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  • 1
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    Reversals in Temperature‐Precipitation Correlations in the Northern Hemisphere Extratropics During the Holocene (2022)
    Herzschuh, Ulrike ; Böhmer, Thomas ; Li, Chenzhi ; [et al.]
    Details
    Publication Date: 2022-12-07
    Description: Future precipitation levels remain uncertain because climate models have struggled to reproduce observed variations in temperature‐precipitation correlations. Our analyses of Holocene proxy‐based temperature‐precipitation correlations and hydrological sensitivities from 2,237 Northern Hemisphere extratropical pollen records reveal a significant latitudinal dependence and temporal variations among the early, middle, and late Holocene. These proxy‐based variations are largely consistent with patterns obtained from transient climate simulations (TraCE21k). While high latitudes and subtropical monsoon areas show mainly stable positive correlations throughout the Holocene, the mid‐latitude pattern is temporally and spatially more variable. In particular, we identified a reversal from positive to negative temperature‐precipitation correlations in the eastern North American and European mid‐latitudes from the early to mid‐Holocene that mainly related to slowed down westerlies and a switch to moisture‐limited convection under a warm climate. Our palaeoevidence of past temperature‐precipitation correlation shifts identifies those regions where simulating past and future precipitation levels might be particularly challenging.
    Description: Plain Language Summary: Predicting future precipitation levels reliably is more challenging than predicting temperature change. Accordingly, we need to understand the relationship between temperature and precipitation and its changes in space and time. We used climate proxy‐data derived from 2,237 pollen records from lake sediments and peats from the Northern Hemisphere extratropics for the early, middle, and late Holocene (i.e., 12,000–8,000, 8,000–4,000, 4,000–0 years before present, respectively). Our results reveal a significant latitudinal dependence and temporal variation of the temperature‐precipitation relationship. These proxy‐based variations are largely consistent with patterns obtained from simulations using climate models. While high latitudes and subtropical monsoon areas show mainly stable positive correlations throughout the Holocene (i.e., warm conditions co‐occur with wet conditions), the mid‐latitude pattern is temporally and spatially more variable. In particular, we identified a reversal to negative temperature‐precipitation correlations in the eastern North American and European mid‐latitudes from the early to middle Holocene. We hypothesize that weak westerly circulation, warm climate, and climate‐soil feedbacks limited evaporation and as such reduced convection during the middle Holocene which led to a negative relationship between temperature and precipitation. Our analysis of past temperature‐precipitation correlation shifts identifies regions where past changes in the temperature‐precipitation relationships are variable and thus where predicting precipitation might be particularly challenging in a warming climate.
    Description: Key Points: We analyzed Holocene temperature‐precipitation correlations and hydrological sensitivities using climate proxy (pollen) and model data from Northern Hemisphere extratropics. We found reversals to negative temperature‐precipitation correlations from the cold early Holocene to the warm mid‐Holocene likely related to moisture‐limited convection. Correlations and hydrological sensitivities were mostly stable positive in polar and extratropical monsoon‐areas.
    Description: EC European Research Council http://dx.doi.org/10.13039/501100000781
    Description: PALMOD
    Description: China Scholarship Council http://dx.doi.org/10.13039/501100004543
    Description: https://doi.pangaea.de/10.1594/PANGAEA.930512
    Description: https://doi.org/10.5281/zenodo.5910989
    Description: https://zenodo.org/record/7038402%23.YxBL1uzP3V8
    Keywords: ddc:551 ; ddc:561 ; Holocene ; pollen ; Northern Hemisphere ; temperature-precipation correlations
    Language: English
    Type: doc-type:article
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  • 2
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    Pollen-climate relationships in time (9 ka, 6 ka, 0 ka) and space (upland vs. lowland) in eastern continental Asia (2017)
    Elsevier
    In:  Quaternary Science Reviews, 156 . pp. 1-11.
