ALBERT

Description: 〈b〉Spiky Fluctuations and Scaling in High-Resolution EPICA Ice Core Dust Fluxes〈/b〉〈br〉 Shaun Lovejoy and Fabrice Lambert〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2018-171,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 We analyze the statistical properties of the eight past glacial-interglacial cycles as well as subsections of a generic glacial cycle using the high-resolution dust flux dataset from the Antarctic EPICA Dome C ice core. We show that the high southern latitude climate during glacial maxima, interglacial, and glacial inception is generally more stable but more drought-prone than during mid-glacial conditions.

Description: 〈b〉A reconstruction of warm water inflow to Upernavik Isstrøm since AD 1925 and its relation to glacier retreat〈/b〉〈br〉 Flor Vermassen, Nanna Andreasen, David J. Wangner, Nicolas Thibault, Marit-Solveig Seidenkrantz, Rebecca Jackson, Sabine M. Schmidt, Kurt H. Kjær, and Camilla S. Andresen〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2018-174,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 This study shows that warming of ocean waters is related to the retreat of Upernavik Isstrøm, a glacier in Northwest Greenland. We show that in the 1930s and after 2000 the waters in the fjord warmed and the glacier retreated. We found this by investigating microfossils from sediments in Upernavik Fjord; different species occur in response to warmer waters.

Description: 〈b〉Inconsistencies between observed, reconstructed, and simulated precipitation indices for England since the year 1650 CE〈/b〉〈br〉 Oliver Bothe, Sebastian Wagner, and Eduardo Zorita〈br〉 Clim. Past, 15, 307-334, https://doi.org/10.5194/cp-15-307-2019, 2019〈br〉 Our understanding of future climate changes increases if different sources of information agree on past climate variations. Changing climates particularly impact local scales for which future changes in precipitation are highly uncertain. Here, we use information from observations, model simulations, and climate reconstructions for regional precipitation over the British Isles. We find these do not agree well on precipitation variations over the past few centuries.

Description: 〈b〉How large are temporal representativeness errors in paleoclimatology?〈/b〉〈br〉 Daniel E. Amrhein〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-10,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 〈p〉Ongoing work in paleoclimate reconstruction prioritizes understanding the origins and magnitudes of errors that arise when comparing models and data. One class of such errors arises from assumptions of proxy temporal representativeness – broadly, the time scales over which paleoclimate proxy measurements are associated with climate variables. In the case of estimating time mean values over an interval, errors can arise when the time interval over which data are averaged and the interval that is being studied have different lengths, or if those intervals are offset from one another in time. Because it is challenging to tailor proxy measurements to precise time intervals, such errors are expected to be common in model-data and data-data comparisons, but how large and prevalent they are is unclear. The goal of this work is to provide a framework for first-order quantification of temporal representativity errors and to study the interacting effects of sampling error, archive smoothing (e.g. by bioturbation in sediment cores), chronological offsets and errors (e.g. arising from radiocarbon dating), and the spectral character of the climate process being sampled.〈/p〉 〈p〉 In some cases, particularly for small values of target intervals 〈i〉τ〈/i〉〈sub〉〈i〉x〈/i〉〈/sub〉 relative to sample intervals 〈i〉τ〈/i〉〈sub〉〈i〉y〈/i〉〈/sub〉, errors can be large relative to signals of interest. Errors from mismatches in 〈i〉τ〈/i〉〈sub〉〈i〉x〈/i〉〈/sub〉 and 〈i〉τ〈/i〉〈sub〉〈i〉y〈/i〉〈/sub〉 can have magnitudes comparable to those from chronological uncertainty. Archive smoothing can reduce sampling errors by acting as an anti-aliasing filter, but destroys high-frequency climate information. An extension of the approach to paleoclimate time series, which are sequences of time-average values, shows that measurement intervals shorter than the spacing between samples lead to errors, absent compensating effects from archive smoothing. Including these sources of uncertainty will improve accuracy in model-data comparisons and data comparisions and syntheses. Moreover, because sampling procedures emerge as important parameters in uncertainty quantification, reporting salient information about how records are processed and assessments of archive smoothing and chronological uncertainties alongside published data is important to be able to use records to their maximum potential in paleoclimate reconstruction and data assimilation.〈/p〉

Description: 〈b〉Diatom-oxygen isotope record from high-altitude Lake Petit (2200 m a.s.l.) in the Mediterranean Alps: shedding light on a climatic pulse at 4.2 ka〈/b〉〈br〉 Rosine Cartier, Florence Sylvestre, Christine Paillès, Corinne Sonzogni, Martine Couapel, Anne Alexandre, Jean-Charles Mazur, Elodie Brisset, Cécile Miramont, and Frédéric Guiter〈br〉 Clim. Past, 15, 253-263, https://doi.org/10.5194/cp-15-253-2019, 2019〈br〉 A major environmental change, 4200 years ago, was recorded in the lacustrine sediments of Lake Petit (Mediterranean Alps). The regime shift was described by a modification in erosion processes in the watershed and aquatic species in the lake. This study, based on the analysis of the lake water balance by using oxygen isotopes in diatoms, revealed that these environmental responses were due to a rapid change in precipitation regime, lasting ca. 500 years.

Description: 〈b〉Central Tethyan platform-top hypoxia during Oceanic Anoxic Event 1a〈/b〉〈br〉 Alexander Hueter, Stefan Huck, Stéphane Bodin, Ulrich Heimhofer, Stefan Weyer, Klaus P. Jochum, and Adrian Immenhauser〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-3,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 4 comments)〈br〉 In this multi-proxy study we present and critically discuss the hypothesis that during the Early Aptian, platform-top hypoxia temporarily established in some of the vast epeiric seas of the Central Tethys and triggered significant changes in reefal ecosystems. Data shown here shed light on the driving mechanisms that control poorly understood faunal patterns during OAE 1a in the neritic realm and provide evidence on the intricate relation between basinal and platform-top water masses.

Description: 〈b〉Contribution of sea ice albedo and insulation effects to Arctic amplification in the EC-Earth Pliocene simulation〈/b〉〈br〉 Jianqiu Zheng, Qiong Zhang, Qiang Li, Qiang Zhang, and Ming Cai〈br〉 Clim. Past, 15, 291-305, https://doi.org/10.5194/cp-15-291-2019, 2019〈br〉 This paper addresses two important issues with the EC-Earth Pliocene simulation, including the following: (1) quantification of albedo and insulation effects of Arctic sea ice on interface heat exchange and (2) an explanation as to why Arctic amplification in surface air temperature (SST) peaks in winter while there is maximum SST warming in summer. These issues provide potential implications for researching Arctic amplification and climate change.

Description: 〈b〉Climate changes in interior semi-arid Spain from the last interglacial to the late Holocene〈/b〉〈br〉 Dongyang Wei, Penélope González-Sampériz, Graciela Gil-Romera, Sandy P. Harrison, and I. Colin Prentice〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-16,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 El Cañizar de Villarquemado provides a pollen record from semi-arid Spain since before the last interglacial. We use modern pollen–climate relationships to reconstruct changes in seasonal temperature and moisture, accounting for CO〈sub〉2〈/sub〉 effects on plants, and show coherent climate changes on glacial–interglacial and orbital timescales. The low glacial CO〈sub〉2〈/sub〉 means moisture changes are less extreme than suggested by the vegetation shifts, and driven by evapotranspiration rather than rainfall changes.

Description: 〈b〉Annually resolved δ〈sup〉2〈/sup〉H tree-ring chronology of the lignin methoxyl groups from Germany reflects averaged Western European surface air temperature changes〈/b〉〈br〉 Tobias Anhäuser, Birgit Sehls, Werner Thomas, Claudia Hartl, Markus Greule, Denis Scholz, Jan Esper, and Frank Keppler〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-8,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 〈p〉Stable hydrogen isotopes ratios of lignin methoxyl groups (expressed as δ〈sup〉2〈/sup〉H〈sub〉LM〈/sub〉) of wood have been shown to reflect the climate-sensitive δ〈sup〉2〈/sup〉H values of precipitation (expressed as δ〈sup〉2〈/sup〉H〈sub〉precip〈/sub〉) modulated by a large uniform negative isotope fractionation. However, a detailed calibration study among temporal variabilities of δ〈sup〉2〈/sup〉H〈sub〉LM〈/sub〉 in tree-ring series, site-specific δ〈sup〉2〈/sup〉H〈sub〉precip〈/sub〉 and climate parameters has not been performed yet. Here, we present annually resolved δ〈sup〉2〈/sup〉H〈sub〉LM〈/sub〉 values from nine tree-ring series (derived from four 〈i〉Fagus sylvatica〈/i〉 L. trees) collected near stations of the Global Isotope Network of Isotopes in Precipitation (GNIP) and the Deutsche Wetterdienst (DWD) meteorological observatory at Hohenpeißenberg (southern Germany; ~ 48° N, 11° E). The measured nine δ〈sup〉2〈/sup〉H〈sub〉LM〈/sub〉 tree-ring series (common period of overlap 1916-2015) show a strong coherency as indicated by highly significant (〈i〉p〈/i〉 2H〈sub〉LM〈/sub〉 chronology shows highest correlations with annually averaged data of δ〈sup〉2〈/sup〉H〈sub〉precip〈/sub〉 as well as temperature particularly when using the year defined from previous September to current August (〈i〉r〈/i〉 = 0.73 and 0.56, respectively, 〈i〉p〈/i〉 2H〈sub〉LM〈/sub〉 chronology shows enhanced correlations with land and sea surface air temperature for multiple (broadly combined) areas across Western Europe (〈i〉r〈/i〉 〉 0.6, 〈i〉p〈/i〉 2H〈sub〉LM〈/sub〉 chronology (〈i〉r〈/i〉 = 0.71, 〈i〉p〈/i〉 n = 100). Therefore, δ〈sup〉2〈/sup〉H〈sub〉LM〈/sub〉 values of mid-latitudinal tree-ring archives are considered suitable for large-scale mean annual temperature reconstructions and are therefore able to improve the paleoclimatic potential of Late Holocene tree-ring archives.〈/p〉

Description: 〈b〉Deglacial evolution of regional Antarctic climate and Southern Ocean conditions in transient climate simulations〈/b〉〈br〉 Daniel P. Lowry, Nicholas R. Golledge, Laurie Menviel, and Nancy A. N. Bertler〈br〉 Clim. Past, 15, 189-215, https://doi.org/10.5194/cp-15-189-2019, 2019〈br〉 Using two climate models, we seek to better understand changes in Antarctic climate and Southern Ocean conditions during the last deglaciation. We highlight the importance of sea ice and ice topography changes for Antarctic surface temperatures and snow accumulation as well as the sensitivity of Southern Ocean temperatures to meltwater fluxes. The results demonstrate that climate model simulations of the deglaciation could be greatly improved by considering ice–ocean interactions and feedbacks.

Description: 〈b〉Precipitation 〈i〉δ〈/i〉〈sup〉18〈/sup〉O on the Himalaya–Tibet orogeny and its relationship to surface elevation〈/b〉〈br〉 Hong Shen and Christopher J. Poulsen〈br〉 Clim. Past, 15, 169-187, https://doi.org/10.5194/cp-15-169-2019, 2019〈br〉 The stable isotopic composition of water (δ〈sup〉18〈/sup〉O) preserved in terrestrial sediments has been used to reconstruct surface elevations. The method is based on the observed decrease in δ〈sup〉18〈/sup〉O with elevation, attributed to rainout during air mass ascent. We use a climate model to test the δ〈sup〉18〈/sup〉O–elevation relationship during Tibetan–Himalayan uplift. We show that δ〈sup〉18〈/sup〉O is a poor indicator of past elevation over most of the region, as processes other than rainout are important when elevations are lower.

Description: 〈b〉Dynamic climate-driven controls on the deposition of the Kimmeridge Clay Formation in the Cleveland Basin, Yorkshire, UK〈/b〉〈br〉 Elizabeth Atar, Christian März, Andrew Aplin, Olaf Dellwig, Liam Herringshaw, Violaine Lamoureux-Var, Melanie J. Leng, Bernhard Schnetger, and Thomas Wagner〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2018-172,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 1 comment)〈br〉 We present a geochemical and petrographic study of the Kimmeridge Clay Formation from the Cleveland Basin (Yorkshire, UK). Our results indicate that deposition during this interval was very dynamic and oscillated between three distinct modes of sedimentation. In line with recent modelling results, we propose that these highly dynamic conditions were driven by changes in climate, which affected continental weathering, enhanced primary productivity, and led to organic carbon enrichment.

Description: 〈b〉Causes for increased flood frequency in central Europe in the 19th century〈/b〉〈br〉 Stefan Brönnimann, Luca Frigerio, Mikhaël Schwander, Marco Rohrer, Peter Stucki, and Jörg Franke〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-17,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 During the 19th century flood frequency was high in Central Europe, but it was low in the mid-20th century. This paper tracks these decadal changes in flood frequency for the case of Switzerland from peak discharge data back to precipitation data and daily weather reconstructions. We find an increased frequency of flood-prone weather types during large parts of the 19th century and decreased frequency in the mid-20th century. Sea-surface temperature anomalies can only explain a small part of it.

Description: 〈b〉Palaeoenvironmental response of mid-latitudinal wetlands to PETM climate change (Schöningen lignite deposits, Germany)〈/b〉〈br〉 Katharina Methner, Olaf Lenz, Walter Riegel, Volker Wilde, and Andreas Mulch〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-20,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 We describe the presence of a carbon isotope excursion (CIE) in Paleogene lignites (Schöningen, DE) that we attribute to reflect the Paleocene-Eocene Thermal Maximum (PETM). Palynological data point to minor PETM-related paleovegetation changes. Paleo-North Sea wetland deposits (Schöningen; Cobham; Vasterival) therefore, share major characteristics of increased fire activity (pre-PETM), minor plant species changes, a reduced CIE, and drowning of near-coastal mires during the PETM.

