How to cite: Schmittner, A., Khatiwala, S., and Cliff, E.: Glacial Ocean Carbon and Oxygen Cycles: Biological Pump or Disequilibrium?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1224, https://doi.org/10.5194/egusphere-egu22-1224, 2022.

Jet streams control hydroclimate variability in the mid-latitudes with important impacts on water availability and human societies. According to future projections, global warming will change jet stream characteristics, including its mean position. Variability of these characteristics on hourly-to-daily timescales is key to understanding the mid-latitudes circulation. Therefore, most analysis methods of present-day jet streams are designed for 6-hourly data. By modelling the climate since the Last Glacial Maximum, we can investigate the long-term drivers of jet stream characteristics. However, for transient simulations of the last deglaciation, 3d wind fields are only archived with a monthly resolution due to storage limitations. Hence, jet variability at shorter timescales cannot be identified, and established methods can’t be used.

Here, we study to what extent changes of jet stream characteristics can be inferred from monthly wind fields. Therefore, we compare latitudinal jet stream positions, strength, tilt and their variability from daily and monthly wind fields in reanalysis data and for LGM and PI simulations. We test three different methods to construct jet stream typologies and metrics. This comparison identifies to which extend these jet stream characteristics can be robustly studied from monthly wind fields. In addition, our analysis assesses the added value of archived daily data for future research. Once the limitations of monthly wind output are known, jet stream characteristics in transient simulations of the last deglaciation can be analysed. This analysis provides new insights on jet stream changes on decadal-to-orbital timescales and identifies the factors controlling these changes.

How to cite: Schoch, P., Baudouin, J.-P., Weitzel, N., Kapsch, M., Kleinen, T., and Rehfeld, K.: Characterising simulated changes of jet streams since the Last Glacial Maximum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5069, https://doi.org/10.5194/egusphere-egu22-5069, 2022.

How to cite: Ivanovic, R., Gregoire, L., Astfalk, L., Williamson, D., Gandy, N., Burke, A., and Schmidt, D.: Quantifying uncertainties in global monthly mean sea surface temperature and sea ice at the Last Glacial Maximum , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2496, https://doi.org/10.5194/egusphere-egu22-2496, 2022.

The Last Glacial Maximum (23,000 – 19,000 years ago; LGM) is the most recent time when Earth’s climate was fundamentally different from today. The LGM hence remains a prime target to evaluate climate models outside current boundary conditions. Evaluation of paleoclimate simulations is usually done using proxy-based reconstructions. However, such reconstructions are indirect and associated with marked uncertainty, which often renders model-data comparison equivocal. Here we take a different approach and use macro-ecological patterns preserved in fossil marine zooplankton to evaluate simulations of LGM near-surface ocean temperature.

We utilise the distance-decay pattern in planktonic foraminifera to evaluate modelled temperature gradients. Distance decay emerges because of differences in habitat preferences among species that cause the compositional similarity between assemblages to decrease the further apart they are from each other in environmental space. Distance decay is a fundamental concept in ecology and is observed in many different taxa and ecosystems, including planktonic foraminifera that show a monotonous decrease in similarity with increasing difference in temperature. Because the ecological niches of planktonic foraminifera are unlikely to have changed since the LGM, the distance-decay relationship based on simulated LGM temperatures and LGM assemblages should in principle be identical to the modern distance decay pattern. Thus we can use fossil planktonic foraminifera species assemblages to evaluate climate model simulations based on ecological principles.

Our analysis is based on an extended new LGM planktonic foraminifera database (2,085 assemblages from 647 unique sites) and a suite of 10 simulations from state-of-the-art climate models (PMIP3 and 4). We find markedly different planktonic foraminifera distributions during the LGM, primarily due to the equatorward expansion of polar assemblages at the expense of transitional assemblages. The distance-decay pattern that emerges when the LGM assemblages are combined with simulated ocean temperatures is different from the modern pattern. All simulations suggest large thermal gradients between regions where the planktonic foraminifera indicate no, or only weak, gradients. This pattern arises from the pronounced shift to polar species assemblages in the North Atlantic where the simulations predict only moderate cooling. In general, the models predict spatially rather uniform cooling, whereas the microfossil evidence suggests more pronounced regional differences in the temperature change. The difference between reconstructions and the simulations reaches up to 10 K in the North Atlantic.

