However, in the Bronze Age and during the Middle Ages and modern times, forest clearing and land use are indicated by pollen and archaeological data. These human activities have clearly affected the sediment record of flood activity, and they can partially explain the amplitude of the increases in flood activity.

Description: Modern isotope hydrology and controls on δD of plant leaf waxes at Lake El'gygytgyn, NE Russia Climate of the Past Discussions, 8, 3719-3764, 2012 Author(s): K. M. K. Wilkie, B. Chapligin, H. Meyer, S. Burns, S. Petsch, and J. Brigham-Grette Stable isotope data from lipid biomarkers and diatom silica recovered from lake sediment cores hold great promise for paleoclimate and paleohydrological reconstructions. However, these records rely on accurate calibration with modern precipitation and hydrologic processes. Here we investigate the stable isotopic composition of modern precipitation, streams, lake water and ice cover, and use these data to constrain isotope systematics of the Lake El'gygytgyn basin hydrology. Compound specific hydrogen isotope ratios determined from modern vegetation are compared with modern precipitation and lake core top sediments. Multi-species net (apparent) fractionation values between source water (precipitation) and leaf wax lipids (mean value is −105 ± 13‰) agree with previous results in arid environments and provide a basis for application of this proxy downcore. We conclude that although there may be some bias towards winter precipitation signal, overall leaf wax lipids record annual average precipitation within the El'gygytgyn Basin.

Description: Lake El'gygytgyn water and sediment balance components overview and its implications for the sedimentary record Climate of the Past Discussions, 8, 3977-4001, 2012 Author(s): G. Fedorov, M. Nolan, J. Brigham-Grette, D. Bolshiyanov, G. Schwamborn, and O. Juschus Modern process studies of the hydrologic balance and sediment flux into and out of Lake El'gygytgyn, central Chukotka, provide important quantitative estimates for better understanding the lacustrine paleoclimate record from this basin. Formed ca. 3.6 Ma as a result of a meteorite impact, the basin contains one of the longest paleoclimate records in the Arctic. Fluvial activity into the basin today is concentrated over the short snowmelt period. Recent lake-level changes have a slow regressive character due to active underground runoff, even during winter. The residence time of the lake-water is estimated to be about 100 yr. The main source of clastic material are incoming steams. The amount of sediment discharge through the outflow river is insignificant and completely compensated even by little aeolian input.

Description: Stable isotopic evidence of El Niño-like atmospheric circulation in the Pliocene Western United States Climate of the Past Discussions, 8, 5083-5108, 2012 Author(s): M. J. Winnick, J. M. Welker, and C. P. Chamberlain Understanding how the hydrologic cycle has responded to warmer global temperatures in the past is especially important today as concentrations of CO 2 in the atmosphere continue to increase due to human activities. The Pliocene offers an ideal window into a climate system that has equilibrated with current atmospheric p CO 2 . During the Pliocene the Western United States was wetter than modern, an observation at odds with our current understanding of future warming scenarios, which involve the expansion and poleward migration of the subtropical dry zone. Here we compare Pliocene oxygen isotope profiles of pedogenic carbonates across the Western US to modern isotopic anomalies in precipitation between phases of the El Niño Southern Oscillation (ENSO). We find that when accounting for seasonality of carbonate formation, isotopic changes through the late Pliocene match modern precipitation isotopic anomalies in El Niño years. Furthermore, isotopic shifts through the late Pliocene mirror changes through the early Pleistocene, which likely represents the southward migration of the westerly storm track caused by growth of the Laurentide Ice Sheet. We propose that the westerly storm track migrated northward through the late Pliocene with the development of the modern Cold Tongue in the East Equatorial Pacific, then returned southward with widespread glaciation in the Northern Hemisphere – a scenario supported by terrestrial climate proxies across the US. Together these data support the proposed existence of background El Niño-like conditions in Western North America during the warm Pliocene. If the Earth behaves similarly with future warming, this observation has important implications with regard to the amount and distribution of precipitation in Western North America.

Description: Spatial gradients of temperature, accumulation and δ 18 O-ice in Greenland over a series of Dansgaard-Oeschger events Climate of the Past Discussions, 8, 5209-5261, 2012 Author(s): M. Guillevic, L. Bazin, A. Landais, P. Kindler, A. Orsi, V. Masson-Delmotte, T. Blunier, S. L. Buchardt, E. Capron, M. Leuenberger, P. Martinerie, F. Prié, and B. M. Vinther Air and water stable isotope measurements from three Greenland deep ice cores (GISP2, NGRIP and NEEM) are investigated over a series of Dansgaard-Oeschger events (DO 8-9-10) which are representative of glacial millennial scale variability. Combined with firn modeling, air isotope data allow to quantify abrupt temperature increases for each drill site. Our data show that the magnitude of stadial-interstadial temperature increase is up to 3 °C larger in Central and North Greenland than in North West Greenland. The temporal water isotope (δ 18 O) – temperature relationship varies between 0.3 and 0.6 ± 0.08‰ °C −1 and is systematically larger at NEEM, possibly due to limited changes in precipitation seasonality compared to GISP2 or NGRIP. The gas age-ice age difference of warming events represented in water and air isotopes can only be modeled when assuming a 26% (NGRIP) to 34% (NEEM) lower accumulation than derived from a Dansgaard-Johnsen ice flow model.