Description: 〈b〉Physical processes of cooling and mega-drought during the 4.2 ka BP event: results from TraCE-21ka simulations〈/b〉〈br〉 Mi Yan and Jian Liu〈br〉 Clim. Past, 15, 265-277, https://doi.org/10.5194/cp-15-265-2019, 2019〈br〉 〈p〉It is widely believed that multi-decadal to centennial cooling and drought occurred from 4500 to 3900 BP, known as the 4.2 ka BP event that triggered the collapse of several cultures. However, whether this event was a global event or a regional event and what caused this event remains unclear. In this study, we investigated the spatiotemporal characteristics, the possible causes and the related physical processes of the event using a set of long-term climate simulations, including one all-forcing experiment and four single-forcing experiments. The results derived from the all-forcing experiment show that this event occurs over most parts of the Northern Hemisphere (NH), indicating that this event could have been a hemispheric event. The cooler NH and warmer Southern Hemisphere (SH) illustrate that this event could be related to the slowdown of the Atlantic Meridional Overturning Circulation (AMOC). The comparison between the all-forcing experiment and the single-forcing experiments indicates that this event might have been caused by internal variability, while external forcings such as orbital and greenhouse gases might have modulation effects. A positive North Atlantic Oscillation (NAO)-like pattern in the atmosphere (low troposphere) triggered a negative Atlantic Multi-decadal Oscillation (AMO)-like pattern in the ocean, which then triggered a circum-global teleconnection (CGT)-like wave train pattern in the atmosphere (high troposphere). The positive NAO-like pattern and the CGT-like pattern are the direct physical processes that led to the NH cooling and mega-drought. The AMO-like pattern played a “bridge” role in maintaining this barotropic structure in the atmosphere at a multi-decadal–centennial timescale. Our work provides a global image and dynamic background to help better understand the 4.2 ka BP event.〈/p〉

Description: 〈b〉Extreme droughts/floods and their impacts on harvest derived from historical documents in Eastern China during 801–1910〈/b〉〈br〉 Zhixin Hao, Maowei Wu, Jingyun Zheng, Jiewei Chen, Xuezhen Zhang, and Shiwei Luo〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-14,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 Due to the data limitation, the long-term impacts of climate change on crop harvest did not get more attention, in particularly before the 1950s. However, the rain fed agriculture was dominated during the historical time, and no more technique intervention, thus it has true statistics significance when we discuss the Extreme droughts/floods and their impacts on harvest for the past 1000 years.

Description: 〈b〉The 4.2 ka event in the vegetation record of the central Mediterranean〈/b〉〈br〉 Federico Di Rita and Donatella Magri〈br〉 Clim. Past, 15, 237-251, https://doi.org/10.5194/cp-15-237-2019, 2019〈br〉 〈p〉In this paper, the variation in forest cover in the central Mediterranean region, reflected by percentage changes in the arboreal pollen record, has been examined in relation to the 4.2 ka event. A total of 36 well-dated and detailed pollen records from latitudes between 45 and 36〈span〉〈sup〉∘〈/sup〉〈/span〉 N were selected and their vegetation dynamics between 5 and 3 ka examined in relation to the physiographic and climatic features of the study area and to the influence of human activity on past vegetation, as suggested by anthropogenic pollen indicators. We have found that the sites located between 43 and 45〈span〉〈sup〉∘〈/sup〉〈/span〉 N do not show any significant vegetation change in correspondence with the 4.2 ka event. Several sites located on the Italian Peninsula between 39 and 43〈span〉〈sup〉∘〈/sup〉〈/span〉 N show a marked opening of the forest, suggesting a vegetation response to the climate instability of the 4.2 ka event. Between 36 and 39〈span〉〈sup〉∘〈/sup〉〈/span〉 N, a forest decline is always visible around 4.2 ka, and in some cases it is dramatic. This indicates that this region was severely affected by a climate change towards arid conditions that lasted a few hundred years and was followed by a recovery of forest vegetation in the Middle Bronze Age. Human activity, especially intense in southern Italy, may have been favored by this natural opening of vegetation. In Sardinia and Corsica, no clear change in vegetation is observed at the same time. We suggest that during the 4.2 ka event southern Italy and Tunisia were under the prevalent influence of a north African climate system characterized by a persistent high-pressure cell.〈/p〉

Description: 〈b〉Mercury anomalies across the Palaeocene–Eocene Thermal Maximum〈/b〉〈br〉 Morgan T. Jones, Lawrence M. E. Percival, Ella W. Stokke, Joost Frieling, Tamsin A. Mather, Lars Riber, Brian A. Schubert, Bo Schultz, Christian Tegner, Sverre Planke, and Henrik H. Svensen〈br〉 Clim. Past, 15, 217-236, https://doi.org/10.5194/cp-15-217-2019, 2019〈br〉 Mercury anomalies in sedimentary rocks are used to assess whether there were periods of elevated volcanism in the geological record. We focus on five sites that cover the Palaeocene–Eocene Thermal Maximum, an extreme global warming event that occurred 55.8 million years ago. We find that sites close to the eruptions from the North Atlantic Igneous Province display significant mercury anomalies across this time interval, suggesting that magmatism played a role in the global warming event.

Description: 〈b〉Centennial-scale precipitation anomalies in the southern Altiplano (18° S) suggest an extra-tropical driver for the South American Summer Monsoon during the late Holocene〈/b〉〈br〉 Ignacio A. Jara, Antonio Maldonado, Leticia González, Armand Hernández, Alberto Sáez, Santiago Giralt, Roberto Bao, and Blas Valero-Garcés〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-13,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 The South American Summer Monsoon (SASM) is the most significant climate feature of South America. However, little is known about SASM variability in the past. Here we present a new SASM reconstruction from Lago Chungará in the southern Altiplano (18° S). We show important changes in SASM strength at centennial timescales. Our results suggest that SASM variability in the southern Altiplano was not only controlled by tropical features, but also influenced by an extra-tropical precipitation.

Description: 〈b〉China's historical record when searching for tropical cyclones corresponding to Intertropical Convergence Zone (ITCZ) shifts over the past 2 kyr〈/b〉〈br〉 Huei-Fen Chen, Yen-Chu Liu, Chih-Wen Chiang, Xingqi Liu, Yu-Min Chou, and Hui-Juan Pan〈br〉 Clim. Past, 15, 279-289, https://doi.org/10.5194/cp-15-279-2019, 2019〈br〉 China’s historical typhoon data show that (1) the tracks of typhoons correspond to the north–southward migration of the Intertropical Convergence Zone and phase of the North Atlantic Oscillation, which show a nearly 30-year and a 60-year cycle during the Little Ice Age (LIA) and (2) paleotyphoons made landfall in mainland China 1 month earlier during the Medieval Warm Period than during the LIA.

Description: 〈b〉Comparing the spatial patterns of climate change in the 9th and 5th millennia BP from TRACE-21 model simulations〈/b〉〈br〉 Liang Ning, Jian Liu, Raymond S. Bradley, and Mi Yan〈br〉 Clim. Past, 15, 41-52, https://doi.org/10.5194/cp-15-41-2019, 2019〈br〉 〈p〉The spatial patterns of global temperature and precipitation changes, as well as corresponding large-scale circulation patterns during the latter part of the 9th and 5th millennia BP (4800–4500 versus 4500–4000 BP and 9200–8800 versus 8800–8000 BP) are compared through a group of transient simulations using the Community Climate System Model version 3 (CCSM3). Both periods are characterized by significant sea surface temperature (SST) decreases over the North Atlantic, south of Iceland. Temperatures were also colder across the Northern Hemisphere but warmer in the Southern Hemisphere. Significant precipitation decreases are seen over most of the Northern Hemisphere, especially over Eurasia and the Asian monsoon regions, indicating a weaker summer monsoon. Large precipitation anomalies over northern South America and adjacent ocean regions are related to a southward displacement of the Intertropical Convergence Zone (ITCZ) in that region. Climate changes in the late 9th millennium BP (the “8.2 ka event”) are widely considered to have been caused by a large freshwater discharge into the northern Atlantic, which is confirmed in a meltwater forcing sensitivity experiment, but this was not the cause of changes occurring between the early and latter halves of the 5th millennium BP. Model simulations suggest that a combination of factors, led by long-term changes in insolation, drove a steady decline in SSTs across the North Atlantic and a reduction in the North Atlantic Meridional Overturning Circulation (AMOC), over the past 4500 years, with associated teleconnections across the globe, leading to drought in some areas. Multi-century-scale fluctuations in SSTs and AMOC strength were superimposed on this decline. This helps explain the onset of neoglaciation around 5000–4500 BP, followed by a series of neoglacial advances and retreats during recent millennia. The “4.2 ka BP Event” appears to have been one of several late Holocene multi-century fluctuations that were embedded in the long-term, low-frequency change in climate that occurred after 〈span〉∼4.8〈/span〉 ka. Whether these multi-century fluctuations were a response to internal centennial-scale ocean–atmosphere variability or external forcing (such as explosive volcanic eruptions and associated feedbacks) or a combination of such conditions is not known and requires further study.〈/p〉

Description: 〈b〉The weather behind the words. New methodologies for integrated hydrometeorological reconstruction through documentary sources〈/b〉〈br〉 Salvador Gil-Guirado, Juan José Gómez-Navarro, and Juan Pedro Montávez〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-1,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 The historical climatology has remarkable research potentialities. However, this discipline there are methodological limitations. This study presents a new methodology that allows us to perform climate reconstructions with monthly resolution. The variability of the climatic series obtained are coherent with previous studies. The new proposed method is objective and is not affected by social changes, which allows it to perform comparative studies in regions with different languages and cultures.

Description: 〈b〉The onset of neoglaciation in Iceland and the 4.2 ka event〈/b〉〈br〉 Áslaug Geirsdóttir, Gifford H. Miller, John T. Andrews, David J. Harning, Leif S. Anderson, Christopher Florian, Darren J. Larsen, and Thor Thordarson〈br〉 Clim. Past, 15, 25-40, https://doi.org/10.5194/cp-15-25-2019, 2019〈br〉 Compositing climate proxies in sediment from seven Iceland lakes documents abrupt summer cooling between 4.5 and 4.0 ka, statistically indistinguishable from 4.2 ka. Although the decline in summer insolation was an important factor, a combination of superposed changes in ocean circulation and explosive Icelandic volcanism were likely responsible for the abrupt perturbation recorded by our proxies. Lake and catchment proxies recovered to a colder equilibrium state following the perturbation.

Description: 〈b〉On the linearity of the temperature response in Holocene: the spatial and temporal dependence〈/b〉〈br〉 Lingfeng Wan, Zhengyu Liu, Jian Liu, Weiyi Sun, and Bin Liu〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2018-177,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 The linearity of the climate response is strong on the orbital, millennial and centennial scales throughout the Holocene, but decadal is poor. The regions of strong linear response on the millennial scale are mostly consistent with orbital scale. This finding can improve our understanding of the regional climate response to various climate forcings.

Description: 〈b〉Joint inversion of proxy system models to reconstruct paleoenvironmental time series from heterogeneous data〈/b〉〈br〉 Gabriel J. Bowen, Brenden Fisher-Femal, Gert-Jan Reichart, Appy Sluijs, and Caroline H. Lear〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2018-178,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 1 comment)〈br〉 Past climate conditions are reconstructed using indirect and incomplete geological, biological, and geochemical proxy data. We propose that paleoclimate reconstructions are best obtained from such data using statistical models that represent how multiple proxy observations and calibration datasets arise from environmental conditions that vary systematically in time and/or space. These methods can extract additional information from traditional proxies and provide robust estimates of uncertainty.

Description: 〈b〉The 4.2 ka event, ENSO, and coral reef development〈/b〉〈br〉 Lauren T. Toth and Richard B. Aronson〈br〉 Clim. Past, 15, 105-119, https://doi.org/10.5194/cp-15-105-2019, 2019〈br〉 We explore the hypothesis that a shift in global climate 4200 years ago (the 4.2 ka event) was related to the El Niño–Southern Oscillation (ENSO). We summarize records of coral reef development in the tropical eastern Pacific, where intensification of ENSO stalled reef growth for 2500 years starting around 4.2 ka. Because corals are highly sensitive to climatic changes, like ENSO, we suggest that records from coral reefs may provide important clues about the role of ENSO in the 4.2 ka event.

Description: 〈b〉The 405 kyr and 2.4 Myr eccentricity components in Cenozoic carbon isotope records〈/b〉〈br〉 Ilja J. Kocken, Margot J. Cramwinckel, Richard E. Zeebe, Jack J. Middelburg, and Appy Sluijs〈br〉 Clim. Past, 15, 91-104, https://doi.org/10.5194/cp-15-91-2019, 2019〈br〉 Marine organic carbon burial could link the 405 thousand year eccentricity cycle in the long-term carbon cycle to that observed in climate records. Here, we simulate the response of the carbon cycle to astronomical forcing. We find a strong 2.4 million year cycle in the model output, which is present as an amplitude modulator of the 405 and 100 thousand year eccentricity cycles in a newly assembled composite record.

Description: 〈b〉Lignin oxidation products as a vegetation proxy in stalagmite and drip water samples from the Herbstlabyrinth, Germany〈/b〉〈br〉 Inken Heidke, Denis Scholz, and Thorsten Hoffmann〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-5,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 This is the first quantitative study of lignin biomarkers in stalagmites and cave drip water. Lignin is only produced by higher plants, therefore its analysis can be used to reconstruct the vegetation of the past. We compared our lignin results with stable isotope and trace element records from the same samples and found correlations or similarities with P, Ba, U and Mg concentrations as well as 〈i〉δ〈/i〉〈sup〉13〈/sup〉C values. These results can help to better interpret other vegetation proxies.

Description: 〈b〉Pollen-based temperature and precipitation changes in the Ohrid Basin (western Balkans) between 160 and 70 ka〈/b〉〈br〉 Gaia Sinopoli, Odile Peyron, Alessia Masi, Jens Holtvoeth, Alexander Francke, Bernd Wagner, and Laura Sadori〈br〉 Clim. Past, 15, 53-71, https://doi.org/10.5194/cp-15-53-2019, 2019〈br〉 Climate changes occur today as they occurred in the past. This study deals with climate changes reconstructed at Lake Ohrid (Albania and FYROM) between 160 000 and 70 000 years ago. Climate reconstruction, based on a high-resolution pollen study, provides quantitative estimates of past temperature and precipitation. Our data show an alternation of cold/dry and warm/wet periods. The last interglacial appears to be characterized by temperatures higher than nowadays.

Description: 〈b〉The longest homogeneous series of grape harvest dates, Beaune 1354–2018, and its significance for the understanding of past and present climate〈/b〉〈br〉 Thomas Labbé, Christian Pfister, Stefan Brönnimann, Daniel Rousseau, Jörg Franke, and Benjamin Bois〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2018-179,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 2 comments)〈br〉 In this paper we present the longest grape harvest dates (GHD) record reconstructed to date, i.e. Beaune (France, Burgundy) 1354-2018. Drawn on unedited archive material the series is validated using the long Paris temperature series that goes back to 1658 and used to assess April-to-July temperatures from 1354 to 2018. The distribution of extreme early GHD is unevenly distributed over the 664-year-long period of the series and mirrors the rapid global warming from 1988 to 2018.