Importantly, simulations with a reduced AMOC and hence lower North Atlantic near sea surface temperatures, yield a distance-decay pattern that is much more similar to the modern pattern. The planktonic foraminifera assemblages thus question the view of the LGM ocean as an equilibrium response to external forcing.

How to cite: Jonkers, L., Laepple, T., Rillo, M., Dolman, A., Lohman, G., Paul, A., Mix, A., and Kucera, M.: Modelled equilibrium LGM seawater temperatures inconsistent with plankton biodiversity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8364, https://doi.org/10.5194/egusphere-egu22-8364, 2022.

The Atlantic overturning circulation plays a critical role in inter-hemispheric transport of heat, carbon, and nutrients, and its potential collapse under anthropogenic forcing is thought to be a major tipping point in the climate system. As such, painstaking efforts have been dedicated to a better understanding of the Atlantic circulation’s past variability and mean-state under different boundary conditions. Yet, despite decades of research many uncertainties remain regarding the state of the ocean circulation over the past 20,000 years, during which Earth’s climate was propelled out of the last ice age. Here, we employed the Bern3D intermediate complexity model, which is equipped with all major water mass tracers (Δ¹⁴C, δ¹³C, δ¹⁸O, εNd, Pa/Th, nutrients, and temperature), to search for converging constraints on the often conflicting interpretations of paleo-reconstructions from individual proxies focusing on the Last Glacial Maximum (LGM). By varying formation rates of northern- and southern-sourced waters we explore a wide range of circulation states and test their ability to reproduce the spatial patterns of newly compiled proxy data of the LGM. Generally, we find that late-Holocene to LGM anomalies give more consistent pictures of proxy distributions than absolute values, since systematic biases, that plague some of the proxies, cancel out. This has the additional advantage that also systematic model biases are minimized. Considering this, we find that the previously opposing neodymium and stable carbon isotope-based interpretations of the glacial water mass structure can be reconciled when non-conservative effects are appropriately taken into account. Furthermore, combining the information from all proxies indicates some shoaling of glacial northern-sourced water, yet not to the same extent as previous studies suggested.

How to cite: Pöppelmeier, F., Jeltsch-Thömmes, A., Joos, F., Scheen, J., Lippold, J., and Stocker, T.: Converging constraints on the glacial Atlantic overturning circulation from multiple proxies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-784, https://doi.org/10.5194/egusphere-egu22-784, 2022.

Under continued high anthropogenic CO₂ emissions, the atmospheric CO₂ concentration around 2100 will be like that of the Early Eocene Climate Optimum (EECO, 56–48 Ma) hothouse period. Hence, reconstructions of the EECO climate give insight into the workings of the climate system under the possible future CO₂ conditions. Our current understanding of global mean surface temperature (GMST) during the Cenozoic era relies on paleo-proxy estimates of deep-sea temperature (DST) combined with assumed relationships between global mean DST (GMDST), global mean sea-surface temperature (GMSST), and GMST. The validity of these assumptions is essential in our understanding of past and future climate states under hothouse conditions.
We analyse the relationship between these global temperature indicators for the end-of-simulation global mean temperature values in 25 different millennia-long model simulations of the EECO climate under varying CO₂ levels, performed as part of the Deep-Time Model Intercomparison Project (DeepMIP). The model simulations show limited spatial variability in DST, indicating that local DST estimates can be regarded representative of GMDST. Linear regression analysis indicates that GMDST and GMST respond stronger to changes in atmospheric CO₂ than GMSST by factors 1.18 and 1.17, respectively. Consequently, the responses of GMDST and GMST to atmospheric CO₂ changes are similar in magnitude. This model-based analysis indicates that changes in GMDST can be used to estimate changes in GMST during the EECO, validating the assumed relationships. To test the robustness of these results, other Cenozoic climate states besides EECO should be analysed similarly.

How to cite: Goudsmit, B., Lansu, A., von der Heydt, A. S., Zhang, Y., and Ziegler, M.: The relationship between the global mean deep-sea and surface temperature during the Early Eocene , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9897, https://doi.org/10.5194/egusphere-egu22-9897, 2022.