Description: Increasing cloud cover in the 20th century: review and new findings in Spain Climate of the Past Discussions, 8, 1133-1167, 2012 Author(s): A. Sanchez-Lorenzo, J. Calbó, and M. Wild Visual observations of clouds have been performed since the establishment of meteorological observatories during the early instrumental period, and have become more systematic and reliable after the mid-19th century due to the establishment of the first national weather services. During the last decades a large number of studies have documented the trends of the total cloud cover (TCC) and cloudy types; most of these studies are focused on the trends since the second half of the 20th century. Due to the lower reliability of former observations, and the fact that most of this data is not accessible in digital format, there is a lack of studies focusing on the trends of cloudiness since the mid-19th century. In the first part, this work attempts to review the previous studies analyzing TCC changes with information covering at least the first half of the 20th century. Then, the study analyses a database of cloudiness observations in Southern Europe (Spain) since the second third of the 19th century. Specifically, monthly TCC series were reconstructed since 1866 by means of a so-called parameter of cloudiness, calculated from the number of cloudless and overcast days. This estimated TCC series show a high interannual and decadal correlation with the observed TCC series originally measured in oktas. After assessing the temporal homogeneity of the estimated TCC series, the mean annual and seasonal series for the whole of Spain and several subregions were calculated. The mean annual TCC shows a general tendency to increase from the beginning of the series until the 1960s; at this point, the trend becomes negative. The linear trend for the annual mean series, estimated over the 1866–2010 period, is a highly remarkable (and statistically significant) increase of +0.44% per decade, which implies an overall increase of more than +6% during the analyzed period. These results are in line with the major part of the previous trends observed at many areas of the World, especially for the records before the 1950s, when a widespread increase of TCC can been considered as a common feature.

Description: A 250 ka oxygen isotope record from diatoms at Lake El'gygytgyn, far east Russian Arctic Climate of the Past Discussions, 8, 1169-1207, 2012 Author(s): B. Chapligin, H. Meyer, G. E. A. Swann, C. Meyer-Jacob, and H.-W. Hubberten In 2003 sediment core Lz1024 was drilled at Lake El'gygytgyn, far east Russian Arctic, in an area of the Northern Hemisphere which has not been glaciated for the last 3.6 Ma. Biogenic silica was used for analysing the oxygen isotope composition (δ 18 O diatom ) in the upper 13 m long section dating back about 250 ka with samples dominated by one taxa in the

Description: A model-data comparison for a multi-model ensemble of early Eocene atmosphere-ocean simulations: EoMIP Climate of the Past Discussions, 8, 1229-1273, 2012 Author(s): D. J. Lunt, T. Dunkley Jones, M. Heinemann, M. Huber, A. LeGrande, A. Winguth, C. Loptson, J. Marotzke, J. Tindall, P. Valdes, and C. Winguth The early Eocene (~55 to 50 Ma) is a time period which has been explored in a large number of modelling and data studies. Here, using an ensemble of previously published model results, making up "EoMIP" – the Eocene Modelling Intercomparison Project, and syntheses of early Eocene terrestrial and SST temperature data, we present a self-consistent inter-model and model-data comparison. This shows that the previous modelling studies exhibit a very wide inter-model variability, but that at high CO 2 , there is good agreement between models and data for this period, particularly if possible seasonal biases in some of the proxies are considered. An energy balance analysis explores the reasons for the differences between the model results, and suggests that differences in surface albedo feedbacks, water vapour and lapse rate feedbacks, and prescribed aerosol loading are the dominant cause for the different results seen in the models, rather than inconsistencies in other prescribed boundary conditions or differences in cloud feedbacks. The CO 2 level which would give optimal early Eocene model-data agreement, based on those models which have carried out simulations with more than one CO 2 level, is in the range 2000 ppmv to 6500 ppmv. Given the spread of model results, tighter bounds on proxy estimates of atmospheric CO 2 during this time period will allow a quantitative assessment of the skill of the models at simulating warm climates, which could be used as a metric for weighting future climate predictions.