Description: 〈b〉Heinrich events show two-stage climate response in transient glacial simulations〈/b〉〈br〉 Florian Andreas Ziemen, Marie-Luise Kapsch, Marlene Klockmann, and Uwe Mikolajewicz〈br〉 Clim. Past, 15, 153-168, https://doi.org/10.5194/cp-15-153-2019, 2019〈br〉 Heinrich events are among the dominant modes of glacial climate variability. They are caused by massive ice discharges from the Laurentide Ice Sheet into the North Atlantic. In previous studies, the climate changes were either seen as resulting from freshwater released from the melt of the discharged icebergs or by ice sheet elevation changes. With a coupled ice sheet–climate model, we show that both effects are relevant with the freshwater effects preceding the ice sheet elevation effects.

Description: 〈b〉The drivers of late Quaternary climate variability in eastern South Africa〈/b〉〈br〉 Charlotte Miller, Jemma Finch, Trevor Hill, Francien Peterse, Marc Humphries, Matthias Zabel, and Enno Schefuß〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-4,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 1 comment)〈br〉 Here we reconstruct vegetation and precipitation, in eastern South Africa, over the last 32 thousand years, by measuring the stable carbon and hydrogen isotope composition of plant-waxes from Mfabeni peatbog (KwaZulu-Natal). Our results indicate that late Quaternary climate in eastern South Africa did not respond directly to orbital forcing nor to changes in sea surface temperatures. Our findings stress the influence of the southern hemisphere westerlies in driving climate change in the region.

Description: 〈b〉The Antarctic Ice Sheet response to glacial millennial-scale variability〈/b〉〈br〉 Javier Blasco, Ilaria Tabone, Jorge Alvarez-Solas, Alexander Robinson, and Marisa Montoya〈br〉 Clim. Past, 15, 121-133, https://doi.org/10.5194/cp-15-121-2019, 2019〈br〉 The LGP is a period punctuated by the presence of several abrupt climate events and sea-level variations of up to 20 m at millennial timescales. The origin of those fluctuations is attributed to NH paleo ice sheets, but a contribution from the AIS cannot be excluded. Here, for the first time, we investigate the response of the AIS to millennial climate variability using an ice sheet–shelf model. We shows that the AIS produces substantial sea-level rises and grounding line migrations.

Description: 〈b〉The 4.2 ka event in the central Mediterranean: new data from a Corchia speleothem (Apuan Alps, central Italy)〈/b〉〈br〉 Ilaria Isola, Giovanni Zanchetta, Russell N. Drysdale, Eleonora Regattieri, Monica Bini, Petra Bajo, John C. Hellstrom, Ilaria Baneschi, Piero Lionello, Jon Woodhead, and Alan Greig〈br〉 Clim. Past, 15, 135-151, https://doi.org/10.5194/cp-15-135-2019, 2019〈br〉 To understand the natural variability in the climate system, the hydrological aspect (dry and wet conditions) is particularly important for its impact on our societies. The reconstruction of past precipitation regimes can provide a useful tool for forecasting future climate changes. We use multi-proxy time series (oxygen and carbon isotopes, trace elements) from a speleothem to investigate circulation pattern variations and seasonality effects during the dry 4.2 ka event in central Italy.

Description: 〈b〉Is there evidence for a 4.2 ka BP event in the northern North Atlantic region?〈/b〉〈br〉 Raymond Bradley and Jostein Bakke〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2018-162,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 2 comments)〈br〉 We review paleoceanographic and paleoclimatic records from the northern North Atlantic to assess the nature of climatic conditions at 4.2 ka BP. There was a general decline in temperatures after ~5 ka BP, which led to the onset of Neoglaciation. Although a few records do show a distinct anomaly around 4.2 ka BP (associated with a glacial advance), this is not widespread and we interpret it as a local manifestation of the overall climatic deterioration that characterized the late Holocene.

Description: 〈b〉Drought during early European exploration and colonization of North America, 1500–1610CE: A comparison of evidence from the archives of societies and the archives of nature〈/b〉〈br〉 Sam White〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2019-2,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 1 comment)〈br〉 Researchers usually reconstruct droughts using natural proxies, such as tree-ring width, or else records left by people. This study examines parts of North America during the first European expeditions there (1510–1610CE). It compares drought reconstructions based on tree rings to records left by those expeditions and finds they mostly agree. Thus early colonial expeditions can provide reliable climate information and tree-ring reconstructions can capture variations relevant to human history.

Description: 〈b〉Indian winter and summer monsoon strength over the 4.2 ka BP event in foraminifer isotope records from the Indus River delta in the Arabian Sea〈/b〉〈br〉 Alena Giesche, Michael Staubwasser, Cameron A. Petrie, and David A. Hodell〈br〉 Clim. Past, 15, 73-90, https://doi.org/10.5194/cp-15-73-2019, 2019〈br〉 A foraminifer oxygen isotope record from the northeastern Arabian Sea was used to reconstruct winter and summer monsoon strength from 5.4 to 3.0 ka. We found a 200-year period of strengthened winter monsoon (4.5–4.3 ka) that coincides with the earliest phase of the Mature Harappan period of the Indus Civilization, followed by weakened winter and summer monsoons by 4.1 ka. Aridity spanning both rainfall seasons at 4.1 ka may help to explain some of the observed archaeological shifts.

Description: 〈b〉Extreme droughts and human responses to them: the Czech Lands in the pre-instrumental period〈/b〉〈br〉 Rudolf Brázdil, Petr Dobrovolný, Miroslav Trnka, Ladislava Řezníčková, Lukáš Dolák, and Oldřich Kotyza〈br〉 Clim. Past, 15, 1-24, https://doi.org/10.5194/cp-15-1-2019, 2019〈br〉 The paper analyses extreme droughts of the pre-instrumental period (1501–1803) over the territory of the recent Czech Republic. In total, 16 droughts were selected for spring, summer and autumn each and 14 droughts for summer half-year (Apr–Sep). They are characterized by very low values of drought indices, high temperatures, low precipitation and by the influence of high-pressure situations. Selected extreme droughts are described in more detail. Effect of droughts on grain prices are studied.

Description: 〈b〉The HadCM3 contribution to PlioMIP Phase 2 Part 1: Core and Tier 1 experiments〈/b〉〈br〉 Stephen J. Hunter, Alan M. Haywood, Aisling M. Dolan, and Julia C. Tindall〈br〉 Clim. Past Discuss., https//doi.org/10.5194/cp-2018-180,2019〈br〉 〈b〉Manuscript under review for CP〈/b〉 (discussion: open, 0 comments)〈br〉 In this paper we model climate of the mid-Piacenzian warm period (mPWP; 3 million years ago), a geological analogue for contemporary climate. Using the HadCM3 climate model we show how changes in CO〈sub〉2〈/sub〉 and geography contributed to mPWP climate. We find mPWP warmth focussed in the high-latitudes, geography-driven precipitation changes and wetter-gets-wetter paradigm under increasing CO〈sub〉2〈/sub〉, complex changes in sea surface temperature and intensified overturning in the North Atlantic (AMOC).

Description: Enhanced summer insolation during the early and mid-Holocene drove increased precipitation and widespread expansion of vegetation across the Sahara during the African Humid Period (AHP). While changes in atmospheric dynamics during this time have been a major focus of palaeoclimate modelling efforts, the transient nature of the shift back to the modern desert state at the end of this period is less well understood. Reconstructions reveal a spatially and temporally complex end of the AHP, with an earlier end in the north than in the south and in the east than in the west. Some records suggest a rather abrupt end, whereas others indicate a gradual decline in moisture availability. Here we investigate the end of the AHP based on a transient simulation of the last 7850 years with the comprehensive Earth System Model MPI-ESM1.2. The model largely reproduces the time-transgressive end of the AHP evident in proxy data, and indicates that it is due to the regionally varying dynamical controls on precipitation. The impact of the main rain-bringing systems, i.e. the summer monsoon and extratropical troughs, varies spatially, leading to heterogeneous seasonal rainfall cycles that impose regionally different responses to the Holocene insolation decrease. An increase in extratropical troughs that interact with the tropical mean flow and transport moisture to the Western Sahara during mid-Holocene delays the end of the AHP in that region. Along the coast, this interaction maintains humid conditions for a longer time than further inland. Drying in this area occurs when this interaction becomes too weak to sustain precipitation. In the lower latitudes of West Africa, where the rainfall is only influenced by the summer monsoon dynamics, the end of the AHP coincides with the retreat of the monsoonal rainbelt. The model results clearly demonstrate that non-monsoonal dynamics can also play an important role in forming the precipitation signal and should therefore not be neglected in analyses of North African rainfall trends.

Description: Limited research has focussed on historical droughts during the pre-instrumental weather-recording period in semi-arid to arid human-inhabited environments. Here we describe the unique nature of droughts over semi-arid central Namibia (southern Africa) between 1850 and 1920. More particularly, our intention is to establish temporal shifts of influence and impact that historical droughts had on society and the environment during this period. This is achived through scrutinizing documentary records sourced from a variety of archives and libraries. The primary source of information comes from misssonary diaries, letters and reports. These missionaries were based at a variety of stations across the central Namibian region and thus collectively provide insight to sub-regional (or site specific) differences in hydro-meteorological conditions, and drought impacts and responses. Earliest instrumental rainfall records (1891–1913) from several missionary stations or settlements are used to quantify hydro-meteorological conditions and compare with documentary sources. The work demonstrates strong-sub-regional contrasts in drought conditions during some given drought events and the dire implications of failed rain seasons, the consequences of which lasted many months to several years. The paper advocates that human experience and associated reporting of drought events depends strongly on social, environmental, spatial and societal developmental situations and perspectives. To this end, the reported experiences, impacts and responses to drought over this 70 year period portray both common and changeable attributes through time.

Description: Droughts pose a climatic hazard that can have a profound impacts on past societies. Using documentary sources, this paper studies the occurrence and impacts of spring-summer droughts in pre-industrial England from 1200 to 1700. The types of records, source availability and changes in record keeping over time are described, and an overview of droughts in those 500 years is provided. The focus lies on a structural survey over the drought impacts most relevant to human livelihood. This includes the agricultural and pastoral sectors of agrarian production, health, the fire risk to settlements and the drop in water levels or dwindling of water supplies. Whereas due the specific characteristics of wheat cultivation in medieval and early modern England, the grain production was comparatively resilient to drought, livestock farming was under threat when rainfall fell noticeably below average. The most important problem in warm and dry summers, however, was the risk to health. Partly steeply raised mortality levels were associated with these conditions during the study period, because malaria, gastrointestinal disease and plague showed an affinity to heat and drought. Adaptation strategies to reduce the stress posed by summer droughts are included in the study.

Description: The present article deals with the reconstruction of drought time series in Germany since 1500. The reconstructions are based on written records from the historical climate and environmental database tambora.org, early and official instrument data as well as precipitation and temperature indices. From the historical descriptions of the weather climatic processes and their effects and consequences for the environment and society, action paths and drought categories are derived. Furthermore, a historical precipitation index (HPI) is calculated and correlated with the SPI index. These are correlated and quantified in a rating scheme with modern rainfall indices and recent drought categories. Finally, a Historical Drought Index (HDI) and a Historical Wet Index (HWI) derived from the hygric indices are presented. On this basis, the long-term development of dryness and drought and significant accumulations and extremes in Germany since 1500 are discussed.

Description: The climate of the Southern Hemisphere (SH) is strongly influenced by variations in the El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Due to the temporally very limited instrumental records in most parts of the SH, very little is known about the relationship between these two key modes of variability and its stability over time. Here, we use proxy-based reconstructions and climate model simulations to quantify changes in tropical-extratropical SH teleconnections as represented by the correlation between the ENSO and SAM indices. Reconstructions indicate mostly negative correlations back to around 1400 CE confirming the pattern seen in the instrumental record over the last few decades. An ensemble of last millennium simulations of the model CESM1 confirms this pattern with very stable ensemble mean correlations around −0.3. Individual forced simulations, the pre-industrial control run and the proxy-based reconstructions indicate intermittent periods of positive correlations and particularly strong negative correlations. The fluctuations of the ENSO-SAM correlations are not significantly related to solar nor volcanic forcing in both proxy and model data, indicating that they are driven by internal variability in the climate system. Pseudoproxy experiments indicate that the currently available proxy records are able to reproduce the tropical-extratropical teleconnection patterns back to around 1600 CE. We analyse the spatial temperature and sea level pressure patterns during periods of positive and particularly strong negative teleconnections in the CESM model. Results indicate no consistent pattern during periods where the ENSO-SAM teleconnection changes its sign. However, periods of very strong negative SH teleconnections are associated with negative temperature anomalies across large fractions of the extra-tropical Pacific and a strengthening of the Aleutian Low.

Description: Global and regional climate changed dramatically with the expansion of the Antarctic Ice sheet at the Eocene-Oligocene Transition (EOT). These large-scale changes are generally linked to declining atmospheric pCO2 levels and/or changes in Southern Ocean gateways such as the Drake Passage around this time. To better understand the Southern Hemisphere regional climatic changes and the impact of glaciation on the Earth’s oceans and atmosphere at the EOT, we compiled a database of sea and land surface temperature reconstructions from a range of proxy records and compared this with a series of fully-coupled climate model simulations. Regional patterns in the proxy records of temperature show that cooling across the EOT was less at high latitudes and greater at mid-latitudes. Climate model simulations have some issues in capturing the zonal mean latitudinal temperature profiles shown by the proxy data, but certain simulations do show moderate-good performance at recreating the temperature patterns shown in the data. When taking into account the absolute temperature before and after the EOT, as well as the change in temperature across it, simulations with a closed Drake Passage before and after the EOT or with an opening of the Drake Passage across the EOT perform poorly, whereas simulations with a drop in atmospheric pCO2 in combination with ice growth generally perform better. This provides further support to previous research that changes in atmospheric pCO2 are more likely to have been the driver of the EOT climatic changes, as opposed to opening of the Drake Passage.

Description: The occurrence of two severe droughts in Northeastern China since CE 2000 has raised attention in the risk presented by droughts. This paper presents a historic drought series for Shenyang in the Liaoning province, NE China since CE 1200 to present, with a reconstructed long precipitation series (1906–2015), augmented with historical documentary accounts. Analysis of the instrumental series using a standardised precipitation index (SPI) and extending it using historical records has produced a combined series spanning over eight centuries. The combined long series was analysed for patterns in drought frequency, severity and typology. Three droughts comparable to those since CE 2000 occur in the instrumental series during early twentieth century (e.g. 1907, 1916–18 and 1920–21), and coeval archival sources reveal the human impacts of these severe droughts. The archival sources demonstrate how reduced vulnerability resulting from societal and cultural changes in the early twentieth century helped prevent the loss of life experienced during comparable severe droughts at the end of the nineteenth century (1887 and 1891). Incorporating a longer temporal perspective to drought analysis shows that onset is often earlier than is documented explicitly within the archives, and so combined SPI series for a region could provide an early warning of drought development expressed as a water deficit in the previous year. Analysis of archival data provides a rich historical description of impacts and societal responses to severe drought. The archives provide a rich historical description of drought impacts and responses at the personal and community level, whilst also detailing the different roles played by communities, state and international organisations in responding to events.