It is well accepted that global circulation models equipped with stable water isotopologues help to better understand the relationships between atmospheric circulation changes and isotope records in paleoclimate archives. Still, isotope-enabled models do not allow to precisely understand the processes affecting precipitation isotopic compositions, such as changes in precipitation amounts or moisture sources. Furthermore, the relevance of this model-oriented approach relies on the realism of modeled isotope results, that would support the interpretation of the records in terms of modeled climate changes. In order to alleviate these limitations, the newly developed WRF-Hydro-iso-tag, that is the version of the isotope-enabled regional coupled model WRF-Hydro-iso enhanced with an isotope tracing procedure, is presented. Physics-based WRF-Hydro-iso-tag ensembles are used to regionally downscale the isotope-enabled Community Earth System Model for Southern Africa, for two 10-year slices of mid-Holocene and pre-industrial times. The isotope tracing procedure is tailored in order to assess the origin of the hydrogen-isotope deuterium contained in Southern African precipitation, between two moisture sources that are the Atlantic and Indian Oceans. In comparison to the global model, WRF-Hydro-iso-tag simulates lower precipitation amounts with more regional details, and mid-Holocene-to-pre-industrial changes in precipitation isotopic compositions that better match plant-wax deuterium records from two marine sediment cores off the Orange and Limpopo River basins. Linear relationships between mid-Holocene-to-pre-industrial changes in temperature, precipitation amount, moisture source and precipitation deuterium compositions are derived from the ensembles results.

How to cite: Arnault, J., Niezgoda, K., Jung, G., Hahn, A., Zabel, M., Schefuss, E., and Kunstmann, H.: Disentangling the contribution of moisture source change to isotopic proxy signatures: Deuterium tracing with WRF-Hydro-iso-tag and application to Southern African Holocene sediment archives, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-913, https://doi.org/10.5194/egusphere-egu22-913, 2022.

The climate of the last 2500 years is documented in natural (speleothems, tree rings, sediments and pollen) and human-historical archives. Proxy records and subsequent climate reconstructions can be subject to a considerable amount of uncertainty, as the proxies can only capture a fraction of the entire variability. Climate model simulations can contribute to the interpretation of variations observed in the paleoclimate data and better understanding of dynamics, mechanisms and procedures. The state-of-the-art simulations following the CMIP6-protocol are highly resolved in time but still present a rather coarse horizontal resolution (200 km or more) to adequately address regional paleoclimate questions/hypotheses. Dynamical downscaling can close the gap between the regional archives and the coarsely resolved Earth System Models (ESMs). Using regional climate models to downscale ESM output requires a consistent implementation of the climate forcings in the regional model used also for the driving ESM. State-of-the-art and CMIP6 compliant reconstructions of volcanic (stratospheric aerosol optical depth), orbital (eccentricity, obliquity, precession), solar (irradiance), land-use and greenhouse-gas changes used for the MPI-ESM are therefore implemented in the regional climate model COSMO-CLM (CCLM, COSMO 5.0 clm16). The functionality of each implemented forcing is tested separately and in combination for the period (1255-1265) that covers the Samalas volcanic eruption of 1257. The orbital forcing is found to have the largest impact in general and the volcanic forcing has a strong but short-lasting effect after the eruption. The other climate forcings only show very small impact in the chosen period. At the moment, a transient CCLM simulation with all forcings implemented with a horizontal resolution of 50 km is running for the last 2500 years in the Eastern Mediterranean, the Middle East and the Nile River basin.

How to cite: Hartmann, E., Zhang, M., Xoplaki, E., Wagner, S., and Adakudlu, M.: Implementation of Climate Forcings (volcanic, orbital, solar, LUC, GHG) for Paleoclimate Simulations (500BCE-2000CE) in the COSMO-CLM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8423, https://doi.org/10.5194/egusphere-egu22-8423, 2022.