Description: "OAE 3" – a low- to mid-latitude Atlantic oceanic event during the Coniacian-Santonian Climate of the Past Discussions, 8, 1209-1227, 2012 Author(s): M. Wagreich The Coniacian-Santonian time interval is the inferred time of oceanic anoxic event 3 (OAE 3), the last of the Cretaceous OAEs. A detailed look on the temporal and spatial distribution of organic-rich deposits attributed to OAE 3 suggests that black shale occurrences are restricted to the equatorial to mid-latitudinal Atlantic and adjacent basins, shelves and epicontinental seas like parts of the Caribbean, the Maracaibo Basin and the Western Interior Basin, and are largely absent in the Tethys, the North Atlantic, the southern South Atlantic, and the Pacific. Here, oxic bottom waters prevailed as indicated by the widespread occurrence of red deep-marine CORBs (Cretaceous Oceanic Red Beds). Widespread CORB sedimentation started during the Turonian after Oceanic Anoxic Event 2 (OAE 2) except in the Atlantic realm where organic-rich strata continue up to the Santonian. The temporal distribution of black shales attributed to OAE 3 indicates that organic-rich strata do not define a single and distinct short-time event, but are distributed over a longer time span and occur in different basins during different times. This suggests intermittent and regional anoxic conditions from the Coniacian to the Santonian. A comparison of time-correlated high-resolution δ 13 C curves for this interval indicates several minor positive excursions of about 0.5 permil, probably as a result of massive organic carbon burial cycles in the Atlantic. Regional wind-induced upwelling and silled deep basins may have contributed to the development of anoxia during a global oxic time interval, thus highlighting the regional character of inferred OAE 3 as an Atlantic anoxic event (AAE).

Description: Simulation of the Indian monsoon and its variability during the last millennium Climate of the Past Discussions, 9, 703-740, 2013 Author(s): S. Polanski, B. Fallah, S. Prasad, and U. Cubasch The general circulation model ECHAM5 has been used to simulate the Indian monsoon and its variability during the Medieval Warm Period (MWP; 900–1100 AD), the Little Ice Age (LIA; 1515–1715 AD) and for recent climate (REC; 1800–2000 AD). The focus is on the analysis of external drivers and internal feedbacks leading to extreme rainfall events over India from interannual to multidecadal time scale. An evaluation of spatiotemporal monsoon patterns with present-day observation data is in agreement with other state-of-the-art monsoon modeling studies. The simulated monsoon intensity on multidecadal time scale is weakened (enhanced) in summer (winter) due to colder (warmer) SSTs in the Indian Ocean. Variations in solar insolation are the main drivers for these SST anomalies, verified by very strong temporal anticorrelations between Total Solar Irradiance and All-India-Monsoon-Rainfall in summer monsoon months. The external solar forcing is coupled and overlain by internal climate modes of the ocean (ENSO and IOD) with asynchronous intensities and lengths of periods. In addition, the model simulations have been compared with a relative moisture index derived from paleoclimatic reconstructions based on various proxies and archives in India. In this context, the Lonar record in Central India has been highlighted and evaluated the first time. The simulated relative annual rainfall anomalies in comparison to present-day climate are in agreement (disagreement) with the reconstructed moisture index for MWP (LIA) climate. In order to investigate the interannual monsoon variability with respect to monsoon failures, dry summer monsoon composites for 30-yr-long periods of MWP, LIA and REC have been further analysed. Within dry years of LIA, the summer rainfall over India and surrounding oceans is less than in MWP indicating stronger drying conditions due to a stronger summer solar insolation forcing coupled with variations in ENSO. To quantify the ECHAM5 simulated long-term drought conditions within Monsoon Asia, the Palmer Drought Severity Index has been additionally estimated for recent climate showing strong pattern correlation between global SST anomalies and EOF variability signal of the drought index, whereas the temporal relationship is weak.

Description: Peak glacial 14 C ventilation ages suggest major draw-down of carbon into the abyssal ocean Climate of the Past Discussions, 9, 925-965, 2013 Author(s): M. Sarnthein, B. Schneider, and P. M. Grootes Ice core records demonstrate a glacial-interglacial atmospheric CO 2 increase of ~ 100 ppm. A transfer of ~ 530 Gt C is required to produce the deglacial rise of carbon in the atmosphere and terrestrial biosphere. This amount is usually ascribed to oceanic carbon release, although the actual mechanisms remained elusive, since an adequately old and carbon-enriched deep-ocean reservoir seemed unlikely. Here we present a new, though still fragmentary, ocean-wide 14 C dataset showing that during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS-1) the 14 C age difference between ocean deep waters and the atmosphere exceeded the modern values by up to 1500 14 C yr, in the extreme reaching 5100 yr. Below 2000 m depth the 14 C ventilation age of modern ocean waters is directly linked to the concentration of dissolved inorganic carbon (DIC). We assume that the range of regression slopes of DIC vs. Δ 14 C remained constant for LGM times, which implies that an average LGM aging by ~ 600 14 C yr corresponded to a global rise by ~ 85–115 μmol DIC kg −1 in the deep ocean. Thus, the prolonged residence time of ocean deep waters indeed made it possible to absorb an additional ~ 730–980 Gt DIC, ~ 1/3 of which transferred from intermediate waters. We infer that LGM deep-water O 2 dropped to suboxic values of 〈 10 μmol kg −1 in the Atlantic sector of the Southern ocean, possibly also in the subpolar North Pacific. The transfer of aged deep-ocean carbon to the atmosphere and the ocean-atmosphere exchange are sufficient to account for the 190-‰ drop in atmospheric 14 C during the so-called HS-1 "Mystery Interval".

