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  • 1
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    Late Cretaceous climate simulations with different CO2 levels and subarctic gateway configurations: A model – data comparison (2017)
    Wiley
    Details
    Publication Date: 2017-08-29
    Description: We investigate the impact of different CO 2 levels and different subarctic gateway configurations on the surface temperatures during the latest Cretaceous using the Earth System Model COSMOS. The simulated temperatures are compared with the surface temperature reconstructions based on a recent compilation of the latest Cretaceous proxies. In our numerical experiments, the CO 2 level ranges from 1 to 6 x the pre-industrial (PI) CO 2 level of 280 ppm. On a global scale, the most reasonable match between modelling and proxy data is obtained for the experiments with 3 to 5 x PI CO 2 concentrations. However, the simulated low (high) latitude temperatures are too high (low) as compared to the proxy data. The moderate CO 2 levels scenarios might be more realistic, if we take into account proxy data and the dead zone effect criterion. Furthermore, we test if the model-data discrepancies can be caused by too simplistic proxy-data interpretations. This is distinctly seen at high latitudes, where most proxies are biased towards summer temperatures. Additional sensitivity experiments with different ocean gateway configurations and constant CO 2 level indicate only minor surface temperatures changes (〈~1°C) on a global scale, with higher values (up to ~8°C) on a regional scale. These findings imply that modelled and reconstructed temperature gradients are to a large degree only qualitatively comparable, providing challenges for the interpretation of proxy data and/or model sensitivity. With respect to the latter, our results suggest that an assessment of greenhouse worlds is best constrained by temperatures in the mid latitudes.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Enhanced North Pacific deep-ocean stratification by stronger intermediate water formation during Heinrich Stadial 1 (2019)
    Springer Nature
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    Publication Date: 2019-02-08
    Electronic ISSN: 2041-1723
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 3
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    Climate-vegetation modelling and fossil plant data suggest low atmospheric CO2 in the late Miocene (2015)
    Copernicus
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    Publication Date: 2015-12-16
    Description: There is an increasing need to understand the pre-Quaternary warm climates, how climate–vegetation interactions functioned in the past, and how we can use this information to understand the present. Here we report vegetation modelling results for the Late Miocene (11–7 Ma) to study the mechanisms of vegetation dynamics and the role of different forcing factors that influence the spatial patterns of vegetation coverage. One of the key uncertainties is the atmospheric concentration of CO2 during past climates. Estimates for the last 20 million years range from 280 to 500 ppm. We simulated Late Miocene vegetation using two plausible CO2 concentrations, 280 ppm CO2 and 450 ppm CO2, with a dynamic global vegetation model (LPJ-GUESS) driven by climate input from a coupled AOGCM (Atmosphere-Ocean General Circulation Model). The simulated vegetation was compared to existing plant fossil data for the whole Northern Hemisphere. For the comparison we developed a novel approach that uses information of the relative dominance of different plant functional types (PFTs) in the palaeobotanical data to provide a quantitative estimate of the agreement between the simulated and reconstructed vegetation. Based on this quantitative assessment we find that pre-industrial CO2 levels are largely consistent with the presence of seasonal temperate forests in Europe (suggested by fossil data) and open vegetation in North America (suggested by multiple lines of evidence). This suggests that during the Late Miocene the CO2 levels have been relatively low, or that other factors that are not included in the models maintained the seasonal temperate forests and open vegetation.