The use of paleoclimates to constrain the equilibrium climate sensitivity (ECS) has seen a growing interest. In particular, the Last Glacial Maximum (LGM) and the mid-Pliocene Warm Period have been used in emergent constraint approaches using simulations from the Paleoclimate Modelling Intercomparison Project (PMIP). Despite lower uncertainties regarding geological proxy data for the LGM in comparison with the Pliocene, the robustness of the emergent constraint between LGM temperature and ECS is weaker at both global and regional scales. Here, we investigate the climate of the LGM in models through different PMIP generations, and how various factors contribute to the spread of the model ensemble. Certain factors have large impact on an emergent constraint, such as state-dependency in climate feedbacks or model-dependency on ice sheet forcing. Other factors, such as models being out of energetic balance and sea-surface temperature not responding below -1.8°C in polar regions have a restricted influence. We quantify some of the contributions and show they mostly have extratropical origins, which contribute to a weak global constraint, and remotely impact tropical temperatures. Statistically, PMIP model generations do not differ substantially, unlike what has been previously suggested. Furthermore, we find that the lack of high or low ECS models in the ensembles critically limits the strength and reliability of the emergent constraints.

How to cite: Renoult, M., Sagoo, N., Zhu, J., and Mauritsen, T.: Causes of the weak relationship between modeled Last Glacial Maximum cooling and climate sensitivity, with consequences for emergent constraints, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4582, https://doi.org/10.5194/egusphere-egu22-4582, 2022.

Paleoclimatic changes in South Africa, especially around the southern Cape region, are of intense interdisciplinary interest; as this is an important area in the context of human evolution, hosting a number of prominent archaeological sites such as Klipdrift Shelter and Blombos Cave (both located near today’s shoreline). Questions surrounding how large-scale and local variability (and change) influenced the local human populations are abundant. Here we present results from downscaling simulations performed for southern Africa, with a high resolution (12 km) regional climate model (WRF), forced by a global earth system model (NorESM). We focus on two time-slices, 82 and 70 ka BP, when orbital parameters and global sea level were markedly different from each other. Changes from 82 to 70 ka BP are generally in line with orbital forcing; indicating, for example, a widespread and significant (> 40%) increase in summer precipitation over inland southern Africa (south of 15°S) due to higher insolation at 70 ka BP compared to 82 ka BP. In contrast, the western and southern Cape coasts became drier at 70 ka BP, owing in part to a 40 m lower sea level, as the coastline shifted and the paleo-Agulhas plain got exposed. The effect of the coastline shift on temperatures in the southern Cape region is evident from the significant (up to 6°C) increases (decreases) in maximum (minimum) temperatures, which are strong enough to overwhelm changes arising from orbital forcing. These inferences are further supported with a separate set of coastline-sensitivity simulations at 70 ka BP, which indicate not only drying, but also larger diurnal and interseasonal temperature ranges when the coastline extends southwards, and once-coastal areas become more continental. For instance, at the archaeological site of Blombos Cave, temperature extremes (1st and 99th percentiles) of the modelled marine climate become 25 to 50-fold more probable to occur as the coastline shift leads to a continental climate. Our results indicate that regional to local-scale processes, which tend to not be represented in most coarse resolution global models, have a strong influence on the paleoclimate of southern Africa, highlighting both the coastal-inland contrasts and the importance of changes in coastline position. 

How to cite: Göktürk, O. M., Sobolowski, S. P., Simon, M. H., Zhang, Z., and Jansen, E.: Modelling the regional paleoclimate of southern Africa: Sub-orbital-scale changes and sensitivity to coastline shifts, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10449, https://doi.org/10.5194/egusphere-egu22-10449, 2022.

The interpretation of East Asian monsoon speleothem δ¹⁸O records is heavily debated in the paleoclimate community. Besides developing new speleothem proxies, the use of isotope-enabled climate simulations is one of the key tools to enhance our understanding of speleothem δ¹⁸O records. Here we present results from novel climate simulations performed with the fully coupled isotope-enabled Community Earth System Model (iCESM1.2), which simulates global variations in water isotopes in the atmosphere, land, ocean, and sea ice. The model closely captures the major observed features of the isotopic compositions in precipitation over East Asia for the present-day conditions. To better understand the physical mechanisms causing interannual to orbital timescale variations in δ¹⁸O in East Asian speleothems, we ran a series of experiments with iCESM. We perturbed solar, orbital, bathymetry, ice-sheet, and greenhouse gas radiative forcings. The simulations supporting of observations/reconstructed records (GNIP/SISAL) from East Asia, help understand the controls on the isotope composition of East Asian monsoon rainfall and how speleothem δ¹⁸O records may be interpreted in terms of climate. The study provides new insights into the mechanisms of East Asian monsoon changes on different timescales.