Description: Volcanic forcing for climate modeling: a new microphysics-based dataset covering years 1600–present Climate of the Past Discussions, 9, 967-1012, 2013 Author(s): F. Arfeuille, D. Weisenstein, H. Mack, E. Rozanov, T. Peter, and S. Brönnimann As the understanding and representation of the impacts of volcanic eruptions on climate have improved in the last decades, uncertainties in the stratospheric aerosol forcing from large eruptions are now not only linked to visible optical depth estimates on a global scale but also to details on the size, latitude and altitude distributions of the stratospheric aerosols. Based on our understanding of these uncertainties, we propose a new model-based approach to generating a volcanic forcing for General-Circulation-Model (GCM) and Chemistry-Climate-Model (CCM) simulations. This new volcanic forcing, covering the 1600–present period, uses an aerosol microphysical model to provide a realistic, physically consistent treatment of the stratospheric sulfate aerosols. Twenty-six eruptions were modeled individually using the latest available ice cores aerosol mass estimates and historical data on the latitude and date of eruptions. The evolution of aerosol spatial and size distribution after the sulfur dioxide discharge are hence characterized for each volcanic eruption. Large variations are seen in hemispheric partitioning and size distributions in relation to location/date of eruptions and injected SO 2 masses. Results for recent eruptions are in good agreement with observations. By providing accurate amplitude and spatial distributions of shortwave and longwave radiative perturbations by volcanic sulfate aerosols, we argue that this volcanic forcing may help refine the climate model responses to the large volcanic eruptions since 1600. The final dataset consists of 3-D values (with constant longitude) of spectrally resolved extinction coefficients, single scattering albedos and asymmetry factors calculated for different wavelength bands upon request. Surface area densities for heterogeneous chemistry are also provided.

Description: Inter-annual tropical Pacific climate variability in an isotope-enabled CGCM: implications for interpreting coral stable oxygen isotope records of ENSO Climate of the Past Discussions, 9, 741-773, 2013 Author(s): T. Russon, A. W. Tudhope, G. C. Hegerl, M. Collins, and J. Tindall Water isotope-enabled coupled atmosphere/ocean climate models allow for exploration of the relative contributions to coral stable oxygen isotope (δ 18 O coral ) variability arising from Sea Surface Temperature (SST) and the isotopic composition of seawater (δ 18 O sw ). The unforced behaviour of the isotope-enabled HadCM3 Coupled General Circulation Model affirms that the extent to which inter-annual δ 18 O sw variability contributes to that in model δ 18 O coral is strongly spatially dependent, ranging from being negligible in the eastern equatorial Pacific to accounting for 50% of δ 18 O coral variance in parts of the western Pacific. In these latter cases, a significant component of the inter-annual δ 18 O sw variability is correlated to that in SST, meaning that local calibrations of the effective local δ 18 O coral –SST relationships are likely to be essential. Furthermore, the relationship between δ 18 O sw and SST in the central and western equatorial Pacific is non-linear, such that the interpretation of model δ 18 O coral in the context of a linear dependence on SST alone may lead to overestimation (by up to 20%) of the SST anomalies associated with large El-Niño events. Intra-model evaluation of a salinity-based pseudo-coral approach shows that such an approach captures the first-order features of the model δ 18 O sw behaviour. However, the utility of the pseudo-corals is limited by the extent of spatial variability seen within the modelled slopes of the temporal salinity–δ 18 O sw relationship.