    Details
    Publication Date: 2020-02-06
    Description: Temporal and spatial stability of the vegetation–climate relationship is a basic ecological assumption for pollen-based quantitative inferences of past climate change and for predicting future vegetation. We explore this assumption for the Holocene in eastern continental Asia (China, Mongolia). Boosted regression trees (BRT) between fossil pollen taxa percentages (Abies, Artemisia, Betula, Chenopodiaceae, Cyperaceae, Ephedra, Picea, Pinus, Poaceae and Quercus) and climate model outputs of mean annual precipitation (Pann) and mean temperature of the warmest month (Mtwa) for 9 and 6 ka (ka = thousand years before present) were set up and results compared to those obtained from relating modern pollen to modern climate. Overall, our results reveal only slight temporal differences in the pollen–climate relationships. Our analyses suggest that the importance of Pann compared with Mtwa for taxa distribution is higher today than it was at 6 ka and 9 ka. In particular, the relevance of Pann for Picea and Pinus increases and has become the main determinant. This change in the climate–tree pollen relationship parallels a widespread tree pollen decrease in north-central China and the eastern Tibetan Plateau. We assume that this is at least partly related to vegetation–climate disequilibrium originating from human impact. Increased atmospheric CO2 concentration may have permitted the expansion of moisture-loving herb taxa (Cyperaceae and Poaceae) during the late Holocene into arid/semi-arid areas. We furthermore find that the pollen–climate relationship between north-central China and the eastern Tibetan Plateau is generally similar, but that regional differences are larger than temporal differences. In summary, vegetation–climate relationships in China are generally stable in space and time, and pollen-based climate reconstructions can be applied to the Holocene. Regional differences imply the calibration-set should be restricted spatially.
    Type: Article , PeerReviewed
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  • 3
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    Biome changes and their inferred climatic drivers in northern and eastern continental Asia at selected times since 40 cal ka bp (2018)
    Springer
    In:  Vegetation History and Archaeobotany, 27 (2). pp. 365-379.
    Details
    Publication Date: 2018-12-17
    Description: Recent global warming is pronounced in high-latitude regions (e.g. northern Asia), and will cause the vegetation to change. Future vegetation trends (e.g. the “arctic greening”) will feed back into atmospheric circulation and the global climate system. Understanding the nature and causes of past vegetation changes is important for predicting the composition and distribution of future vegetation communities. Fossil pollen records from 468 sites in northern and eastern Asia were biomised at selected times between 40 cal ka bp and today. Biomes were also simulated using a climate-driven biome model and results from the two approaches compared in order to help understand the mechanisms behind the observed vegetation changes. The consistent biome results inferred by both approaches reveal that long-term and broad-scale vegetation patterns reflect global- to hemispheric-scale climate changes. Forest biomes increase around the beginning of the late deglaciation, become more widespread during the early and middle Holocene, and decrease in the late Holocene in fringe areas of the Asian Summer Monsoon. At the southern and southwestern margins of the taiga, forest increases in the early Holocene and shows notable species succession, which may have been caused by winter warming at ca. 7 cal ka bp. At the northeastern taiga margin (central Yakutia and northeastern Siberia), shrub expansion during the last deglaciation appears to prevent the permafrost from thawing and hinders the northward expansion of evergreen needle-leaved species until ca. 7 cal ka bp. The vegetation-climate disequilibrium during the early Holocene in the taiga-tundra transition zone suggests that projected climate warming will not cause a northward expansion of evergreen needle-leaved species.
    Type: Article , PeerReviewed
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  • 4
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    Harmonizing plant functional type distributions for evaluating Earth System Models (2018)
    Copernicus
    In:  Climate of the Past Discussions . pp. 1-51.
    Details
    Publication Date: 2018-09-05
    Description: Dynamic vegetation models simulate global vegetation in terms of fractional coverages of a few plant functional types (PFTs). Although these models often share the same concept, they differ with respect to the number and kind of PFTs, complicating the comparability of simulated vegetation distributions. Pollen-based reconstructions are initially only available in form of time-series of individual taxa that are not distinguished in the models. Thus, to evaluate simulated vegetation distributions, the modelling results and pollen-based reconstructions have to be converted into a comparable format. The classical approach is the method of biomisation, but hitherto, PFT-based biomisation methods were only available for individual models. We introduce and evaluate a simple, universally applicable technique to harmonize PFT-distributions by assigning them into nine mega-biomes that follow the definitions commonly used for vegetation reconstructions. The method works well for all state-of the art dynamic vegetation models, independent of the spatial resolution or the complexity of the models. Large biome belts (such as tropical forest) are well represented, but regionally confined biomes (warm-mixed forest, Savanna) are only partly captured. Overall, the PFT-based biomisation is able to keep up with the conventional biomisation approach of forcing biome models (here: BIOME1) with the background climate states. The new method has, however, the advantage that it allows a more direct comparison and evaluation of the vegetation distributions simulated by Earth System Models. Thereby, the new method provides a powerful tool for the evaluation of Earth System Models in general.