Description: Droughts derive from a deficit of precipitation and belong to the most dangerous natural hazards for human societies. Documentary data of the pre-modern and early modern times contain direct and indirect information on precipitation that allow the production of reconstructions with the methods of historical climatology. For this study, two drought indices have been created on the basis of documentary data produced in Bern, Switzerland (DIB) and in Rouen, France (DIR) for the period from 1315 to 1715. These two indices have been compared to a third supra-regional drought index for Switzerland (SDI), Germany, France, the Netherlands, and Belgium synthesised from precipitation reconstruction based on historical climatology. The results of the study show that the documentary data from Bern mainly contain summer droughts, whereas the data from Rouen rather allow the reconstruction of spring droughts. The comparison of the three indices shows that the DIB and the DIR most probably do not contain all actual drought events, but they also detect droughts that do not appear in the SDI. This fact suggests that more documentary data from single places, such as historical city archives, should be examined in the future and added to larger reconstructions in order to obtain more complete drought reconstructions.

Description: We investigate the changes in terrestrial natural methane emissions between the Last Glacial Maximum (LGM) and preindustrial (PI) by performing time-slice experiments with a methane-enabled version of MPI-ESM, the Max Planck Institute for Meteorology Earth System Model. We consider all natural sources of methane except for emissions from wild animals and geological sources, i.e. emissions from wetlands, fires, and termites. Changes are dominated by changes in tropical wetland emissions, with mid-to-high latitude wetlands playing a secondary role, and all other natural sources being of minor importance. The emissions are determined by the interplay of vegetation productivity, a function of CO2 and temperature, source area size, affected by sea level and ice sheet extent, and the state of the West African Monsoon, with increased emissions from north Africa during strong monsoon phases. We show that it is possible to explain the difference in atmospheric methane between LGM and PI purely by changes in emissions. As emissions more than double between LGM and PI, changes in the atmospheric lifetime of CH4, as proposed in other studies, are not required.

Description: Volcanic eruptions are important drivers of climate variability on both seasonal and multi-decadal time scales as a result of atmosphere-ocean coupling. While the direct response after equatorial eruptions emerges as the positive phase of the North Atlantic Oscillation in the first two years after an eruption, less is known about high latitude northern hemisphere eruptions. In this study we assess the difference between equatorial and high latitude volcanic eruptions through the reconstructed atmospheric circulation and stable water isotope records of Greenland ice cores for the last millennia (1241–1979 CE), where the coupling mechanism behind the long-term response is addressed. The atmospheric circulation is studied through the four main modes of climate variability in the North Atlantic, the Atlanti Ridge (AtR), Scandinavian Blocking (ScB) and the positive and negative phase of the North Atlantic Oscillation (NAO+/NAO−). We report a difference in the atmospheric circulation response after equatorial eruptions compared to the response after high latitude eruptions, where NAO+ and AtR seem to be more associated with equatorial eruptions while NAO- and ScB seems to follow high latitude eruptions. This response is present during the first five years and then again in years 8–12 after both equatorial and high latitude eruptions. Such a prolonged response is evidence of an ocean-atmosphere coupling that is initiated through different mechanisms, where we suspect sea ice to play a key role.

Description: Imposing freshwater flux (FWF) variations in the North Atlantic is an effective method to cause reorganizations of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. Through this approach, models have been able to reproduce the abrupt climate changes of the last glacial period. Such exercises have been useful for gaining insight into a wealth of processes regarding the widespread climatic consequences of AMOC variations. However, an issue that has passed unnoticed is the fact that the timing of the FWF applied in these studies is inconsistent with reconstructions. Here we focus on the deglaciation to show that imposing a FWF that is derived from the sea-level record results in a simulated AMOC evolution in a poor fit with the data, revealing an inconsistency between the generally accepted FWF mechanism and the resulting climatic impacts. Based on these negative results, we propose that the trigger of deglacial abrupt climate changes is not yet fully identified and that mechanisms other than FWF forcing should be explored more than ever.

Description: The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland Currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late Deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7,000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening and sea ice cover on the Labrador Shelf during the last 6,000 years. Our records reveal that the LC underwent three major oceanographic phases from the Mid- to Late Holocene. From 6.2 to 5.6 ka BP, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka BP, possibly associated with the late Holocene Thermal Maximum. Although surface waters on the Labrador Shelf cooled gradually after 3 ka BP in response to the Neoglaciation, Labrador Shelf subsurface/bottom waters show a shift to warmer temperatures after 2.1 ka BP. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last two millennia compared to the mid-Holocene, it remains difficult to assess whether hydrographic conditions in the LC have had a significant impact on Labrador Sea deep-water formation.

Description: Over the past few decades, paleoenvironmental studies in the Indian Summer Monsoon (ISM) region have mainly focused on precipitation change, with few published terrestrial temperature records from the region. We analyzed the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) in the sediments of Lake Chenghai in southwest China across the Pleistocene–Holocene transition, to extract both regional hydrological and temperature signals for this important transition period. Lake-level was reconstructed from the relative abundance of crenarchaeol in isoGDGTs (%cren) and the crenarchaeol'/crenarchaeol ratio. The %cren-inferred lake-level identified a single lowstand (15.4–14.4 cal ka BP), while the crenarchaeol'/crenarchaeol ratio suggests relatively lower lake-level between 15.4–14.4 cal ka BP and 12.5–11.7 cal ka BP, corresponding to periods of weakened ISM during the Heinrich 1 (H1) and Younger Dryas (YD) cold event. A filtered TetraEther indeX consisting of 86 carbon atoms (TEX86 index) revealed that lake surface temperature reached present-day values during the YD cold event, and suggests a substantial warming of ~ 4 °C from the early Holocene to the mid-Holocene. Our paleotemperature record is generally consistent with other records in southwest China, suggesting that the distribution of isoGDGTs in Lake Chenghai sediments has potential for quantitative paleotemperature reconstruction.

Description: Previous studies reconstructing summer temperature from an ice core relied on statistical relationship between melt feature and instrumental temperature observed at a nearby station. This study demonstrates a novel method to reconstruct summer temperature from ice layer thickness in an ice core using an energy balance model, in which heat conduction and refreezing of meltwater in firn are taken into account. Using seasonal patterns of the ERA-Interim reanalysis dataset for an ice core site, we calculated amounts of refreezing water within firn under various settings of summer mean temperature (SMT) and annual precipitation, and prepared lookup tables containing these three variables. We then estimated SMT from the refreezing amount and annual accumulation, both available in an ice core. We applied this method to four ice cores drilled in the sites of different climates; two sites on the Greenland Ice Sheet, one in Alaska, and one in Russian Altai Mountains. Reconstructed SMTs show comparable variations to the temperatures observed at nearby stations. Relationships between SMT and ice layer thickness differ site by site, indicating that a single approximation cannot be applicable to estimate SMT. Sensitivity analyses suggest that annual temperature range, amount of annual precipitation and firn albedo setting significantly affect the relationship between SMT and ice layer thickness. This new method provides alternative and independent estimation of SMT from ice cores affected by melting.

Description: During the late Palaeocene to the middle Eocene (57.5 to 46.5 Ma) a total of 39 hyperthermals – periods of rapid global warming recorded by prominent negative carbon isotope excursions (NCIEs) as well as peaks in iron content – have been recognized in marine cores. Understanding how the Earth system responded to rapid warming during these hyperthermals is fundamental because they represent potential analogues, in the geological record, to the ongoing anthropogenic modification of global climate. However, while hyperthermals have been well documented in the marine sedimentary record, only few have been recognized and described in continental deposits, thereby limiting our ability to understand the effect and record of global warming on terrestrial surficial systems. Hyperthermals in the continental record could be a powerful correlation tool to help connect marine and continental records, addressing issues of environmental signal propagation from land to sea. In this study, we generate new stable carbon isotope data (δ13C values) across the well-exposed and time-constrained fluvial sedimentary succession of the early Eocene Castissent Formation in the South-Central Pyrenees (Spain). The δ13C values of pedogenic carbonate reveal – similarly to the global records – stepped NCIEs, culminating in a minimum δ13C value that we correlate with the hyperthermal event U at ca. 50 Ma. This general trend towards more negative values is most probably linked to higher primary productivity leading to an overall higher respiration of soil organic matter during these climatic events. The relative enrichment in immobile elements (Zr, Ti, Al) and higher estimates of mean annual precipitation together with the occurrence of small iron-oxides/hydroxides nodules during the NCIEs suggest intensification of chemical weathering and/or longer exposure of soils in a highly seasonal climate. The results show that even relatively small-scale hyperthermals compared with their prominent counterparts, such as PETM, ETM2 and 3, have left a recognizable trace in the stratigraphic record, providing insights into the dynamics of the carbon cycle in continental environments during these events.

Description: In this study, the dead carbon fraction (DCF) variations in stalagmite M1-5 from Socotra Island in the western Arabian Sea were investigated through a new set of high-precision U-series and radiocarbon (14C) dates. The data reveal an extreme case of very high and also climate dependent DCF values. For M1-5 an average DCF of 56.2 ± 3.4 % is observed between 27 and 18 kyr BP. Such high DCF values indicate a high influence of aged soil organic matter (SOM) and nearly completely closed system carbonate dissolution conditions. Towards the end of the last glacial period decreasing Mg/Ca ratios suggest an increase in precipitation which caused a marked change in the soil carbon cycling as indicated by sharply decreasing DCF. This is in contrast to the relation of soil infiltration and reservoir age observed in stalagmites from temperate zones. For Socotra Island, which is influenced by the East African–Indian monsoon, we propose that more humid conditions and enhanced net-infiltration after the LGM led to denser vegetation and thus lowered the DCF by increased 14CO2 input into the soil zone. The onset of the Younger Dryas (YD) is represented in the record by the end of DCF decrease with a sudden change to much higher and extremely variable reservoir ages. Our study highlights the dramatic variability of soil carbon cycling processes and vegetation feedback on Socotra Island manifested in stalagmite reservoir ages on both long-term trends and sub-centennial timescales, thus providing evidence for climate influence on stalagmite radiocarbon. This is of particular importance for studies focussing on 14C calibration and atmospheric reconstruction through stalagmites which relies on largely climate independent soil carbon cycling above the cave.

Description: A detailed and accurate reconstruction of the past climate is essential in understanding the interactions between ecosystems and their environment through time. We know that climatic drivers have shaped the distribution and evolution of species, including our own, and their habitats. Yet, spatially-detailed climate reconstructions that continuously cover the Quaternary do not exist. This is mainly because no paleoclimate model can reconstruct regional-scale dynamics over geological time scales. Here we develop a statistical emulator, the Global Climate Model Emulator (GCMET), which reconstructs the climate of the last 800 000 years with unprecedented spatial detail. GCMET captures the temporal dynamics of glacial-interglacial climates as an Earth System Model of Intermediate Complexity would whilst resolving the local dynamics with the accuracy of a Global Climate Model. It provides a new, unique resource to explore the climate of the Quaternary, which we use to investigate the long-term stability of major habitat types. We identify a number of stable pockets of habitat that have remained unchanged over the last 800 thousand years, acting as potential long-term evolutionary refugia. Thus, the highly detailed, comprehensive overview of climatic changes through time delivered by GCMET provides the needed resolution to quantify the role of long term habitat change and fragmentation in an ecological and anthropological context.

Description: During the latter stages of the Holocene, and prior to anthropogenic global warming, the Earth underwent a period of cooling called the neoglacial. The neoglacial was associated with declining summer insolation and changes to Earth surface albedo. Although impacts varied globally, in China the neoglacial was generally associated with cooler, more arid climate, which led to renewed permafrost formation, and shifts in vegetation composition. Few studies in central China, however, have explored the impact of neoglacial cooling on freshwater diversity, especially in remote alpine regions. Here we take a palaeolimnological approach to characterise multidecadal variability in diatom community composition, beta-diversity, and flux-inferred productivity over the past 3,500 years in the Qinling Mountains, biodiversity hotspot. We investigate the impact of long-term cooling on primary producers in an alpine lake, which are fundamental to overall aquatic ecosystem function. We show that trends in beta-diversity and shifts in ecological guilds likely reflect changing lake-catchment resource availability, linked to both long-term attenuation of the Asian summer monsoon, and abrupt cool events, linked to a strengthened Siberian High. Important diatom community and productivity responses to the Medieval Climatic Optimum and the Little Ice Age are all apparent in our record, although impact from previous centennial-scale, cool-events are less evident.

Description: The oxygen and hydrogen isotopic compositions of water in fluid inclusions in speleothems are important hydroclimate proxies because they provide information on the isotopic compositions of rainwater in the past. Moreover, because isotopic differences between fluid inclusion water and the host calcite provide information on the past isotopic fractionation factor, they are also useful for quantitative estimation of past temperature changes. The oxygen isotope ratio of inclusion water (δ18Ofi), however, may be affected by isotopic exchange between the water and the host carbonate. Thus, it is necessary to estimate the bias caused by this post-depositional effect for precise reconstruction of palaeo-temperatures. Here, we evaluate the isotopic exchange reaction between inclusion water and host calcite based on a laboratory experiment involving a natural stalagmite. Multiple stalagmite samples cut from the same depth interval were heated at 105 °C in the laboratory for 0–80 hours. Then, the isotopic compositions of the inclusion water were measured. In the 105 °C heating experiments, the δ18Ofi values increased from the initial value by 0.7 ‰ and then remained stable after ca. 20 hours. The hydrogen isotope ratio of water showed no trend in response to the heating experiments, suggesting that the hydrogen isotopic composition of fluid inclusion water effectively reflects the composition of past dripwater. We then evaluated the process behind the observed isotopic variations using a partial equilibration model. The experimental results are best explained when we assumed that a thin CaCO3 layer surrounding the inclusion reacted with the water. The amount of CaCO3 that reacted with the water is equivalent to 2 % of the water inclusions in molar terms. These results suggest that the magnitude of the isotopic exchange effect has a minor influence on palaeo-temperature estimates for the Quaternary climate reconstructions.