Description: Using paleo-climate comparisons to constrain future projections in CMIP5 Climate of the Past Discussions, 9, 775-835, 2013 Author(s): G. A. Schmidt, J. D. Annan, P. J. Bartlein, B. I. Cook, E. Guilyardi, J. C. Hargreaves, S. P. Harrison, M. Kageyama, A. N. LeGrande, B. Konecky, S. Lovejoy, M. E. Mann, V. Masson-Delmotte, C. Risi, D. Thompson, A. Timmermann, L.-B. Tremblay, and P. Yiou We present a description of the theoretical framework and "best practice" for using the paleo-climate model component of the Coupled Model Intercomparison Project (Phase 5) (CMIP5) to constrain future projections of climate using the same models. The constraints arise from measures of skill in hindcasting paleo-climate changes from the present over 3 periods: the Last Glacial Maximum (LGM) (21 thousand years before present, ka), the mid-Holocene (MH) (6 ka) and the Last Millennium (LM) (850–1850 CE). The skill measures may be used to validate robust patterns of climate change across scenarios or to distinguish between models that have differing outcomes in future scenarios. We find that the multi-model ensemble of paleo-simulations is adequate for addressing at least some of these issues. For example, selected benchmarks for the LGM and MH are correlated to the rank of future projections of precipitation/temperature or sea ice extent to indicate that models that produce the best agreement with paleoclimate information give demonstrably different future results than the rest of the models. We also find that some comparisons, for instance associated with model variability, are strongly dependent on uncertain forcing timeseries, or show time dependent behaviour, making direct inferences for the future problematic. Overall, we demonstrate that there is a strong potential for the paleo-climate simulations to help inform the future projections and urge all the modeling groups to complete this subset of the CMIP5 runs.

Description: Global and regional sea surface temperature trends during Marine Isotope Stage 11 Climate of the Past Discussions, 9, 837-890, 2013 Author(s): Y. Milker, R. Rachmayani, M. Weinkauf, M. Prange, M. Raitzsch, M. Schulz, and M. Kučera The Marine Isotope Stage (MIS) 11 (424–374 ka) was characterized by a protracted deglaciation and an unusually long climatic optimum. It remains unclear to what degree the climate development during this interglacial reflects the unusually weak orbital forcing or greenhouse gas trends. Previously, arguments about the duration and timing of the MIS11 climatic optimum and about the pace of the deglacial warming were based on a small number of key records, which appear to show regional differences. In order to obtain a global signal of climate evolution during MIS11, we compiled a database of 78 sea surface temperature (SST) records from 57 sites spanning MIS11, aligned these individually on the basis of benthic ( N = 28) or planktonic ( N = 31) stable oxygen isotope curves to a common time-frame and subjected 48 of them to an Empirical Orthogonal Function (EOF) analysis. The analysis revealed a high commonality among all records, with the principal SST trend explaining almost 49% of the variability. This trend indicates that on the global scale, the surface ocean underwent rapid deglacial warming during Termination V, in pace with carbon dioxide rise, followed by a broad SST optimum centered at ~ 410 kyr. The second EOF, which explained 19% of the variability, revealed the existence of a different SST trend, characterized by a delayed onset of the temperature optimum during MIS11 at ~ 398 kyr, followed by a prolonged warm period lasting beyond 380 kyr. This trend is most consistently manifested in the mid-latitude North Atlantic and Mediterranean Sea and is here attributed to the strength of the Atlantic meridional overturning circulation. A sensitivity analysis indicates that these results are robust to record selection and to age-model uncertainties of up to 3–6 kyr, but more sensitive to SST seasonal attribution and SST uncertainties 〉 1 °C. In order to assess the effect of orbital forcing on MIS11 SST trends, the annual and seasonal SST anomalies recorded in a total of 74 proxy records were compared with CCSM3 (Community Climate System Model, version 3) runs for three time slices representing orbital configuration extremes during the peak interglacial of MIS11. The modeled SST anomalies are characterized by a significantly lower variance compared to the reconstructions. Nevertheless, significant correlations between proxy and model data are found in comparisons on the seasonal basis, indicating that the model captures part of the long-term variability induced by astronomical forcing, which appears to have left a detectable signature in SST trends.

Description: Large spatial variations in coastal 14 C reservoir age – a case study from the Baltic Sea Climate of the Past Discussions, 9, 891-923, 2013 Author(s): B. C. Lougheed, H. L. Filipsson, and I. Snowball Coastal locations are highly influenced by input from freshwater river runoff, including sources of terrestrial carbon, which can be expected to modify the 14 C reservoir age, or R(t) , associated with marine water. In this Baltic Sea case study, pre-bomb museum collection mollusc shells of known calendar age, from 30 locations across a strategic salinity transect of the Baltic Sea, were analysed for 14 C, δ 13 C and δ 18 O. R(t) was calculated for all 30 locations. Seven locations, of which six are within close proximity of the coast, were found to have relatively higher R(t) values, indicative of hard-water effects. δ 13 C aragonite values were found to be indicative of hard-water influence only for certain locations, suggesting the possibility of different sources of old carbon in different locations. Whenever possible, the Macoma genus of mollusc was selected from the museum collections, in order to exclude species specific reservoir age effects as much as possible. When the Macoma samples are exclusively considered, and samples from hard-water locations excluded, a statistically significant correlation between Macoma R(t) and average salinity is found, indicating a two end-member linear mixing model between 14 C marine and 14 C runoff . A map of Baltic Sea Macoma aragonite R(t) for the late 19th and early 20th centuries is produced. Such a map can provide an estimate for contemporary Baltic Sea Macoma R(t) , although one must exercise caution when applying such estimates back in time or to 14 C dates obtained from different sample material. A statistically significant correlation is also found between δ 18 O aragonite and Macoma R(t) , suggesting that δ 18 O aragonite can be used to estimate Macoma palaeo- R(t) . The results of this Baltic Sea case study, which show that R(t) is affected by hydrographic conditions and local carbon inputs, have important consequences for other coastal and estuarine locations, where R(t) is also likely to significantly vary on spatial and temporal bases.