How to cite: Sinha, N., Timmermann, A., Wessenburg, J. A., and Lee, S.-S.: Understanding climate, precipitation and δ18O linkages over Eastern Asia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10715, https://doi.org/10.5194/egusphere-egu22-10715, 2022.

How to cite: Ziegler, E., Wirths, C., Andres, H., Gregoire, L., Ivanovic, R., Kapsch, M.-L., Kutterolf, S., Mikolajewicz, U., Schindlbeck-Belo, J. C., Toohey, M., Valdes, P. J., Weitzel, N., and Rehfeld, K.: Temperature and precipitation distribution changes in response to global warming – results from transient simulations of the Last Deglaciation from a hierarchy of climate models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8788, https://doi.org/10.5194/egusphere-egu22-8788, 2022.

The instability with respect to global glaciation is a fundamental property of the climate system caused by the positive ice-albedo feedback. The atmospheric concentration of carbon dioxide (CO₂) at which this Snowball bifurcation occurs changes through Earth’s history because of the slowly increasing solar luminosity. Quantifying this critical CO₂concentration is not only interesting from a climate dynamics perspective, but also an important prerequisite for understanding past "snowball Earth" episodes and the conditions for habitability on Earth and other planets. Earlier studies are limited to investigations with very simple climate models for Earth’s entire history or studies of individual time slices carried out with a variety of more complex models and for different boundary conditions, making comparisons difficult. Here we use a coupled climate model of intermediate complexity to trace the Snowball bifurcation of an aquaplanet through Earth’s history in one consistent model framework. We find that the critical CO₂concentration decreases more or less logarithmically with increasing solar luminosity until about 1 billion years ago, but drops faster in more recent times. Furthermore, there is a fundamental shift in the dynamics of the critical state about 1.8 billion years ago, driven by the interplay of wind-driven sea-ice dynamics and the surface energy balance.

How to cite: Feulner, G. and Bukenberger, M.: Tracing the snowball bifurcation of aquaplanets through time reveals a fundamental shift in critical-state dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11090, https://doi.org/10.5194/egusphere-egu22-11090, 2022.

Glacial inception represents a bifurcation transition between interglacial and glacial states and is governed by the non-linear dynamics of the climate-cryosphere system. It has been previously proposed that to trigger glacial inception, the orbital forcing defined as the maximum of summer insolation at 65^oN and determined by Earth’s orbital parameters must be lower than a critical level. This critical level depends on the atmospheric CO₂ concentration. While paleoclimatic data do not constrain the critical dependence, its accurate estimation is of fundamental importance for predicting future glaciations and the effect that anthropogenic CO₂ emissions might have on them. 

In this study we use the new Earth system model of intermediate complexity CLIMBER-X (which includes modules for atmosphere, ocean, land surface, sea ice and the new version of the 3-D polythermal ice sheet model SICOPOLIS) to estimate the critical orbital forcing - CO₂ relationship for triggering glacial inception. We perform a series of experiments in which different combinations of orbital forcing and atmospheric CO₂ concentration are maintained constant in time. Each model simulation is run for 1 million years using an acceleration technique with asynchronous coupling between the climate and ice sheet model components. SICOPOLIS is applied only to the Northern Hemisphere with a 40 km horizontal resolution.

We analyse for which combinations of orbital forcing and CO₂ glacial inception occurs and trace the critical relationship between them, separating conditions under which glacial inception is possible from those where glacial inception is not materialised. We study how adequate it is to use the maximum summer insolation at 65°N as a single metric for orbital forcing, as well as consider the differential effect each one of Earth’s orbital parameters might have. In addition, we investigate the spatial and temporal patterns of ice cover during glacial inception under different orbital forcings.

How to cite: Talento, S., Willeit, M., Calov, R., Höning, D., and Ganopolski, A.: New estimation of critical orbital forcing – CO2 relationship for triggering of glacial inception, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7379, https://doi.org/10.5194/egusphere-egu22-7379, 2022.