Description: We measured the methane mixing ratios of enclosed air in five ice core sections drilled on the East Antarctic plateau. Our work aims to study two effects that affect the recorded gas concentrations in ice cores: layered gas trapping artifacts and firn smoothing. Layered gas trapping artifacts are due to the heterogeneous nature of polar firn, where some strata might close early and trap abnormally old gases that appear as spurious values during measurements. The smoothing is due to the combined effects of diffusive mixing in the firn and the progressive closure of bubbles at the bottom of the firn. Consequently, the gases trapped in a given ice layer span a distribution of ages. Concentration measurements thus only measure the average value in the ice layer, which removes the fast variability from the record. We focus on the study of East Antarctic plateau ice cores, as these low accumulation ice cores are particularly affected by both layering and smoothing. Our results suggest that the presence of layering artifacts in deep ice cores is linked with the chemical content of the ice. We use high-resolution methane data to parametrize a simple model reproducing the layered gas trapping artifacts for different accumulation conditions typical of the East Antarctic plateau. We also use the high-resolution methane measurements to estimate the gas age distributions of the enclosed air in the five newly measured ice core sections. It appears that for accumulations below 2 cm ie yr−1(ice equivalent) the gas records experience nearly the same degree of smoothing. We therefore propose to use a single gas age distribution to represent the firn smoothing observed in the glacial ice cores of the East Antarctic plateau. Finally, we used the layered gas trapping model and the estimation of glacial firn smoothing to estimate their potential impacts on a million-and-a-half years old ice core from the East Antarctic plateau. Our results indicate that layering artifacts are no longer individually resolved in the case of very thinned ice near the bedrock. They nonetheless contribute to slight biases of the measured signal (less than 10 ppbv and 0.5 ppmv in the case of methane and carbon dioxide). However, these biases are small compared to the dampening experienced by the record due to firn smoothing.

Description: Differences between paleoclimatic reconstructions are caused by two main factors, the method and the input data. While many studies compare methods, we will focus in this study on the consequences of the input data choice in a state-of-the-art paleo data assimilation approach. We evaluate reconstruction quality based on three collections of tree-ring records: (1) 54 of the best temperature sensitive tree-ring chronologies chosen by experts; (2) 415 temperature sensitive tree-ring records chosen less strictly by regional working groups and statistical screening; (3) 2287 tree-ring series that are not screened for climate sensitivity. The three data sets cover the range from small sample size, small spatial coverage and strict screening for temperature sensitivity to large sample size and spatial coverage but no screening. Additionally, we explore a combination of these data sets plus screening methods to improve the reconstruction quality. Neither a large, unscreened collection of proxy data nor the small expert selection leads to the best possible climate field reconstruction. A large collection of unscreened data leads to a poor reconstruction skill. The few best temperature proxies allow for a skillful high latitude temperature reconstruction but fail to provide improved reconstructions for other regions and other variables. We achieve the best reconstruction skill across all variables and regions by combing all available input data but rejecting records with small, insignificant information and removing duplicate records. In case of assimilating tree ring proxies, it appeared to be important to use a tree-ring proxy system model that includes both major growth limitations, temperature and moisture.

Description: A climate event at 7.5–7.0 cal ka BP (thousand calibrated years before present) has been recognized. This event is important for foreseeing the possible response of the climate system to global warming and for interpreting considerable societal change, but it has heretofore lacked a systematic review. Here, we summarize previously published paleoclimate records spanning this event from 47 sites around the world. The proxy evidence from a variety of paleo-archives, including lake sediments, speleothems, marine sediments, and ice cores, provides a clear picture of this climate change. The synthesis results show a weaker Asian summer monsoon, in contrast to a stronger South American summer monsoon during the event. The event also involves dramatic cooling and wetter conditions in north-central Europe and in western North America, widespread aridity across Africa, contrasting patterns of precipitation variability throughout the Mediterranean, and notable cooling over the polar region, suggesting that it is a worldwide climate event. Comparison of paleoclimate records with climate-forcing time series gives likely climate controls for the event. The close correspondence in time of solar irradiance minima, strong volcanic eruptions, the meltwater flux into the North Atlantic, an orbitally induced decrease in solar insolation, and climate changes indicated by proxy data suggest possible linkages. More quantitative reconstructions and higher resolution climate records are needed to fully capture the magnitude, timing, duration, and nature of this event, which will be of considerable relevance to modeling.

Description: Measurements of carbon isotope variations in climate archives and isotope-enabled climate modelling foster the understanding of the carbon cycle. Perturbations in atmospheric CO2, and in its isotopic ratios (δ13C, ∆14C) are removed on different time scales and by partly different processes. We investigate these differences on timescales of up to 100,000 years in idealized pulse release experiments with the Bern3D-LPX Earth system model of intermediate complexity and by analytical solutions from a box model. Isotopic perturbations are initially removed much faster from the atmosphere than perturbations in CO2 as explained by aquatic carbonate chemistry. On longer time scales, the CO2 perturbation is removed by carbonate compensation and silicate rock weathering. In contrast, the δ13C perturbation is removed by the relentless flux of organic and calcium carbonate particles buried in sediments. The associated removal rate is significantly modified by spatial δ13C gradients within the ocean influencing the isotopic perturbation of the burial flux. Space-time variations in ocean δ13C perturbations are captured by three Principal Components and Empirical Orthogonal Functions. Analytical impulse response functions for atmospheric CO2 and δ13CO2 are provided. Our results show that changes in terrestrial carbon storage are unlikely the sole cause for the abrupt, centennial CO2 and δ13C variations recorded in ice during Heinrich Stadials HS1 and HS4 of the last glacial period. Ocean processes likely played a significant role. The δ113 offset between the penultimate and last glacial maximum reconstructed for the ocean and atmosphere is most likely caused by imbalances between weathering, volcanism and burial fluxes.

Description: Landscapes in high northern latitudes are assumed to be highly sensitive to future global change, but the rates and long-term trajectories of changes are rather uncertain. In the boreal zone, fires are an important factor in climate–vegetation–interactions and biogeochemical cycles. Fire regimes are characterized by small, frequent, low-intensity fires within summergreen boreal forests dominated by larch, whereas evergreen boreal forests dominated by spruce and pine burn large areas less frequently, but at higher intensities. Here, we explore the potential of the monosaccharide anhydrides (MA) levoglucosan, mannosan, and galactosan to serve as proxies of low-intensity biomass burning in glacial-to-interglacial lake sediments from the high northern latitudes. We use sediments from Lake El’gygytgyn (cores PG 1351 and ICDP 5011-1), located in the far north-east of Russia, and study glacial and interglacial samples of the last 430 kyrs (marine isotope stages 5e, 6, 7e, 8, 11c, 12) that had different climate and biome configurations. Combined with pollen and non-pollen palynomorph records from the same samples, we assess past relationships between fire, climate, and vegetation on orbital to centennial time scales. We find that MAs were well-preserved in up to 430 kyrs old sediments with higher influxes from low-intensity biomass burning in interglacials compared to glacials. MA influxes significantly increase when summergreen boreal forest spreads closer to the lake, whereas they decrease when tundra-steppe environments and, especially, Sphagnum peatlands spread. This suggests that low-temperature fires are a typical property of Siberian larch forest on long timescales. The results also suggest that low-intensity fires would be reduced by vegetation shifts towards very dry environments due to reduced biomass availability, as well as by shifts towards peatlands, which limits fuel dryness. In addition, we observed very low MA ratios, which we interpret as high contributions of galactosan and mannosan from other than currently monitored biomass sources, such as the moss-lichen mats in the understorey of the summergreen boreal forest. Overall, sedimentary MAs can provide a powerful proxy for fire regime reconstructions and extend our knowledge on long-term fire–climate–vegetation feedbacks in the high northern latitudes.

Description: Alpine ecosystems of the southern Iberian Peninsula are among the most vulnerable and the first to respond to modern climate change in southwestern Europe. While major environmental shifts have occurred over the last ~ 1500 years in these alpine environments, only changes in the recent centuries have led to extreme responses, but factors imposing the strongest stress have been unclear until now. To understand these environmental responses, here, for the first time, we calibrated algal lipids (long-chain diols) to instrumental data extending alpine temperatures backward 1500 years. These novel results highlight the enhanced effect of greenhouse gases on alpine temperatures during the last ~ 200 years and the long-term modulating role of solar forcing. This study also shows that warming rates during the 20th century (~ 0.18 ºC/decade) increased ~ 2.5 times with respect to the last stage of the Little Ice Age (~ 0.07 ºC/decade), even exceeding temperature trends of the high-altitude Alps during the 20th century. As a consequence, temperature exceeded the pre-industrial threshold in the 1950s, being one of the major forcings of the enhanced recent change in the alpine ecosystems from southern Iberia. Nevertheless, other factors reducing the snow and ice albedo (i.e. atmospheric deposition) may have influenced local glacier loss, since steady climate conditions predominated from middle 19th century to the first decades of the 20th century.

Description: The Pacific-North American (PNA) teleconnection is one of the most important climate modes in the present climate condition, and it enables climate variations in the tropical Pacific to exert significant impacts on North America. Here, we show climate simulations that the PNA teleconnection was largely distorted or broken at the Last Glacial Maximum (LGM). The distorted PNA is caused by a split of the westerly jet stream, which is ultimately forced by the thick and large Laurentide ice sheet at the LGM. Changes in the jet stream greatly alter the extratropical wave guide, distorting wave propagation from the North Pacific to North America. The distorted PNA suggests that climate variability in the tropical Pacific, notably, El Niño and Southern Oscillation (ENSO), would have little direct impact on North American climate at the LGM.

Description: As a continuation of the Pliocene Model Intercomparison Project (PlioMIP), PlioMIP Phase 2 (PlioMIP2) coordinates a wide selection of different climate model experiments aimed at further improving our understanding of the climate and environments during the late Pliocene with updated boundary conditions. Here we report on PlioMIP2 simulations carried out by the two versions of the Norwegian Earth System Model (NorESM), NorESM-L and NorESM1-F, with updated boundary conditions derived from the Pliocene Research, Interpretation and Synoptic Mapping version 4 (PRISM4). The two NorESM versions both produce warmer and wetter Pliocene climate. Relative to the pre-industrial period, the simulated Pliocene global mean surface air temperature is 2.1 ℃ higher with NorESM-L and 1.7 ℃ higher with NorESM1-F, respectively, and the corresponding global mean sea surface temperature enhances by 1.5 ℃ and 1.2 ℃. The simulated precipitation for the Pliocene increases by 0.14 mm day−1 globally in both model versions, with large responses in the tropics and especially in monsoon regions. In the simulated warmer and wetter Pliocene world, Atlantic meridional overturning circulation (AMOC) become deeper and stronger, with the maximum AMOC levels increasing by ~ 9 % (with NorESM-L) and ~ 15 % (with NorESM1-F), while the meridional overturning circulation slightly strengthens in the Pacific and Indian Oceans. Although the two models produce similar Pliocene climates, they also generate some differences, in particular for the Southern Ocean, which should be investigated through the PlioMIP2 in the future. As compared to PlioMIP1, the simulated Pliocene warming with NorESM-L is weaker in PlioMIP2, but otherwise show very similar responses.

Description: Currently, the rate of global warming has led to persistent drought patterns. It is considered to be the preliminary reason affecting socio-economic development under the background of dynamic forecasting of water supply and forest ecosystems in West Asia. However, long-term climate records in the semi-arid Chitral mountains of northern Pakistan are seriously lacking. Therefore, we developed a new tree-ring width chronology of Cedrus deodara spanning the period of 1537–2017. We reconstructed the March-August Palmer Drought Sensitivity Index (PDSI) for the past 424 years back to A.D. 1593. Our reconstruction was featured with nine dry and eight wet periods 1593–1598, 1602–1608, 1631–1645, 1647–1660, 1756–1765, 1785–1800, 1870–1878, 1917–1923, 1981–1995, and 1663–1675, 1687–1708, 1771–1773, 1806–1814, 1844–1852, 1932–1935, 1965–1969 and 1996–2003, respectively. This reconstruction is consistent with other dendroclimatic reconstructions in west Asia, confirming its reliability. The analysis of the Multi-Taper Method and wavelet analysis revealed drought variability at periodicities of 2.1–2.4, 3.3, 6, 16.8, and 34–38 years. The drought patterns could be linked to the broad-scale atmospheric-oceanic variability such as El Niño-Southern Oscillation (ENSO), Atlantic Multi-decadal Oscillation (AMO) and solar activity. In terms of current climate conditions, our findings have important implications for developing drought-resistant policies in communities on the fringes of Hindu Kush mountain Ranges in northern Pakistan.

Description: Previous studies show that the evolution of global mean temperature forced by the total forcing is almost the same as the sum of those forced by individual forcing in the last 21 000 years as simulated in three independent climate models, CCSM3, FAMOUS and LOVECLIM. But how does the linearity of the climate response depend on the spatial and temporal scales quantitatively and in what regions the linear response tends to dominate remain unknown. Here, based on the TraCE-21ka climate simulation outputs, the spatial and temporal dependence of the linear response of temperature evolution in the Holocene is studied using correlation coefficient and a linear error index. The results show that the linear response of global mean temperature is strong on the orbital, millennial and centennial scales throughout the Holocene, but is poor on the decadal scale. The linear response differs significantly between the Northern Hemisphere (NH) and Southern Hemisphere (SH). In the NH, the linear response is strong on millennial scale, while in the SH the linear response is strong on orbital scale, such that the linear response of the global mean orbital variability is dominated by the SH response, but that of the global millennial variability seems dominated by the NH response. Furthermore, at the regional scales, the linear responses differ substantially between the orbital, millennial, centennial and decadal timescales. On the orbital scale, the linear response is dominant for most regions, even at the small area of about a mid-size country like Germany. On the millennial scale, the linear response is still strong in the NH over many regions, albeit weaker than on the orbital scale. However, the millennial variability shows relatively poor linear response over most regions in the SH. On the centennial and decadal timescales, the linear response is no longer significant in almost all the regions. The regions of strong linear response on the millennial scale are mostly consistent with those on the orbital scale, notably western Eurasian, North Africa, subtropical North Pacific, tropical Atlantic and Indian Ocean, because of a large signal-to-noise ratios over these regions. This finding can improve our understanding of the regional climate response to various climate forcings in the Holocene.

Description: Atmospheric variability as a function of scale has been divided in various dynamical "regimes" with alternating increasing and decreasing fluctuations: weather, macroweather, climate, macroclimate, megaclimate. Although a vast amount of data is available at small scales, the larger picture is not well constrained due to the scarcity and low resolution of long paleoclimatic time-series. Using statistical techniques originally developed for the study of turbulence, we analyse the fluctuations of a centimetric resolution dust flux time-series from the EPICA Dome C ice-core in Antarctica that spans the past 800 000 years. The temporal resolution is 5 years over the last 400 kyrs, and 25 years over the last 800 kyrs, enabling the detailed statistical analysis and comparison of eight glaciation cycles, and the subdivision of each cycle into eight consecutive phases. The unique span and resolution of the dataset allows us to analyze the macroweather and climate scales in detail, i.e. fluctuations with periodicities from 1 year to 100 000 years. We find that the interglacial and glacial maximum phases of each cycle showed particularly large macroweather to climate transition scale τc (around 2 kyrs), whereas mid-glacial phases feature centennial transition scales (average of 300 yr). This suggests that interglacials and glacial maxima are exceptionally stable when compared with the rest of a glacial cycle. The Holocene (with τc ≈ 7.9 kyrs) had a particularly large τc but it was not an outlier when compared with the phase 1 and 2 of other cycles. For each phase, we quantified the drift, intermittency, amplitude, and extremeness of the variability. Phases close to the interglacials (1, 2, 8) show low drift, moderate intermittency, and strong extremes, while the "glacial" middle phases 3–7 display strong drift, weak intermittency, and weaker extremes. Our results suggest that despite the large climatic changes occurring during glacial-interglacial transitions, glacial maxima, interglacials, and glacial inceptions were characterized by relatively stable atmospheric conditions, but punctuated by more frequent and severe droughts, than during the more unstable mid-glacial conditions. The low amplitude during phases 6–8 also suggests that the Patagonian ice sheet was not yet fully developed before 30 kyr after glacial inception.