    Print ISSN: 1814-9324
    Electronic ISSN: 1814-9332
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 4
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    Climate–vegetation modelling and fossil plant data suggest low atmospheric CO2 in the late Miocene (2015)
    Copernicus
    Details
    Publication Date: 2015-06-16
    Description: There is increasing need to understand the pre-Quaternary warm climates, how climate–vegetation interactions functioned in the past, and how we can use this information for understanding the present. Here we report vegetation modelling results for the Late Miocene (11–7 Ma) to study the mechanisms of vegetation dynamics and the role of different forcing factors that influence the spatial patterns of vegetation coverage. One of the key uncertainties is the atmospheric concentration of CO2 during past climates. Estimates for the last 20 million years range from 280 to 500 ppm. We simulated Late Miocene vegetation using two plausible CO2 concentrations, 280 and 450 ppm CO2, with a dynamic global vegetation model (LPJ-GUESS) driven by climate input from a coupled AOGCM (Atmosphere–Ocean General Circulation Model). The simulated vegetation was compared to existing plant fossil data for the whole Northern Hemisphere. For the comparison we developed a novel approach that uses information of the relative dominance of different Plant Functional Types (PFTs) in the palaeobotanical data to provide a quantitative estimate of the agreement between the simulated and reconstructed vegetation. Based on this quantitative assessment we find that pre-industrial CO2 levels are largely consistent with the presence of seasonal temperate forests in Europe (suggested by fossil data) and open vegetation in North America (suggested by multiple lines of evidence). This suggests that during the Late Miocene the CO2 levels have been relatively low, or that other factors that are not included in the models maintained the seasonal temperate forests and open vegetation.
    Print ISSN: 1814-9340
    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 5
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    The effect of a dynamic soil scheme on the climate of the mid-Holocene and the Last Glacial Maximum (2016)
    Copernicus
    Details
    Publication Date: 2016-01-29
    Description: In order to account for coupled climate–soil processes, we have developed a soil scheme which is asynchronously coupled to a comprehensive climate model with dynamic vegetation. This scheme considers vegetation as the primary control of changes in physical soil characteristics. We test the scheme for a warmer (mid-Holocene) and colder (Last Glacial Maximum) climate relative to the preindustrial climate. We find that the computed changes in physical soil characteristics lead to significant amplification of global climate anomalies, representing a positive feedback. The inclusion of the soil feedback yields an extra surface warming of 0.24 °C for the mid-Holocene and an additional global cooling of 1.07 °C for the Last Glacial Maximum. Transition zones such as desert–savannah and taiga–tundra exhibit a pronounced response in the model version with dynamic soil properties. Energy balance model analyses reveal that our soil scheme amplifies the temperature anomalies in the mid-to-high northern latitudes via changes in the planetary albedo and the effective longwave emissivity. As a result of the modified soil treatment and the positive feedback to climate, part of the underestimated mid-Holocene temperature response to orbital forcing can be reconciled in the model.
    Print ISSN: 1814-9324
    Electronic ISSN: 1814-9332
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 6
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    Transient Variability of the Miocene Antarctic Ice Sheet Smaller Than Equilibrium Differences (2019)
    American Geophysical Union
    Details
    Publication Date: 2019-04-16
    Description: During the early to mid-Miocene, benthic δ18O records indicate large ice volume fluctuations of the Antarctic ice sheet (AIS) on multiple timescales. Hitherto, research has mainly focused on how CO2 and insolation changes control an equilibrated AIS. However, transient AIS dynamics remain largely unexplored. Here, we study Miocene AIS variability, using an ice sheet-shelf model forced by climate model output with various CO2 levels and orbital conditions. Besides equilibrium simulations, we conduct transient experiments, gradually changing the forcing climate state over time. We show that transient AIS variability is substantially smaller than equilibrium differences. This reduces the contribution of the AIS to δ18O fluctuations by more than two thirds on a 40-kyr timescale, hence requiring a larger contribution by deep-sea-temperature variability. The growth rates are much slower than the decay rates, which ensures variability around a preferred small state. Finally, if the bedrock topography enlarges the West Antarctic land surface, AIS self-sustenance increases. ©2019. American Geophysical Union. All Rights Reserved.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 7