Description: The Irish famine of 1740–1741: causes and effects Climate of the Past Discussions, 9, 1013-1052, 2013 Author(s): S. Engler, J. Luterbacher, F. Mauelshagen, and J. Werner This paper advances the current debate on causes and effects of famines. Since Sen's food entitlement decline theory emerged in the 1980's, climate and environmental factors are widely excluded in famine analysis. Studying the causation and the processes of famines as well as the adaptations to it before the 20th century will enhance modern famine theories and lead to a rethinking of the role of climate/environmental aspects in current research. In our case study, the "Famine Vulnerability Analysis Model" (FVAM) serves as an explanatory model and will open up new perspectives on famines. Special emphasis will be put on the Europe-wide crises of 1740–1741, with a focus on the famine of the "great frost" in Ireland. The interaction of demographic, political, economic and environmental aspects is characteristic in this famine.

Description: A Last Glacial Maximum world-ocean simulation at eddy-permitting resolution – Part 1: Experimental design and basic evaluation Climate of the Past Discussions, 9, 297-328, 2013 Author(s): M. Ballarotta, L. Brodeau, J. Brandefelt, P. Lundberg, and K. Döös Most state-of-the-art climate models include a coarsely resolved oceanic component, which has difficulties in capturing detailed dynamics, and therefore eddy-permitting/eddy-resolving simulations have been developed to reproduce the observed World Ocean. In this study, an eddy-permitting numerical experiment is conducted to simulate the global ocean state for a period of the Last Glacial Maximum (LGM, ~ 26 500 to 19 000 yr ago) and to investigate the improvements due to taking into account these higher spatial scales. The ocean general circulation model is forced by a 49-yr sample of LGM atmospheric fields constructed from a quasi-equilibrated climate-model simulation. The initial state and the bottom boundary condition conform to the Paleoclimate Modelling Intercomparison Project (PMIP) recommendations. Before evaluating the model efficiency in representing the paleo-proxy reconstruction of the surface state, the LGM experiment is in this first part of the investigation, compared with a present-day eddy-permitting hindcast simulation as well as with the available PMIP results. It is shown that the LGM eddy-permitting simulation is consistent with the quasi-equilibrated climate-model simulation, but large discrepancies are found with the PMIP model analyses, probably due to the different equilibration states. The strongest meridional gradients of the sea-surface temperature are located near 40° N and S, this due to particularly large North-Atlantic and Southern-Ocean sea-ice covers. These also modify the locations of the convection sites (where deep-water forms) and most of the LGM Conveyor Belt circulation consequently takes place in a thinner layer than today. Despite some discrepancies with other LGM simulations, a glacial state is captured and the eddy-permitting simulation undertaken here yielded a useful set of data for comparisons with paleo-proxy reconstructions.

Description: Petrophysical characterization of the lacustrine sediment succession drilled in Lake El'gygytgyn, Far East Russian Arctic Climate of the Past Discussions, 9, 351-391, 2013 Author(s): A. C. Gebhardt, A. Francke, J. Kück, M. Sauerbrey, F. Niessen, V. Wennrich, and M. Melles Seismic profiles of Far East Russian Lake El'gygytgyn which was formed by a meteorite impact some 3.6 million years ago show a stratified sediment succession that can be separated into Subunits Ia and Ib at approximately 167 m below lake floor (= ∼ 3.17 Ma). The former is well-stratified, while the latter is acoustically more massive. The sediments are intercalated with frequent mass movement deposits mainly in the proximal parts, while the distal part is almost free of such deposits at least in the upper part. In spring 2009, a long core drilled in the lake center within the framework of the International Continental Scientific Drilling Program (ICDP) penetrated the entire lacustrine sediment succession down to ~ 320 m below lake floor and about 200 m further into the meteorite-impact related bedrock. Downhole logging data down to 390 m below lake floor show that the bedrock and the lacustrine part of the core differ largely in their petrophysical characteristics. The contact between the bedrock and the lacustrine sediments is not abrupt, but rather transitional with a mixture of impact-altered bedrock clasts in a lacustrine matrix with varying percentages. Physical and chemical proxies measured on the cores can be used to divide the lacustrine part into five different clusters. These can be plotted in a redox-condition vs. input type diagram with total organic carbon content and magnetic susceptibility values indicating anoxic or oxic conditions and with the Si/Ti ratio representing more clastic or more biogenic input. Plotting the clusters in this diagram allows identifying clusters that represent glacial phases (Cluster I), super interglacials (Cluster II), and interglacial phases (Clusters III and IV).