Description: We present the UK's input into the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) using the HadCM3 climate model. We outline the process of setting up HadCM3 with the enhanced PRISM4 boundary conditions and discuss in detail the assumptions and choices made. We then present the HadCM3 spin-up process from an initial arbitrary atmosphere, zero-momentum ocean state through to a well-equilibriated climatic state. We present data from the spin-up and final climatological mean state of the pre-industrial and Pliocene experiments. We focus on large-scale climatic and oceanic features. Comparing the control Pliocene experiment to pre-industrial the change in palaeogeography and CO2 combined account for a warming in globally integrated air temperature (sea surface temperature) of 1.4 °C (0.8 °C) and 1.5 °C (1.0 °C). For the pre-industrial and Pliocene we see climate sensitivities (for 2 × CO2) of 3.5 °C and 2.9 °C. We derive an approximation of Earth System Sensitivity of ~ 5.5 °C leading to an ESS / CS ratio ~ 1.90. Precipitation change is more complex, with geographic and land surface changes primarily modifying the geographical extent, and increasing CO2 leading to a general wet-get-wetter response. We see a reduction in summer and winter sea ice extent driven by both geographical – land surface changes and CO2 increase. In our model, the Atlantic Meridional overturning is relatively insensitive to CO2 but is strengthened in the Pliocene (from 15.7 to 19.6 Sv) due to the change in palaeogeography. Understanding the change in Antarctic Circumglobal Current within the Pliocene is problematic given an overly intense modern ACC and palaeogeography-driven changes in barotropic model set-up within the Pliocene. We confirm that the modern orbit used throughout PlioMIP2, is a satisfactory substitute for the Pliocene 3.205 Myr KM5c orbit in terms of large-scale climate and a number of important climatic indices. We also quantify the impact of the total solar irradiance choice (1361 versus 1365 W m−2) on the Pliocene – pre-industrial anomaly and absolute climatic state and highlight climatic systems which may present non-linear responses.

Description: The Kimmeridge Clay Formation (KCF) is a laterally extensive, total organic carbon-rich succession deposited throughout Northwest Europe during the Kimmeridgian–Tithonian (Late Jurassic). Here we present a petrographic and geochemical dataset for a 40 metre-thick section of a well-preserved drill core recovering thermally-immature deposits of the KCF in the Cleveland Basin (Yorkshire, UK), covering an interval of approximately 800 kyr. The new data are discussed in the context of depositional processes, sediment source and supply, transport and dispersal mechanisms, water column redox conditions, and basin restriction. Armstrong et al. (2016) recently postulated that an expanded Hadley Cell, with an intensified but alternating hydrological cycle, heavily influenced sedimentation and total organic carbon (TOC) enrichment, through promoting the primary productivity and organic matter burial, in the UK sectors of the Boreal Seaway. Consistent with such climate boundary conditions, petrographic observations, total organic carbon and carbonate contents, and major and trace element data presented here indicate that the KCF of the Cleveland Basin was deposited in the distal part of the Laurasian Seaway. Depositional conditions alternated between three states that produced a distinct cyclicity in the lithological and geochemical records: lower variability mudstone intervals (LVMIs) which comprise of clay-rich mudstone, TOC-rich sedimentation, and carbonate-rich sedimentation. The lower variability mudstone intervals dominate the studied interval but are punctuated by three ~ 2–4 m thick intervals of alternating TOC-rich and carbonate-rich sedimentation (here termed higher variability mudstone intervals, HVMIs). During the lower variability mudstone intervals, conditions were quiescent with oxic to sub-oxic bottom water conditions. During the higher variability mudstone intervals, highly dynamic conditions resulted in repeated switching of the redox system in a way similar to the modern deep basins of the Baltic Sea. During carbonate-rich sedimentation, oxic conditions prevailed, most likely due to elevated depositional energies at the seafloor by current/wave action. During TOC-rich sedimentation, anoxic-euxinic conditions led to an enrichment of redox sensitive/sulphide forming trace metals at the seafloor and a preservation of organic matter, and an active Mn-Fe particulate shuttle delivered redox sensitive/sulphide forming trace metals to the seafloor. In addition, based on TOC–S–Fe relationships, organic matter sulphurisation appears to have increased organic material preservation in about half of the analysed samples throughout the core, while the remaining samples were either dominated by excess Fe input into the system or experienced pyrite oxidation and sulphur loss during oxygenation events. New Hg/TOC data do not provide evidence of increased volcanism during this time, consistent with previous work. Set in the context of recent climate modelling, our study provides a comprehensive example of the dynamic climate-driven depositional and redox conditions that can control TOC and metal accumulations in the distal part of a shallow epicontinental sea, and is therefore key to understanding the formation of similar deposits throughout Earth's history.

Description: The Paleocene-Eocene Thermal Maximum (PETM) offers insight into massive short-term carbon cycle perturbations that caused significant warming during a high-pCO2 world, affecting both marine and terrestrial ecosystems. PETM records from the marine-terrestrial interface (e.g. estuarine swamps and mire deposits) are, therefore, of great interest as their present-day counterparts are highly vulnerable to future climate and sea level change. Here, we assess paleoenvironmental changes of mid-latitudinal Late Paleocene-Early Eocene peat mire records along the paleo-North Sea coast. We provide carbon isotope data of bulk organic matter (δ13CTOC), organic carbon content (%TOC), and palynological data from an extensive peat mire deposited at a mid-latitudinal (ca. 41 °N) coastal site (Schöningen, Germany). The δ13CTOC data show a carbon isotope excursion (CIE) of −1.7 ‰ coeval with a conspicuous Apectodinium acme, calling for the presence of the PETM in this coastal section. Due to the exceptionally large stratigraphic thickness of the PETM at Schöningen (10 m of section) we established a detailed palynological record that indicates only minor changes in paleovegetation leading to and during the PETM. Instead, paleovegetation changes mostly follow natural successions in response to changes along the marine-terrestrial interface. Compared to other available peat mire records (Cobham, UK; Vasterival, France) it appears that wetland deposits around the Paleogene North Sea have a typical CIE magnitude of ca. −1.3 ‰ in δ13CTOC. Moreover, the Schöningen record shares major characteristics with the Cobham Lignite, including evidence for increased fire activity prior to the PETM, minor PETM-related plant species changes, a reduced CIE in δ13CTOC, and drowning of the mire (marine ingressions) during much of the PETM. This suggests that paleoenvironmental conditions during the Late Paleocene-Early Eocene, including the PETM, consistently affected major segments of the paleo-North Sea coast.

Description: The El Cañizar de Villarquemado sequence provides a palaeoenvironmental record from the western Mediterranean Basin spanning the interval from the last part of MIS6 to the late Holocene. The pollen and sedimentological records provide qualitative information about changes in temperature seasonality and moisture conditions. We use Weighted Averaging Partial Least-Squares (WA-PLS) regression to derive quantitative reconstructions of winter and summer temperature regimes from the pollen data, expressed in terms of the mean temperature of the coldest month (MTCO) and growing degree days above a baseline of 0 °C (GDD0) respectively. We also reconstruct a moisture index (MI), the ratio of annual precipitation to annual potential evapotranspiration, taking account of the effect of low CO2 on water use efficiency. We find a rapid summer warming at the transition to MIS5. Summers were cold during MIS4 and MIS2, but some intervals in MIS3 were characterized by summers as warm as the warmest phases of MIS5 or the Holocene. However, MIS3 was not significantly warmer in winter than other intervals, and there was a gradual decline in winter temperature from MIS4 through MIS3 to MIS2. The pronounced changes in temperature seasonality during MIS5 and MIS1 are consistent with changes in summer insolation. The ecophysiological effects of changing CO2 concentration through the glacial cycle has a significant impact on reconstructed MI. Conditions became progressively more humid during MIS5 and MIS4 was also relatively humid, while MIS3 was more arid. High MI values are reconstructed during the deglaciation and there was a pronounced increase in aridity during the Holocene. Changes in MI are anti-correlated with changes in GDD0, with increased MI during intervals of summer warming indicating a strong influence of temperature on evapotranspiration. Although our main focus here is on longterm changes in climate, the Villarquemado record also shows millennial-scale changes corresponding to Dansgaard-Oeschger cycles.

Description: Very little is known about the impact of atmospheric carbon dioxide pressure (pCO2) on the shaping of biomes. The development of pCO2 throughout the Brunhes Chron may be considered a natural experiment to elucidate relationships between vegetation and pCO2. While the glacial periods show low to very low values (~ 230 to ~ 190 ppmv, respectively), the pCO2 levels of the interglacial periods vary from intermediate to relatively high (~250 to ~ 270, respectively). To study the influence of pCO2 on the Pleistocene development of SE African vegetation, we used the pollen record of a marine core (MD96-2048) retrieved from Maputo Bay south of the Limpopo River mouth in combination with stable isotope and geochemical proxies. Applying endmember analysis, four pollen assemblages could be distinguished representing different biomes: heathland, mountain forest, shrubland and woodland. We find that the vegetation of the Limpopo River catchment and the coastal region of southern Mozambique is not only influenced by hydroclimate but by also temperature and atmospheric pCO2. Our results suggest that the extension of either open ericaceous vegetation including C4 sedges or mountain forest depended on glacial pCO2 levels, and that the main development of woodlands in the area took place after the Mid-Brunhes Event when interglacial pCO2 levels rose over 270 ppmv.

Description: The formation of Paraiba do Sul river delta plain in the coast of Rio de Janeiro state, Brazil, gave rise to diverse lagoons formed under different sea level regimes and climate variations. Sedimentary core lithology, organic matter geochemistry, and isotopic composition (δ13C and δ15N) were analyzed to interpret the sedimentation of the paleoenvironment of the Lagoa Salgada carbonate system. Different lithofacies reflect variations of the depositional environment. The abundance of silt and clay between 5.8 to 3.7 ka B.P., enhances the interpretation of a transgressive system, which promoted the stagnation of coarse sediment deposition due to coast drowning. Geochemistry data from this period (5.8–3.7 ka B.P.) suggest the dominance of a wet climate, with an increase of C3 plant and a marked dry event between 4.2–3.8 ka B.P. This dryer event also matches with previous published records from around the world, indicating a global event at 4.2 ka B.P. Between 3.8–1.5 ka B.P., Lagoa Salgada was isolated, sand and silt arrived at the system by erosion with the retreat of the ocean and less fluvial drainage. Geochemistry from this moment marks the changes to favourable conditions for microorganisms active in the precipitation of carbonates, forming microbial mats and stromatolites in the drier phase.

Description: Precipitation is a key climate driver of vegetation and ecosystems of the Iberian Peninsula. Here, we use a regional pollen-climate calibration model and fossil pollen data from seven sites from different parts of Spain to provide quantitative reconstructions of annual precipitation values for the last 15 000 years. Our records show that in the Late Pleistocene (~ 15 000 to 11 600 cal yr BP) precipitation changes took place markedly in tune with the temperature trends in northern Europe, with higher precipitation during the Greenland interstadial 1 (Bølling-Allerød) and lower precipitation during the Greenland stadial 1 (Younger Dryas). The early Holocene was characterized by a rapid precipitation increase after 11 600 cal yr BP, followed by a slowly declining trend until roughly 8000 cal yr BP. From 8000 to 4000 cal yr BP the reconstructed precipitation values are the highest in most records, with maximum values nearly 100 % higher that the modern reconstructed values. The results suggest a gradually declining precipitation over the last four millennia, although the late-Holocene reconstructions are biased by intensifying human impact on vegetation. In general, our results suggest that the main changes in precipitation in the Iberian Peninsula have occurred in pace with the main temperature changes in the North European-Atlantic region, with warm (cold) periods in the North corresponding with humid (dry) periods in the Iberian Peninsula.

Description: The 692 proxy records of the new PAGES 2k compilation offer an unprecedented opportunity to study regional to global temperature trends associated with orbitally-driven changes in solar irradiance over the past two millennia. Here, we analyse the significance of long-term trends from 1–1800 CE in the PAGES 2k compilation’s tree-ring, ice core, marine and lake sediment records and find, unlike ice-cores, glacier dynamics, marine and lake sediments, no suggestion of a pre-industrial cooling trend in the tree-ring records. To understand why the tree-ring proxies lack a significant pre-industrial cooling, we divide the dendro data by location (high NH latitudes vs. mid latitudes), seasonal response (annual vs. summer), detrending method, and temperature sensitivity (high vs. low). We conclude the ability to detect any pre-industrial, millennial-long cooling in the tree-ring proxies does not increase with latitude, seasonal sensitivity, or detrending method. Consequently, caution is advised when using multi-proxy approaches to reconstruct long-term temperature changes.

Description: Tridacna is the largest marine bivalves in the tropical ocean, and its carbonate shell can shed light on high-resolution paleoclimate reconstruction. In this contribution, δ18Oshell was used to estimate the climatic variation in the Xisha Islands of the South China Sea. We first evaluate the sea surface temperature (SST) and sea surface salinity (SSS) influence on modern rehandled monthly (r-monthly) resolution Tridacna gigas δ18Oshell. The obtained results reveal that δ18Oshell seasonal variation is mainly controlled by SST and appear insensitive to local SSS change. Thus, the δ18O of Tridacna shells can be roughly used as a proxy of the local SST: a 1 ‰ δ18Oshell change is roughly equal to 4.41 °C of SST. R-monthly δ18O of a 40-year Tridacna squamosa (3673 ± 28 BP) from the North Reef of Xisha Islands was analyzed and compared with the modern specimen. The difference between the average δ18O of fossil Tridacna shell (δ18O = −1.34 ‰) and modern Tridacna specimen (δ18O = −1.15 ‰) probably implies a warm climate with roughly 0.84°C higher in 3700 years ago. The seasonal variation in 3700 years ago was slightly decreased compared with that suggested by the instrument data, and the switching between warm and cold-seasons was rapid. Higher amplitude in r-monthly and r-annual reconstructed SST anomalies implies an enhanced climate variability in this past warm period. Investigation of the El Ninõ-Southern Oscillation (ENSO) variation (based on the reconstructed SST series) indicates a reduced ENSO frequency but more extreme El Ninõ events in 3700 years ago.