Changes between icehouse and greenhouse states are known to be the result from non-linear climate responses. However, the magnitudes of these responses are not well constrained. Recent work shows that climate models, specifically the Community Earth System Model version 1 (CESM1), have improved substantially in their capacity to quantify the Last Glacial Maximum (LGM) state. Given that CESM1 can reproduce the LGM well, we consider the combined impacts of estimated ice sheet heights, Quaternary orbits, and greenhouse gas changes for a range of Quaternary climate states. To that end, we conducted two sets of experiments: first, a series of sensitivity experiments on the Preindustrial climate and second, experiments on Quaternary glacial states.

In the first set of the experiments, we show how CESM1 quantifies the impacts of ice height, orbit, and greenhouse gas changes by considering each component incrementally. Then we demonstrate that they combine through non-linear impacts. The analysis is based on seven sensitivity experiments: 1) Late Holocene orbit, 2) Representative Concentration Pathway 8.5 (RCP85) greenhouse gases, 3) LGM orbit, 4) LGM greenhouse gasses, and 5) Greenland icesheet height changes, 6) LGM orbit with Greenland icesheet height changes, and 7) LGM orbit with LGM greenhouse gases and Greenland icesheet height changes. We show that adding individually these component changes do not linearly combine to match the simulations with combined changes.

These non-linear effects guide the second set of experiments, because non-linear systems are predictable due to state dependent outcomes. We use of 4 glacial ice sheet height differences and 4 glacial maximum orbital states (LGM, and Marine Isotopic Stage 4,6, and 8), for a total of 16 sensitivity experiments. These orbits are known glacial maximal states, and the 4 ice sheet heights are within the range of estimated ice volumes. We analyze these simulations in two ways, 1) the explicit effect of changes in orbit while holding the ice sheet constant, and 2) the explicit effect of changes in ice sheet height, while holding the orbit constant.

Our results show that ice sheet heights dominate the changes in climate system, regardless of orbit. But, there are subtle regional effects that orbit has that are not explained by ice sheet height changes. For example, higher ice sheets induce a global temperature increase, but regionally within Europe, there are non-linear changes in warming or cooling that are unexplained by the ice sheets. As the ice sheet height is lowered, the changes in Europe do not linearly change, and are dependent on the orbit configuration.

These results show that there are specific pathways for climate that occur due to the combination of icesheet height and orbit, and theoretically imply a constraint on the real climate state. In a linear system, these 16 states would represent the variability of the Quaternary, but as this is a non-linear system only 1 state is physical for a given orbit. As proxy data spatial and temporal resolution improves for the Quaternary, combined with these modeled climates, we expect substantial constraints on the available realistic climate states.

How to cite: Buzan, J., Russo, E., Kim, W., and Raible, C.: Sensitivity of glacial states to orbits and ice sheet heights in CESM1.2, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11116, https://doi.org/10.5194/egusphere-egu22-11116, 2022.

How to cite: Bretones, A., Nisancioglu, K. H., and Guo, C.: Changes in Arctic Meridional Overturning (ArMOC) under past abrupt warming , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5710, https://doi.org/10.5194/egusphere-egu22-5710, 2022.

The ¹³C/¹²C of dissolved inorganic carbon (δ¹³C DIC ) carries valuable information on ocean
biological C-cycling, air-sea CO₂ exchange, and circulation. Paleo-reconstructions of oceanic ¹³C
from sediment cores provide key insights into past as changes in these three drivers. As a step
toward full inclusion of ¹³C in the next generation of Earth system models, we implemented ¹³C-
cycling in a 1° lateral resolution ocean-ice-biogeochemistry Geophysical Fluid Dynamics
Laboratory (GFDL) model driven by Common Ocean Reference Experiment perpetual year
forcing. The model improved the mean of modern δ¹³C DIC over coarser resolution GFDL-model
implementations, capturing the Southern Ocean decline in surface δ¹³C DIC that propagates to the
deep sea via deep water formation. The model is used here to quantify controls on modern and
anthropogenic δ¹³C DIC as well as to test their sensitivity to wind speed/gas exchange
parameterizations.
We found that reducing the coefficient for air-sea gas exchange following OMIP-CMIP6
protocols reduces deep sea modern δ¹³C DIC by 0.2 permil and improves the depth-integrated
anthropogenic δ¹³C DIC relative to previous gas exchange parameterizations. This is because the
δ¹³C DIC of the endmembers ventilating the deep sea and intermediate waters are highly sensitive to
the wind speed dependence of the air-sea CO₂ gas exchange. Additionally, meridional gradients
of surface modern δ¹³C DIC are better resolved with OMIP-CMIP6. While this model was initially
constructed to study the anthropogenic ¹³C response, it has promising applications toward longer
time scales. For example, BLING 13 C includes controls on the biological C-pump thought to be
important in the glacial ocean: light and iron limitation, and controls on ¹³C of organic matter
formation, and thus on ocean δ¹³C DIC and its vertical gradient, that depend on pCO₂ .