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    North Atlantic Versus Global Control on Dansgaard‐Oeschger Events (2018)
    American Geophysical Union
    Details
    Publication Date: 2018-12-02
    Description: The classic scenario for the generation of Dansgaard-Oeschger (DO) events assumes a link to changes in the Atlantic Meridional Overturning Circulation (AMOC) induced by North Atlantic freshwater perturbations. Recent proxy data emphasize the existence of leads and lags between DO fingerprints in Greenland and Antarctic records, highlighting the potential of a Southern Hemisphere control on these events. Investigating this possibility, we provide a conceptual model resulting from phase space reconstructions based on the northern and southern ice core records. The resulting patterns closely resemble AMOC hysteresis, consistent with a northern abrupt warming linked to gradual global temperature changes. This suggests that rapid DO warmings associated with abrupt AMOC transitions from a relatively weak (cold stadial) state to a stronger (warm interstadial) state can be controlled by global forcing that can be linked to the Southern Hemisphere, rather than by the end of a local temporary forcing in the North Atlantic. ©2018. American Geophysical Union. All Rights Reserved.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 8
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    Land-–ocean connectivity in southernmost East Africa over the past 270,000 years: A combined multiproxy and modelling approach (2016)
    Elsevier
    Details
    Publication Date: 2016-06-01
    Print ISSN: 1040-6182
    Electronic ISSN: 1873-4553
    Topics: Geography , Geosciences
    Published by Elsevier
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  • 9
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    North Atlantic Versus Global Control on Dansgaard‐Oeschger Events (2018)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 45 (23). 12.991-12.998.
    Details
    Publication Date: 2021-02-08
    Description: The classic scenario for the generation of Dansgaard‐Oeschger (DO) events assumes a link to changes in the Atlantic Meridional Overturning Circulation (AMOC) induced by North Atlantic freshwater perturbations. Recent proxy data emphasize the existence of leads and lags between DO fingerprints in Greenland and Antarctic records, highlighting the potential of a Southern Hemisphere control on these events. Investigating this possibility, we provide a conceptual model resulting from phase space reconstructions based on the northern and southern ice core records. The resulting patterns closely resemble AMOC hysteresis, consistent with a northern abrupt warming linked to gradual global temperature changes. This suggests that rapid DO warmings associated with abrupt AMOC transitions from a relatively weak (cold stadial) state to a stronger (warm interstadial) state can be controlled by global forcing that can be linked to the Southern Hemisphere, rather than by the end of a local temporary forcing in the North Atlantic.
    Type: Article , PeerReviewed
    Format: text
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  • 10
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    Abrupt climate change experiments: The role of freshwater, ice sheet and deglacial warming for the Atlantic meridional overturning circulation (2016)
    Deutsche Gesellschaft für Polarforschung; Alfred-Wegener-Institu für Polar- und Meeresforschung
    In:  Polarforschung, 85 (2). pp. 161-170.
    Details
    Publication Date: 2019-02-01
    Description: In this review paper we summarise a series of numerical abrupt climate change experiments in the context deglaciation. The effects of global warming, deglacial freshwater, and ice sheets for the termination of the last ice age are examined in a model of intermediate complexity and a fully coupled, coarse-resolution climate model. We find that gradual deglacial global warming induces an abrupt strengthening of the Atlantic Meridional Overturning Circulation (AMOC). More generally, if the system is in a bistable window, a linear forcing can yield non-linear AMOC changes. In this sense Northern Hemisphere freshwater hosing only modulates the timing of the AMOC onset. Furthermore, Northern Hemisphere freshwater hosing weakens the AMOC with a potential overshoot, after the freshwater forcing has stopped. Therefore, as a further hypothesis the onset of Bølling/Allerød (B/A) interstadial with warming over Greenland could be related to an increase in AMOC, which is induced by a declining freshwater forcing prior to or in parallel with the transition. In contrast, hosing in the Southern Hemisphere has a relatively minor influence on the AMOC. The associated climate signatures and mechanisms are explored and discussed in this study.
    Type: Article , PeerReviewed
    Format: text
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