Description: Understanding the ocean circulation changes associated with last glacial abrupt climate events is key to better assess climate variability and understand its different natural modes. Sedimentary Pa / Th, benthic δ13C and Δ14C are common proxies used to reconstruct past circulation flow rate and ventilation. To overcome the limitations of each proxy taken separately, a better approach is to produce multi-proxy measurements on a single sediment core. Yet, different proxies can provide conflicting information about past ocean circulation. Thus, modelling them in a consistent physical framework has become necessary to assess the geographical pattern, the timing and sequence of the multi-proxy response to abrupt circulation changes. We have implemented a representation of the 231Pa and 230Th tracers into the model of intermediate complexity iLOVECLIM, which already included δ13C and Δ14C. We have further evaluated the response of these three ocean circulation proxies to a classical abrupt circulation reduction obtained by freshwater addition in the Nordic seas under preindustrial boundary conditions. Without a priori guess, the proxy response is shown to cluster in modes that resemble the modern Atlantic water masses. The clearest and most coherent response is obtained in the deep (〉 2,000 m) Northwest Atlantic, where δ13C and Δ14C significantly decrease while Pa / Th increases. This is consistent with observational data across millennial scale events of the last glacial. Interestingly, while in marine records, except in rare instances, the phase relationship between these proxies remains unclear due to large dating uncertainties, in the model the bottom water carbon isotopes (δ13C and Δ14C) response lags the sedimentary Pa / Th response by a few hundred years.

Description: Combining ocean general circulation models with proxy data via data assimilation is a means to obtain estimates of past ocean states that are consistent with model physics as well as with proxy data. The climate during the Last Glacial Maximum (LGM, 19–23 ka) was substantially different from today. Even though boundary conditions are comparatively well known, the large-scale patterns of the ocean circulation during this time remain uncertain. Previous efforts to combine ocean models with proxy data have shown dissimilar results regarding the state of the ocean, in particular of the Atlantic Meridional Overturning Circulation. Here, we present a new LGM ocean state estimate that extents previous estimates by using global benthic as well as planktic data on the oxygen isotopic composition of calcite. It is further constrained by global seasonal and annual sea surface temperature (SST) reconstructions. The estimate shows an Atlantic Ocean that is similar to the Late Holocene Atlantic Ocean but with a reduced formation of Antarctic Bottom Water, in contrast to results of previous studies. The results indicate that SST and oxygen isotopic data alone do not require the presence of a shallower North Atlantic Deep Water and a more extensive Antarctic Bottom Water, and highlight the need for more proxy data of different types to obtain reliable ocean state estimates. Additional adjoint sensitivity experiments reveal that data from the deep North Atlantic and from the global deep Southern Ocean are most important to constrain the Atlantic Meridional Overturning Circulation.

Description: The period 36–18 ka was a dynamic phase of the last glacial, with large climate shifts in both hemispheres. Through the bipolar seesaw, the Antarctic Isotope Maxima and Greenland DO events were part of a global concert of large scale climate changes. The interaction between atmospheric processes and Atlantic meridional overturning circulation (AMOC) is crucial for such shifts, controlling upwelling- and carbon cycle dynamics, and generating climate tipping points. Here we report the first temperature and humidity record for the glacial period from the central South Atlantic (SA). The presented data resolves ambiguities about atmospheric circulation shifts during bipolar climate events recorded in polar ice cores. A unique lake sediment sequence from Nightingale Island at 37° S in the SA, covering 36.4–18.6 ka, exhibits continuous impact of the Southern Hemisphere Westerlies (SHW), recording shifts in their position and strength. The SHW displayed high latitudinal and strength-wise variability 36–31 ka locked to the bipolar seesaw, followed by 4 ka of slightly falling temperatures, decreasing humidity and fairly southern westerlies. After 27.5 ka temperatures decreased 3–4 °C, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly positioned SHW are correlated with periods of increased CO2 outgassing from the ocean. Changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, it shows that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial.

Description: Indonesia is a country composed of several thousand islands, many of them small, low-lying and densely inhabited. These are, in particular, subject to high risk of inundation due to future relative sea level changes. The Spermonde Archipelago, off the coast of Southwest Sulawesi, consists of more than 100 small islands. This study presents a dataset of 24 sea-level index points from fossil microatolls, surveyed on five islands in the Spermonde Archipelago and compares these new results with published data from the same region and with relative sea level predictions from different Glacial Isostatic Adjustment (GIA) models. The newly surveyed fossil microatolls are located around the islands of Tambakulu, Suranti (both ~ 60 km offshore of Makassar city), Bone Batang and Kodingareng Keke (both located in the center of the Archipelago) and Sanrobengi (located ~ 20 km south-southwest of Makassar). Results from the near- and mid-shelf islands indicate that relative sea level between 4 to 6 ka BP was less than one meter above present sea level. The only exception to this pattern is the heavily populated island of Barrang Lompo, where we record a significant subsidence when compared to the other islands. These new results support the conclusions from a previous dataset and are relevant to constrain late Holocene ice melting scenarios. Samples from the two outer islands (Tambakulu and Suranti) yielded ages spanning the Common Era that represent, to our knowledge, the first reported for the entire Southeast Asian region.

Description: Geothermal estimates of the ground surface temperatures for the last glacial cycle in Northern Europe has been analyzed. During the Middle and Late Weichselian (55–12 kyr BP) a substantial part of this area was covered by the Scandinavian Ice Sheet. The analysis of geothermal data has allowed reconstructing limits of the ice sheet extension and its basal thermal state in the Late Weichselian. Ground surface temperatures outside the ice sheet were extremely low (from −8 to −18 °C). Within the ice sheet, there were both thawed and frozen zones. The revealed temperature pattern is generally consistent with the modern one for the ground surface temperatures in Greenland that makes it possible to consider these ice sheets as analogues. The anomalous climatically induced surface heat flux and orbital insolation of the Earth varied consistently outside the glaciation and independently within the limits of the ice sheet.

Description: The mass loss from the Greenland Ice Sheet has increased over the past two decades. Marine-terminating glaciers contribute significantly to this mass loss due to increased melting and ice discharge. Rapid retreat periods of these tidewater glaciers have been linked to the concurrent inflow of warm, Atlantic derived waters. However, little is known about the variability of Atlantic-derived waters within these fjords, due to a lack of multi-annual, in situ measurements. Thus, to better understand the potential role of ocean warming on glacier retreat, reconstructions that characterize the variability of Atlantic water inflow to these fjords are required. Here, we investigate foraminiferal assemblages in a sediment core from Upernavik Fjord, West Greenland, in which the major ice stream Upernavik Isstrøm terminates. We investigate the environmental characteristics that control species diversity and derive that it is predominantly controlled by changes in bottom water variability. Hence, we provide a reconstruction of Atlantic water inflow to Upernavik Fjord, spanning the period 1925–2012. This reconstruction reveals peak Atlantic water inflow during the 1930s and again after 2000, a pattern that is similar to the Atlantic Multidecadal Oscillation (AMO). We compare these results to historical observations of front positions of Upernavik Isstrøm. This reveals that inflow of warm, Atlantic-derived waters indeed likely contributed to high retreat rates in the 1930s and after 2000. However, moderate retreat rates of Upernavik Isstrøm also prevailed in the 1960s/1970s, showing that retreat continued despite reduced Atlantic water inflow, albeit at a lower rate. Considering the link between bottom water variability and the AMO in Upernavik Fjord and the fact that a persistent negative phase of the AMO is expected for the next decade, Atlantic water inflow into the fjord may decrease in the next ~ 10 years.

Description: Chinese historical documents recorded plenty of information related with climate change and grain harvest, which are helpful to explore the impacts of extreme drought/flood on crops and the implications on adaptation for agriculture to more extreme climate probability in the context of global warming. Here, we used the reconstructed extreme drought/flood chronologies and reconstructed grain harvest series derived from historical documents to investigate the connection between the occurrences of extreme drought/flood in eastern China and poor harvest during 801–1910. The results showed that more extreme droughts occurred in 801–870, 1031–1230, 1481–1530 and 1581–1650 over whole eastern China. On regional scale, more extreme droughts occurred in 1031–1100, 1441–1490, 1601–1650 and 1831–1880 in North China, 801–870, 1031–1120, 1161–1220 and 1471–1530 in Jianghuai, 991–1040, 1091–1150, 1171–1230, 1411–1470 and 1481–1530 in Jiangnan. The grain harvest was poor in periods of 801–940, 1251–1650 and 1841 to 1910, but bumper in periods of 951–1250 and 1651–1840 approximately. For entire period of 801–1910, more occurrence of extreme drought in any sub–region of eastern China could significantly reduce harvest in the long term average, but the connection between harvest and extreme flood seemed to be much weaker. The co–occurrence of extreme drought and extreme flood in different sub–regions in the same year had a greater impact on harvest yield. However, the connection between the occurrence of poor harvest and regional extreme drought was weak in the warm epoch of 920–1300 but strong in the cold epoch of 1310–1880, which implicated warm climate might weaken the impact of extreme drought on poor harvest during historical times.

Description: Billions of people depend on the precipitation of the Asian monsoons. The Tibetan Plateau and the Himalayas on the one hand strongly influence the monsoonal circulation pattern and on the other hand represent water towers of humanity. Understanding the dynamics of the Asian monsoons is one of the prime targets in climate research. Modern coupling of atmospheric circulation and hydrological cycle over and on the plateau can be observed and outlined, and lake level controlling factors be identified. Recent monitoring of lakes showed that many of them have grown at least for decades, the causes being higher meltwater inflow or stronger rainfall of different sources, depending on the particular location of a drainage basin. The long-term dynamics, however, can be described best with the aid of high-resolution climate archives. We focus here on the often controversial discussion of Holocene lake development and selected the Bangong Co drainage basin on the western Tibetan Plateau as a case site. The aim of our study is, to identify the factors influencing lake level such as monsoonal or convective precipitation and meltwater. For doing so, shells of the aquatic gastropod genus Radix were collected from an early Middle Holocene sediment sequence in the Nama Chu sub-catchment of the eastern Bangong Co and sclerochronlogical isotope patterns of five shells obtained in weekly to sub-monthly resolution. Our data suggests that during ca. 7.5 ka ago, monsoonal rainfall was higher than today. However, summer precipitation was not continuous but affected the area as extended moisture pulses. This implicates that the northern boundary of the SW Asian monsoon was similar to modern times. We could identify convective rainfall events significantly stronger than today. We relate this to higher soil moisture and larger lake surface areas under higher insolation. The regional meltwater amount corresponds with westerly-derived winter snowfall. The snowfall amount was probably similar to modern times. Exceptionally heavy δ13C values archived in the shells were likely, at least partly, triggered by biogenic methane production. We suggest that our approach is suitable to study other lake systems on the Tibetan Plateau from which fossil Radix shells can be obtained. It may thus help to infer palaeo-weather patterns across the plateau.

Description: Although quantitative isotopic data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to use the speleothem data for data-model comparisons. Here, we accomplish this using 456 globally-distributed speleothem δ18O records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates procuring large numbers of records if data-model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotopic values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model’s ability to simulate spatial isotopic trends and the degree to which the model reproduces the observed environmental controls of isotopic spatial variability. Our analyses provide a protocol for using speleothem isotopic data for model evaluation, including screening the observations, the optimum period for the modern observational baseline, and the selection of an appropriate time-window for creating means of the isotope data for palaeo time slices.

Description: Ongoing work in paleoclimate reconstruction prioritizes understanding the origins and magnitudes of errors that arise when comparing models and data. One class of such errors arises from assumptions of proxy temporal representativeness – broadly, the time scales over which paleoclimate proxy measurements are associated with climate variables. In the case of estimating time mean values over an interval, errors can arise when the time interval over which data are averaged and the interval that is being studied have different lengths, or if those intervals are offset from one another in time. Because it is challenging to tailor proxy measurements to precise time intervals, such errors are expected to be common in model-data and data-data comparisons, but how large and prevalent they are is unclear. The goal of this work is to provide a framework for first-order quantification of temporal representativity errors and to study the interacting effects of sampling error, archive smoothing (e.g. by bioturbation in sediment cores), chronological offsets and errors (e.g. arising from radiocarbon dating), and the spectral character of the climate process being sampled. In some cases, particularly for small values of target intervals τx relative to sample intervals τy, errors can be large relative to signals of interest. Errors from mismatches in τx and τy can have magnitudes comparable to those from chronological uncertainty. Archive smoothing can reduce sampling errors by acting as an anti-aliasing filter, but destroys high-frequency climate information. An extension of the approach to paleoclimate time series, which are sequences of time-average values, shows that measurement intervals shorter than the spacing between samples lead to errors, absent compensating effects from archive smoothing. Including these sources of uncertainty will improve accuracy in model-data comparisons and data comparisions and syntheses. Moreover, because sampling procedures emerge as important parameters in uncertainty quantification, reporting salient information about how records are processed and assessments of archive smoothing and chronological uncertainties alongside published data is important to be able to use records to their maximum potential in paleoclimate reconstruction and data assimilation.

Description: All major mountain ranges are assumed to have been subject to increased uplifting processes during the late Miocene and Pliocene. Previous work has demonstrated that African uplift is an important element to explain Benguela upper-ocean cooling in the late Miocene/Pliocene. According to proxy records, a surface ocean cooling also occurred in other Eastern Boundary upwelling regions during the late Neogene. Here we investigate a set of sensitivity experiments altering topography in major mountain regions (Andes, North American Cordillera and South/East African mountains) separately with regard to the potential impact on the intensity of near-coastal low-level winds, Ekman transport and Ekman pumping as well as upper-ocean cooling. The simulations show that mountain uplift is important for upper-ocean temperature evolution in the area of Eastern Boundary Currents. The impact is primarily on the atmospheric circulation which is then acting on upper-ocean temperatures through changes in strengths of upwelling, horizontal heat advection and surface heat fluxes. Different atmosphere-ocean feedbacks additionally alter the sea surface temperature response to uplift. The relative importance of the different feedback mechanisms depends on the region, but is most likely also influenced by model and model resolution.