How to cite: Sonnerup, R. and Claret, M.: A Next Generation Ocean Carbon Isotope Model for Climate Studies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8892, https://doi.org/10.5194/egusphere-egu22-8892, 2022.

The main objective of this study is to develop and test a method of bias correction for paleoclimate model simulations using the “Cumulative Distribution Functions – transform” (CDF-t) method. The CDF-t is a quantile-mapping based method, extended to account for climate change signal. Here we apply the CDF-t to climate model outputs for the Mid-Holocene and the Last Glacial Maximum, simulated by the climate model of intermediate complexity iLOVECLIM at 5.625° resolution. Additionally, we test the proposed methodology on iLOVECLIM model outputs dynamically downscaled on a  0.25° resolution.

The results are validated through inverse and forward modelling approaches. The inverse approach implies comparing the obtained results with proxy-based reconstructed climatic variables. Here we use temperature and precipitation reconstructions, obtained with inverse modelling methods from pollen data. In this study, both gridded and point-based multi-proxy reconstruction datasets were used for the analysis.

The forward approach includes a further step of vegetation modelling, using the climatologies derived from bias-corrected outputs of the iLOVECLIM model in CARAIB (CARbon Assimilation In the Biosphere) global dynamic vegetation model. The modelled biomes are evaluated in comparison with pollen-based biome reconstructions BIOME6000.

The findings of this study indicate that the use of the proposed methodology results in significant improvements in climate and vegetation modelling and suggest that the CDF-t method is an valuable approach to reduce biases in paleoclimate modelling.

How to cite: Zapolska, A., Vrac, M., Quiquet, A., Arthur, F., Renssen, H., François, L., and Roche, D. M.: Validation of a CDF-t bias correction method using palaeo-data for the Mid-Holocene and the Last Glacial Maximum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4376, https://doi.org/10.5194/egusphere-egu22-4376, 2022.

Recent progress in modelling the Earth system has made it possible to produce transient climate simulations longer than 10.000 years with comprehensive ESMs. These simulations improve our understanding of slow climatic feedbacks, climate state transitions, and abrupt climate changes. However, assessing the quality and reliability of such paleoclimate simulations is particularly challenging due to the inherent characteristic differences between model data and the climate reconstructions used to validate them.

Here, we present a collection of software packages for inter-model and model-data comparisons called Palaeo ToolBox (PTBox). Its first intent is to evaluate transient simulations of the PalMod project (deglaciation, glacial inception, MIS3) using several proxy data syntheses. Various variables are evaluated (including temperature, precipitation, oxygen isotopes, vegetation, carbon storages and fluxes), across a range of timescales (from decadal to multi-millenial). PTBox provides integrated model-data workflows, from data pre-processing to visualisations, organised into a series of (mostly R) packages. So far, PTBox includes 1) tools for pre-processing simulations and proxy data, 2) ensemble and pseudo-proxy methods to bridge the gap between simulations and proxies and to quantify uncertainties, 3) spectral methods to analyse timescale-dependent climate variability, and 4) newly developed metrics for spatio-temporal model-data comparisons.

Finally, PTBox is accompanied by a website (http://palmodapp.cloud.dkrz.de/) with examples on how to use PTBox and interactive visualisations of the datasets produced in the PalMod project.