Description: Investigating the past and recent δ 18 O-accumulation relationship seen in Greenland ice cores Climate of the Past Discussions, 8, 4105-4120, 2012 Author(s): S. L. Buchardt, H. B. Clausen, B. M. Vinther, and D. Dahl-Jensen Decadal means of δ 18 O and accumulation rates from 52 ice core sites in Greenland are presented. The accumulation rates are derived from annual layers determined in the δ 18 O curve. Investigation of the δ 18 O-accumulation relationship across the ice divide reveals a significant Foehn effect with anticorrelation of δ 18 O and accumulation on the lee side of the divide in Southern Greenland, while no effect is seen in Central Greenland. Furthermore, the sensitivity of accumulation rate to changes in temperature is found to be smaller in Northern Greenland than in the central and southern parts. Four sites in the data set contain sufficient recent data that the period of observed temperature rise from the 1990's and onwards can be investigated. All four sites are located close to the ice divide in Northern Greenland and while three sites show increased temperatures, none show evidence of increased accumulation.

Description: Vegetation dynamics in the Northeastern Mediterranean region during the past 23 000 yr: insight from a new pollen record from the Sea of Marmara (core MD01-2430) Climate of the Past Discussions, 8, 4183-4221, 2012 Author(s): V. Valsecchi, M. F. Sanchez Goñi, and L. Londeix High-resolution pollen analysis of core MD01-2430 from the Sea of Marmara (40°47.81′ N, 27°43.51′ E) allows us to reconstruct the vegetation response to climatic changes during the past 23 cal ka in the Northeastern Mediterranean. Variation in mesic/temperate forest cover indicates major climatic shifts connected to Heinrich Stadial 1, Bölling-Allerød, Younger Dryas and to the onset of the Holocene. Pollen-anthropogenic indicator approach was used to recognize human-induced landscape changes in the Sea of Marmara. The pollen-inferred onset of the Holocene occurs at ca. 11.5 cal ka, indicating that the Northeastern Mediterranean region represents a transitional zone where higher moisture availability supported an earlier forest expansion than the borderlands of the Aegean Sea and Black Sea. Three major forest retreats occurred during the Holocene at ca. 9.2, 5.5, and 2.1 cal ka while a weaker forest setback was detected at 7.9 cal ka. The Holocene forest setbacks are in phase with previously published alkenone-inferred sea-surface temperature decreases in the Sea of Marmara (core MD01-2430) reconstructed from the same archive. Our new pollen record testifies the sensitivity of Mediterranean forests to changes in moisture availability which is driven by changes in high-latitude climate drivers (North Atlantic Oscillations and/or Siberian High).

Description: Temperature response to external forcing in simulations and reconstructions of the last millennium Climate of the Past Discussions, 8, 4003-4073, 2012 Author(s): L. Fernández-Donado, J. F. González-Rouco, C. C. Raible, C. M. Ammann, D. Barriopedro, E. García-Bustamante, J. H. Jungclaus, S. J. Lorenz, J. Luterbacher, S. J. Phipps, J. Servonnat, D. Swingedouw, S. F. B. Tett, S. Wagner, P. Yiou, and E. Zorita The understanding of natural climate variability and its driving factors is crucial to assess future climate change. Therefore, comparing proxy-based climate reconstructions with forcing factors as well as comparing these with paleoclimate model simulations is key to gain insights into the relative roles of internal versus forced variability. A review of the state of modeling of the last millennium climate previous to the CMIP5-PMIP3 coordinated effort is presented and compared to the available temperature reconstructions. Simulations and reconstructions broadly agree on reproducing the major temperature changes and suggest an overall linear response to external forcing on multidecadal or longer timescales. Internal variability is found to have an important influence at hemispheric and global scales. The spatial distribution of simulated temperature changes during the transition of the Medieval Climate Anomaly to the Little Ice Age disagrees with that found in the reconstructions, thus advocating for internal variability as a possible major player in shaping temperature changes through the millennium. A paleo transient climate response (PTCR) is defined to provide a quantitative framework for analysing the consistency between simulated and reconstructed climate. Beyond an overall agreement between simulated and reconstructed PTCR ranges, this analysis is able to single out specific discrepancies between some reconstructions and the ensemble of simulations. The disagreement is found in the cases where the reconstructions show reduced covariability with external forcings or when they present high rates of temperature change.