Description: In order to investigate the relation between ice sheets and climate in a warmer-than-present world, recent research has focussed on the Late Pliocene, 3.6 to 2.58 million years ago. It is the most recent period in Earth history when such a climate state existed for a significant duration of time. Marine Isotope Stage (MIS) M2 (~ 3.3 Myr ago) is a strong positive excursion in benthic oxygen records in the middle of the otherwise warm and relatively stable Late Pliocene. However, the relative contributions to the benthic δ18O signal from deep-ocean cooling and growing ice sheets are still uncertain. Here, we present results from simulations of the late Pliocene with a hybrid ice-sheet–climate model, showing a reconstruction of ice sheet geometry, sea-level and atmospheric CO2. Initial experiments simulating the last four glacial cycles indicate that this model yields results which are in good agreement with proxy records in terms of global mean sea level, benthic oxygen isotope abundance, ice core-derived surface temperature and atmospheric CO2 concentration. For the Late Pliocene, our results show an atmospheric CO2 concentration during MIS M2 of 233–249 ppmv, and a drop in global mean sea level of 10 to 25 m. Uncertainties are larger during the warmer periods leading up to and following MIS M2. CO2 concentrations during the warm intervals in the Pliocene, with sea-level high stands of 8–14 m above present-day, varied between 320 and 400 ppmv, lower than indicated by some proxy records but in line with earlier model reconstructions.

Description: Global climate cooled from the early Eocene hothouse (~ 52–50 Ma) to the latest Eocene (~ 34 Ma). At the same time, the tectonic evolution of the Southern Ocean was characterized by the opening and deepening of circum-Antarctic gateways, which affected both surface- and deep-ocean circulation. The Tasman Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasman Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on physico-chemical properties of the water masses in which these organisms thrived. The degree to which the geographic path of surface currents (primarily controlled by tectonism) or their physico-chemical properties (significantly impacted by climate) have controlled the composition of the fossil assemblages has, however, remained unclear. In fact, it is yet poorly understood to what extent oceanographic response as a whole was dictated by climate change, independent of tectonics-induced oceanographic changes that operate on longer time scales. To disentangle the effects of tectonism and climate in the southwest Pacific Ocean, we target a climatic deviation from the long-term Eocene cooling trend, a 500 thousand year long global warming phase termed the Middle Eocene Climatic Optimum (MECO; ~ 40 Ma). The MECO warming is unrelated to regional tectonism, and thus provides a test case to investigate the oceans physiochemical response to climate change only. We reconstruct changes in surface-water circulation and temperature in and around the Tasman Gateway during the MECO through new palynological and organic geochemical records from the central Tasman Gateway (Ocean Drilling Program Site 1170), the Otway Basin (southeastern Australia) and the Hampden Section (New Zealand). Our results confirm that dinocyst communities track tectonically driven circulation patterns, yet the variability within these communities can be driven by superimposed temperature change. Together with published results from the east of the Tasman Gateway, our results suggest that as surface-ocean temperatures rose, the East Australian Current extended further southward during the peak of MECO warmth. Simultaneous with high sea-surface temperatures in the Tasman Gateway area, pollen assemblages indicate warm temperate rainforests with paratropical elements along the southeastern margin of Australia. Finally, based on new age constraints we suggest that a regional southeast Australian transgression might have been caused by sea-level rise during MECO.

Description: The Marine Isotope Stage 3 (MIS 3) is considered a period of persistent and rapid climate and sea level variabilities during which eustatic sea level is observed to have varied by tens of meters. Constraints on local sea level during this time are critical for further estimates of these variabilities. We here present constraints on relative sea level in the Marmara and Black Sea regions in the northeastern Mediterranean, inferred from reconstructions of the history of the connections and disconnections (partial or total) of these seas together with the global ocean. We use a set of independent data from seismic imaging and core-analyses to infer that the Marmara and Black Seas remained connected persistent freshwater lakes that outflowed to the global ocean during the majority of MIS 3. Marine water intrusion during the early MIS-3 stage may have occurred into the Marmara Sea-Lake but not the Black Sea-Lake. This suggests that the relative sea level was near the paleo-elevation of the Bosporus sill and possibly slightly above the Dardanelles paleo-elevation, ~80 mbsl. The Eustatic sea level may have been even lower, considering the isostatic effects of the Eurasian ice sheet would have locally uplifted the topography of the northeastern Mediterrranean.

Description: The increasingly nonlinear response of the climate-cryosphere system to insolation forcing during the Pliocene and Pleistocene, as recorded in benthic foraminiferal stable oxygen isotope ratios (δ18O), is marked by a distinct evolution in ice-age cycle frequency, amplitude, phase, and geometry. To date, very few studies have thoroughly investigated the nonsinusoidal shape of these climate cycles, leaving precious information unused to further unravel the complex dynamics of the Earth's system. Here, we present higher-order spectral analyses of the LR04 δ18O stack that describe coupling and energy exchanges among astronomically-paced climate cycles during the Pliocene and Pleistocene. These advanced bispectral computations show how energy is passed from precession-paced to obliquity-paced climate cycles during the Early Pleistocene (~ 2,500–~ 750 ka), and ultimately to eccentricity-paced climate cycles during the Middle and Late Pleistocene (from ~ 750 ka onward). They also show how energy is transferred among many cycles that have no primary astronomical origin. We hypothesize that the change of obliquity-paced climate cycles during the mid-Pleistocene transition (~ 1,200–~ 600 ka), from being a net sink into a net source of energy, is indicative of the passing of a land-ice mass-loading threshold in the Northern Hemisphere (NH), after which cycles of crustal depression and rebound started to resonate with the ~ 110-kyr eccentricity modulation of precession. However, precession-paced climate cycles remain persistent energy providers throughout the Late Pliocene and entire Pleistocene, which is supportive of a dominant and continuous fuelling of the NH ice ages by insolation in the (sub-) tropical zones, and the control it exerts on meridional heat and moisture transport through atmospheric and oceanic circulation.

Description: The South Pole Ice Core (SPICEcore) was drilled in 2014–2016 to provide a detailed multi-proxy archive of paleoclimate conditions in East Antarctica during the Holocene and late Pleistocene. Interpretation of these records requires an accurate depth-age relationship. Here, we present the SP19 timescale for the age of the ice of SPICEcore. SP19 is synchronized to the WD2014 chronology from the West Antarctic Ice Sheet Divide (WAIS Divide) ice core using stratigraphic matching of 251 volcanic events. These events indicate an age of 54 302 ± 519 years BP (before the year 1950) at the bottom of SPICEcore. Annual layers identified in sodium and magnesium ions to 11 341 BP were used to interpolate between stratigraphic volcanic tie points, yielding an annually-resolved chronology through the Holocene. Estimated timescale uncertainty during the Holocene is less than 18 years relative to WD2014, with the exception of the interval between 1800 to 3100 BP when uncertainty estimates reach ± 25 years due to widely spaced volcanic tie points. Prior to the Holocene, uncertainties remain within 124 years relative to WD2014. Results show an average Holocene accumulation rate of 7.4 cm/yr (water equivalent). The time variability of accumulation rate is consistent with expectations for steady-state ice flow through the modern spatial pattern of accumulation rate. Time variations in nitrate concentration, nitrate seasonal amplitude, and δ15N of N2 in turn are as expected for the accumulation-rate variations. The highly variable yet well-constrained Holocene accumulation history at the site can help improve scientific understanding of deposition-sensitive climate proxies such as δ15N of N2 and photolyzed chemical compounds.

Description: We present here the first results, for the pre-industrial and mid-Holocene climatological periods, of the newly developed isotope-enhanced version of the fully coupled Earth system model MPI-ESM, called hereafter MPI-ESM-wiso. The water stable isotopes H216O, H218O and HDO have been implemented into all components of the coupled model setup: the atmosphere model ECHAM6, the land/soil vegetation model JSBACH, and the ocean/sea ice model MPIOM. The exchanges of the related isotope masses between the atmosphere and the ocean are made via the coupler OASIS3. The mid-Holocene, one of the PMIP4-CMIP6 entry cards to evaluate the performance of the latest generation of fully-coupled General Circulation Models, provides the opportunity to evaluate the model response to changes in the seasonal and latitudinal distribution of insolation induced by different orbital forcing conditions. The results of our equilibrium simulations allow to evaluate the performance of the isotopic model in simulating the spatial and temporal variations of water isotopes in the different compartments of the hydrological system for warm climates. It represents a first necessary step before simulating other climatological interglacial periods or transient Holocene experiment. For pre-industrial climate, MPI-ESM-wiso reproduces very well the observed spatial distribution of isotopic content in precipitation, in link with the spatial variations in temperature and precipitation rate. We find also a good model-data agreement with the observed distribution of isotopic composition in surface seawater, but a bias with too depleted surface seawater is present in the Arctic Ocean. All these results are improved compared to the previous model version ECHAM5/MPIOM. The spatial relationships of water isotopic composition with temperature, precipitation rate and salinity are consistent with observational data. For the pre-industrial climate, the interannual relationships of water isotopes with temperature and salinity are globally lower than the spatial ones, consistent with previous studies. Simulated results under mid-Holocene conditions are in fair agreement with the isotopic measurements from ice cores and continental speleothems. MPI-ESM-wiso simulates a depletion in isotopic composition of precipitation from North Africa to the Tibetan plateau via India due to the enhanced monsoons during mid-Holocene. Over Greenland, our simulation indicates enriched isotopic composition of precipitation over Greenland in link with higher summer temperature and reduction in sea ice, shown by positive isotope-temperature gradient. For the Antarctic continent, the model simulates depleted isotopic values over the East Antarctic plateau, in link with the lower temperatures during the mid-Holocene period, while similar or higher isotopic values are modeled over the rest of the continent. While variations of isotopic contents in precipitation over West Antarctica between mid-Holocene and pre-industrial periods are partly controlled by changes in temperature, the transport of relatively enriched water vapor near the coast to the western ice core sites could play a role in the final isotopic composition. The reconstruction of past salinity through isotopic content in sea surface waters can be complicated for regions with strong ocean dynamics, variations in sea ice regimes or significant changes in freshwater budget, giving an extremely variable relationship between isotopic content and salinity of ocean surface waters over small spatial scales. These complicating factors demonstrate the complexity in interpreting water isotopes as past climate signals of warm periods like the mid-Holocene.

Description: In the modern oceans, the relative abundances of Glycerol dialkyl glycerol tetraether (GDGTs) compounds produced by marine archaeal communities show a significant dependence on the local sea surface temperature at the site of formation. When preserved in ancient marine sediments, the measured abundances of these fossil lipid biomarkers thus have the potential to provide a geological record of long-term variability in planetary surface temperatures. Several empirical calibrations have been made between observed GDGT relative abundances in late Holocene core top sediments and modern upper ocean temperatures. These calibrations form the basis of the widely used TEX86 palaeothermometer. There are, however, two outstanding problems with this approach, first the appropriate assignment of uncertainty to estimates of ancient sea surface temperatures based on the relationship of the ancient GDGT assemblage to the modern calibration data set; and second, the problem of making temperature estimates beyond the range of the modern empirical calibrations (〉 30 ºC). Here we apply modern machine-learning tools, including Gaussian Process Emulators and forward modelling, to develop a new mathematical approach we call OPTiMAL (Optimised Palaeothermometry from Tetraethers via MAchine Learning) to improve temperature estimation and the representation of uncertainty based on the relationship between ancient GDGT assemblage data and the structure of the modern calibration data set. We reduce the root mean square uncertainty on temperature predictions (validated using the modern data set) from ~ ±6 ºC using TEX86 based estimators to ±3.6 ºC using Gaussian Process estimators for temperatures below 30 ºC. We also provide a new but simple quantitative measure of the distance between an ancient GDGT assemblage and the nearest neighbour within the modern calibration dataset, as a test for significant non-analogue behaviour. Finally, we advocate against the use of temperature estimates beyond the range of the modern empirical calibration dataset, given the absence – to date – of a robust predictive biological model or extensive and reproducible mesocosm experimental data in this elevated temperature range.

Description: As one of the first transcontinental polities that led to widespread anthropogenic modification of the environment, the influence of the Roman Empire on European climate has been studied for more than 20 years. Recent advances in our understanding of past land use and aerosol-climate interactions make it valuable to revisit the way humans may have affected the climate of the Roman Era. Here we drive the global aerosol-enabled climate model ECHAM-HAM-SALSA with land use maps and novel estimates of anthropogenic aerosol emissions from the Roman Empire at its apogee to quantify the effect of humans on regional climate. In a factorial study, we used the HYDE and KK11 anthropogenic land cover change scenarios with three estimates (low, medium, high) of aerosol emissions from fuel combustion and burning of agricultural land. Land use effects on climate varied from no influence using the HYDE scenario to a significant warming over land of 0.15 K with KK11 relative to a no-land use control. This warming is primarily caused by regional decreases in turbulent fluxes, in contrast to previous studies that emphasised changes in albedo and evapotranspiration. Aerosol emissions from agricultural burning were greater than those from fuel consumption, but on the same order of magnitude. All emissions scenarios result in an enhanced cooling effect of clouds over a no-emissions control scenario. As a consequence, the land surface temperature averaged over our entire study domain decreased significantly by 0.17 K, 0.23 K, and 0.46 K for the low, the intermediate, and the high emissions scenarios, respectively. Cooling caused by aerosol emissions is largest over Central and Eastern Europe, while warming caused by land use occurs in parts of North Africa and the Middle East. Our results suggest that the influence of Roman Era anthropogenic aerosol emissions on European climate may have been as important as that of deforestation and other forms of land use. Our model may overestimate aerosol-effective radiative forcing, however, and our results are very sensitive to the inferred seasonal timing of agricultural burning practices and natural aerosol emissions over land (wildfire emissions and biogenic emissions). Nevertheless, it is likely that human influence on land and the atmosphere affected continental-scale climate during Classical Antiquity.

Description: The historical climatology has remarkable potentialities to produce climatic reconstructions with high temporal resolution. However, there are methodological limitations that hinder the spatial development of this discipline. This study presents a new approach to Historical Climatology that overcomes some of the limitations of classical approaches such as the Rogations Method or the Content Analysis: the Cost Opportunity for Small Towns (COST). It analyses historical documents and takes advantage of all sort of meteorological information available in the written documents, not only the most severe events, thereby overcoming the most prominent bottleneck of former approaches. COST relies on the fact that the use paper had a high cost, so its use to describe meteorological conditions is hypothesized to be proportional to the impact they had on society. To prove the validity of this approach to reconstruct climate conditions, this article uses exemplarily the Municipal Chapter Acts of a small town in Southern Spain (Caravaca de la Cruz), which span the period 1600–1900, and allow to obtain reconstructions with monthly resolution. Using the same documentary source, the three approaches have been used to derive respective climate reconstructions, which are then compared to assess the consistency of the climate signal and identify possible caveats in the methods. The three approaches lead to generally coherent series of secular variability in the hydrological conditions and that agree well with the results pointed out by previous studies. Still, COST approach is arguably more objective and less affected by changes in the societal behavior, which allows it to perform comparative studies in regions with different languages and traditions.