How to cite: Baudouin, J.-P., Bothe, O., Chevalier, M., Ellerhoff, B., Adam, M., Schoch, P., Weitzel, N., and Rehfeld, K.: PTBox, a toolbox to facilitate palaeoclimate model-data analyses, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3684, https://doi.org/10.5194/egusphere-egu22-3684, 2022.

An increasing number of Earth System Models has been used to simulate the climatic transition from the Last Glacial Maximum to the Holocene. This creates a demand for benchmarking against environmental proxy records, which have been synthesized for the same time period. Comparing these two data sources in space and time over a period with changing background conditions requires new methods. We employ proxy system modeling for probabilistic quantification of the deviation between temperature reconstructions and transient simulations. Regional and global scores quantify the mismatch in the pattern and magnitude of orbital- as well as millennial-scale temperature variations.

In pseudo-proxy experiments, we test the ability of our algorithm to accurately rank an ensemble of simulations according to their deviation from a prescribed temperature history, dependent upon the amount of added non-climatic noise. To this purpose, noisy pseudo-proxies are constructed by perturbing a reference simulation. We show that the algorithm detects the main features separating the ensemble members. When scores are aggregated spatially, the algorithm ranks simulations robustly and accurately in the presence of uncertainties. In contrast, erroneous rankings occur more often if only a single location is assessed.

Having established the effectiveness of the algorithm in idealized experiments, we apply our method to quantify the deviation between data from the PalMod project: an ensemble of transient deglacial simulations and a global compilation of sea surface temperature reconstructions. No simulation performs consistently well across different regions and components of the temperature evolution which we attribute to the larger spatial heterogeneity in reconstructions. Our work provides a basis for a standardized model-data comparison workflow, which can be extended subsequently with additional proxy data, new simulations, and improved representations of uncertainties.

How to cite: Weitzel, N., Andres, H., Baudouin, J.-P., Bothe, O., Dolman, A. M., Jonkers, L., Kapsch, M., Kleinen, T., Mikolajewicz, U., Paul, A., and Rehfeld, K.: Towards spatio-temporal comparison of transient simulations and temperature reconstructions for the last deglaciation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6562, https://doi.org/10.5194/egusphere-egu22-6562, 2022.

The forest expansion in the Northern Hemispheric extra-tropics during the deglaciation, i.e. the last some 22,000 years, starts earlier and occurs much faster in our model simulation using the MPI-ESM 1.2 than in the recently published synthesis of biome reconstructions by Cao et al. (2019). As a result, the simulated Northern Hemisphere maximum in forest cover is reached at 11ka in the model, whereas the forest distribution peaks substantially later (at 7ka in the spatial mean) in the reconstructions. The model-data mismatch is largest in Asia, particularly in Siberia and the East Asian monsoon margin. The simulated temperature trend is in line with pollen-independent temperature reconstructions for Asia. Since the simulated vegetation adapt to the simulated climate within decades, the temporal model-data mismatch with respect to the forest cover may indicate that pollen records are not in equilibrium with climate on multi-millennial timescales.

Our study has some far-reaching consequences. Pollen-based vegetation and climate reconstructions are commonly used to evaluate Earth System Models against past climate states, but to what extent the reconstructed vegetation is in equilibrium with the climate at the reconstructed time slice is still a matter of discussion. Our results raise the question on which time-scales pollen-based reconstructions are reliable. Although, it is so far not possible to identify the causes of the mismatch between our simulations and the reconstruction, we suggest critical re-assessment of pollen-based climate reconstructions. Last, but not least, our results may also point to a much slower response of forest biomes to current and future ongoing climate changes than Earth System Models currently predict.

References:

Cao, X., Tian, F., Dallmeyer, A. and Herzschuh, U.: Northern Hemisphere biome changes (>30°N) since 40 cal ka BP and their driving factors inferred from model-data comparisons, Quat. Sci. Rev., 220, 291–309, doi:10.1016/j.quascirev.2019.07.034, 2019.

How to cite: Dallmeyer, A., Kleinen, T., Claussen, M., Weitzel, N., Cao, X., and Herzschuh, U.: The deglacial forest conundrum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6979, https://doi.org/10.5194/egusphere-egu22-6979, 2022.