Description: Mismatch between the depth habitat of planktonic foraminifera and the calibration depth of SST transfer functions may bias reconstructions Climate of the Past Discussions, 8, 4075-4103, 2012 Author(s): R. J. Telford, C. Li, and M. Kucera We demonstrate that the temperature signal in the planktonic foraminifera assemblage data from the North Atlantic typically does not originate from near surface waters and argue that this has the potential to bias sea surface temperature reconstructions using transfer functions calibrated against near surface temperatures if the thermal structure of the upper few hundred metres of ocean changes over time. CMIP5 climate models indicate that ocean thermal structure in the N Atlantic changed between the Last Glacial Maximum (LGM) and the pre-industrial (PI), with some regions, mainly in the tropics, of the LGM ocean lacking good thermal analogues in the PI. Transfer functions calibrated against different depths reconstruct a marked subsurface cooling in the tropical Atlantic during the last glacial, in contrast to previous studies that reconstructed only a modest cooling. These possible biases in temperatures reconstructions may affect estimates of climate sensitivity based on the difference between LGM and pre-industrial climate. Quantifying these biases has the potential to alter our understanding of Last Glacial Maximum climate and improve estimates of climate sensitivity.

Description: Variability of the ocean heat content during the last millennium – an assessment with the ECHO-g Model Climate of the Past Discussions, 8, 4223-4267, 2012 Author(s): P. Ortega, M. Montoya, F. González-Rouco, H. Beltrami, and D. Swingedouw Studies addressing climate variability during the last millennium generally focus on variables with a direct influence on climate variability, like the fast thermal response to varying radiative forcing, or the large-scale changes in atmospheric dynamics (e.g. North Atlantic Oscillation). The ocean responds to these variations by slowly integrating in depth the upper heat flux changes, thus producing a delayed influence on ocean heat content (OHC) that can latter impact on low frequency SST variability through reemergence processes. In this study, both the externally and internally driven variations of the OHC are investigated during the last millennium using a set of fully coupled simulations using the model ECHO-G. When compared to observations for the last 55 yr, the model tends to overestimate the global trends, and underestimate the decadal OHC variability. Extending the analysis back to the last one thousand years, the main impact of the radiative forcing is an OHC increase at high latitudes, explained to some extent by a reduction in cloud cover and the subsequent increase of short-wave radiation at the surface. This OHC response is dominated by the effect of volcanism in the preindustrial era, and by the fast increase of GHGs during the last 150 yr. Likewise, salient impacts from internal climate variability are observed at regional scales. For instance, upper temperature in the Equatorial Pacific is controlled by ENSO variability from interannual to multidecadal timescales. Also, both the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) modulate intermittently the interdecadal OHC variability in the North Pacific and Mid Atlantic, respectively. The NAO, through its influence on North Atlantic surface heat fluxes and convection, also plays an important role on the OHC at multiple timescales, leading first to a cooling in the Labrador and Irminger seas, and later on to a North Atlantic warming, associated with a delayed impact on the AMO.

Description: Historical and idealized climate model experiments: an EMIC intercomparison Climate of the Past Discussions, 8, 4121-4181, 2012 Author(s): M. Eby, A. J. Weaver, K. Alexander, K. Zickfeld, A. Abe-Ouchi, A. A. Cimatoribus, E. Crespin, S. S. Drijfhout, N. R. Edwards, A. V. Eliseev, G. Feulner, T. Fichefet, C. E. Forest, H. Goosse, P. B. Holden, F. Joos, M. Kawamiya, D. Kicklighter, H. Kienert, K. Matsumoto, I. I. Mokhov, E. Monier, S. M. Olsen, J. O. P. Pedersen, M. Perrette, G. Philippon-Berthier, A. Ridgwell, A. Schlosser, T. Schneider von Deimling, G. Shaffer, R. S. Smith, R. Spahni, A. P. Sokolov, M. Steinacher, K. Tachiiri, K. Tokos, M. Yoshimori, N. Zeng, and F. Zhao Both historical and idealized climate model experiments are performed with a variety of Earth System Models of Intermediate Complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO 2 , additional greenhouse gases, land-use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes seem to be underestimated. It is possible that recent modelled climate trends or climate-carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO 2 and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2x and 4x CO 2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate-carbon feedbacks. The values from EMICs generally fall within the range given by General Circulation Models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows considerable synergy between land-use change and CO 2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO 2 between the Medieval Climate Anomaly and the Little Ice Age estimated from paleoclimate reconstructions. This in turn could be a result of errors in the reconstructions of volcanic and/or solar radiative forcing used to drive the models or the incomplete representation of certain processes or variability within the models. Given the datasets used in this study, the models calculate significant land-use emissions over the pre-industrial. This implies that land-use emissions might need to be taken into account, when making estimates of climate-carbon feedbacks from paleoclimate reconstructions.