    Type: Article , NonPeerReviewed
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  • 5
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    Biome changes in Asia since the mid-Holocene – an analysis of different transient Earth system model simulations (2017)
    Copernicus Publications (EGU)
    In:  Climate of the Past, 13 (2). pp. 107-134.
    Details
    Publication Date: 2020-02-06
    Description: The large variety of atmospheric circulation systems affecting the eastern Asian climate is reflected by the complex Asian vegetation distribution. Particularly in the transition zones of these circulation systems, vegetation is supposed to be very sensitive to climate change. Since proxy records are scarce, hitherto a mechanistic understanding of the past spatio-temporal climate–vegetation relationship is lacking. To assess the Holocene vegetation change and to obtain an ensemble of potential mid-Holocene biome distributions for eastern Asia, we forced the diagnostic biome model BIOME4 with climate anomalies of different transient Holocene climate simulations performed in coupled atmosphere–ocean(–vegetation) models. The simulated biome changes are compared with pollen-based biome records for different key regions. In all simulations, substantial biome shifts during the last 6000 years are confined to the high northern latitudes and the monsoon–westerly wind transition zone, but the temporal evolution and amplitude of change strongly depend on the climate forcing. Large parts of the southern tundra are replaced by taiga during the mid-Holocene due to a warmer growing season and the boreal treeline in northern Asia is shifted northward by approx. 4° in the ensemble mean, ranging from 1.5 to 6° in the individual simulations, respectively. This simulated treeline shift is in agreement with pollen-based reconstructions from northern Siberia. The desert fraction in the transition zone is reduced by 21 % during the mid-Holocene compared to pre-industrial due to enhanced precipitation. The desert–steppe margin is shifted westward by 5° (1–9° in the individual simulations). The forest biomes are expanded north-westward by 2°, ranging from 0 to 4° in the single simulations. These results corroborate pollen-based reconstructions indicating an extended forest area in north-central China during the mid-Holocene. According to the model, the forest-to-non-forest and steppe-to-desert changes in the climate transition zones are spatially not uniform and not linear since the mid-Holocene.
    Type: Article , PeerReviewed
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  • 6
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    Harmonising plant functional type distributions for evaluating Earth system models (2019)
    Copernicus Publications (EGU)
    In:  Climate of the Past, 15 (1). pp. 335-366.
    Details
    Publication Date: 2022-01-17
    Description: Dynamic vegetation models simulate global vegetation in terms of fractional coverage of a few plant functional types (PFTs). Although these models often share the same concept, they differ with respect to the number and kind of PFTs, complicating the comparability of simulated vegetation distributions. Pollen-based vegetation reconstructions are initially only available in the form of time series of individual taxa that are not distinguished in the models. Thus, to evaluate simulated vegetation distributions, the modelling results and pollen-based vegetation reconstructions have to be converted into a comparable format. The classical approach is the method of biomisation, but hitherto PFT-based biomisation methods were only available for individual models. We introduce and evaluate a simple, universally applicable technique to harmonise PFT distributions by assigning them into nine mega-biomes, using only assumptions on the minimum PFT cover fractions and few bioclimatic constraints (based on the 2 m temperature). These constraints mainly follow the limitation rules used in the classical biome models (here BIOME4). We test the method for six state-of-the-art dynamic vegetation models that are included in Earth system models based on pre-industrial, mid-Holocene and Last Glacial Maximum simulations. The method works well, independent of the spatial resolution or the complexity of the models. Large biome belts (such as tropical forest) are generally better represented than regionally confined biomes (warm–temperate forest, savanna). The comparison with biome distributions inferred via the classical biomisation approach of forcing biome models (here BIOME1) with the simulated climate states shows that the PFT-based biomisation is even able to keep up with the classical method. However, as the new method considers the PFT distributions actually calculated by the Earth system models, it allows for a direct comparison and evaluation of simulated vegetation distributions which the classical method cannot do. Thereby, the new method provides a powerful tool for the evaluation of Earth system models in general.
    Type: Article , PeerReviewed
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  • 7
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    Northern Hemisphere biome changes (〉30°N) since 40 cal ka BP and their driving factors inferred from model-data comparisons (2019)
    Elsevier
    In:  Quaternary Science Reviews, 220 . pp. 291-309.
    Details
    Publication Date: 2022-01-31
    Description: Ongoing and past biome transitions are generally assigned to climate and atmospheric changes (e.g. temperature, precipitation, CO2), but the major regional factors or factor combinations that drive vegetation change often remain unknown. Modelling studies applying ensemble runs can help to partition the effects of the different drivers. Such studies require careful validation with observational data. In this study, fossil pollen records from 741 sites in Europe, 728 sites in North America, and 418 sites in Asia (extracted from terrestrial archives including lake sediments) are used to reconstruct biomes at selected time slices between 40 cal ka BP (calibrated thousand years before present) and today. These results are used to validate Northern Hemisphere biome distributions (〉30°N) simulated by the biome model BIOME4 that has been forced with climate data simulated by a General Circulation model. Quantitative comparisons between pollen- and model-based results show a generally good fit at a broad spatial scale. Mismatches occur in central-arid Asia with a broader extent of grassland throughout the last 40 ka (likely due to the over-representation of Artemisia and Chenopodiaceae pollen) and in Europe with over-estimation of tundra at 0 cal ka BP (likely due to human impacts to some extent). Sensitivity analysis reveals that broad-scale biome changes follow the global signal of major postglacial temperature change, although the climatic variables vary in their regional and temporal importance. Temperature is the dominant variable in Europe and other rather maritime areas for biome changes between 21 and 14 ka, while precipitation is highly important in the arid inland regions of Asia and North America. The ecophysiological effect of changes in the atmospheric CO2-concentration has the highest impact during this transition than in other intervals. With respect to modern vegetation in the course of global warming, our findings imply that vegetation change in the Northern Hemisphere may be strongly limited by effective moisture changes, i.e. the combined effect of temperature and precipitation, particularly in inland areas.
    Type: Article , PeerReviewed
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  • 8
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    Diving into the past – A paleo data-model comparison workshop on the Late Glacial and Holocene (2019)
    AMS (American Meteorological Society)
    In:  Bulletin of the American Meteorological Society, 100 (1). ES1-ES4.
    Details
    Publication Date: 2022-01-31
    Type: Article , PeerReviewed
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  • 9
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    Position and orientation of the westerly jet determined Holocene rainfall patterns in China (2019)
    Nature Research
    In:  Nature Communications, 10 (1). Art.Nr. 2376.
    Details
    Publication Date: 2022-01-31
    Description: Proxy-based reconstructions and modeling of Holocene spatiotemporal precipitation patterns for China and Mongolia have hitherto yielded contradictory results indicating that the basic mechanisms behind the East Asian Summer Monsoon and its interaction with the westerly jet stream remain poorly understood. We present quantitative reconstructions of Holocene precipitation derived from 101 fossil pollen records and analyse them with the help of a minimal empirical model. We show that the westerly jet-stream axis shifted gradually southward and became less tilted since the middle Holocene. This was tracked by the summer monsoon rain band resulting in an early-Holocene precipitation maximum over most of western China, a mid-Holocene maximum in north-central and northeastern China, and a late-Holocene maximum in southeastern China. Our results suggest that a correct simulation of the orientation and position of the westerly jet stream is crucial to the reliable prediction of precipitation patterns in China and Mongolia.
    Type: Article , PeerReviewed
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  • 10
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    The end of the African humid period as seen by a transient comprehensive Earth system model simulation of the last 8000 years (2020)
    Copernicus Publications (EGU)
    Details
    Publication Date: 2023-02-08
    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 it 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 the 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 rain belt. 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.
    Type: Article , PeerReviewed
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