The global carbon cycle is a complex system with many drivers, including slow ones such as the chemical weathering of rocks. At long enough timescales, changes in weathering rates influence CO₂ consumption, but also the river loads of carbon, nutrients, and alkalinity. In particular, the global ocean inventory of alkalinity is a critical driver of carbon sequestration into the ocean. Thus, any transitory imbalance between the sources and sinks of alkalinity can lead to changes in ocean chemistry and impact atmospheric CO₂ concentration. During the last deglaciation (ca. 19-11 ka BP), the Earth’s climate transitioned from cold and arid to comparatively warmer and wetter conditions. Simultaneously, large ice sheets melted and led to a significant rise of sea level (ca. +120 m), which reduced the size of the exposed continental shelves. Loess deposits were also gradually eroded. These changes logically influenced the chemical weathering of rocks because weathering rates depend on climate variables (runoff and temperature), land-sea distribution and lithology. Some modelling studies and proxy reconstructions suggest little net changes over this period. Yet, the deglacial changes of weathering rates remain poorly constrained.

Most Earth System Models do not explicitly represent weathering and the consequent river fluxes. Moreover, the alkalinity inventory is often assumed constant in models, despite the fact that proxy data suggest an elevated total alkalinity at the Last Glacial Maximum (and the likely changes of its sources and sinks). These choices can potentially bias the model representation of the global carbon cycle, whose deglacial variations have been notoriously hard to simulate for decades. In this study, we calculate weathering fluxes of phosphorus and alkalinity (among others) using reconstructed lithological maps, and model results from transient runs of the last deglaciation and/or time-slice runs of the Last Glacial Maximum and pre-industrial period. To improve robustness, we compare the evolution and spatial distribution of weathering fluxes in different models. We demonstrate that while the increase of runoff during deglaciation enhances weathering, the rise of sea level and the erosion of loess deposits tend to have a counterbalancing effect on the river loads. Our model ensemble tends to show inconsistent deglacial changes of some river loads (e.g. for phosphorus), depending both on runoff biases and on the representation of land-sea distribution. Still, all models indicate a significant decrease of river alkalinity from the LGM to the pre-industrial. Using these findings, we discuss the implications of an explicit representation of weathering fluxes for the global carbon cycle in transient runs with Earth System Models.

How to cite: Lhardy, F., Liu, B., Willeit, M., Bouttes, N., Kurahashi-Nakamura, T., Hagemann, S., and Ilyina, T.: Multimodel comparison of weathering fluxes during the last deglaciation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8404, https://doi.org/10.5194/egusphere-egu23-8404, 2023.

The marine biological carbon pump substantially contributes to the glacial-interglacial CO2 change. Compared to the late Holocene, proxy data for the Last Glacial Maximum (LGM) generally agree on an increased export production, associated with an enhanced marine biological carbon pump, in the subantarctic region of the Southern Ocean (SO). By contrast, global export production during the LGM is poorly constrained due to the sparseness and uncertainty of proxy data. The efficiency of the biological pump is mainly controlled by phytoplankton growth, ocean circulation and the sinking and remineralisation of organic matter. Previous modelling studies primarily focused on the sensitivity regarding the former two factors. By far, few studies have discussed the impact of marine particle sinking on glacial ocean biogeochemistry.

In this study, we examine the impact of two different sinking schemes for biogenic particles on the LGM ocean biogeochemistry in the Max Planck Institute Earth System Model (MPI-ESM). In the default sinking scheme, sinking velocities of particulate organic matter (POM), biogenic minerals (CaCO3 and opal) and dust are prescribed and kept the same between LGM and pre-industrial (PI) state. Such a scheme is also widely applied in other ocean biogeochemical models. In a new Microstructure, Multiscale, Mechanistic, Marine Aggregates in the Global Ocean (M4AGO) sinking scheme, the size, microstructure, heterogeneous composition, density and porosity of marine aggregates, consisting of POM, CaCO3, opal and dust, are explicitly represented, and the sinking speed is prognostically computed. We discuss the effect of the two particle sinking schemes under two LGM circulation states: “deep LGM AMOC” with a similar NADW/AABW boundary compared to PI, which is produced in many existing models, and “shallow LGM AMOC” with a shallower NADW/AABW boundary, which agrees better with proxy data. Furthermore, we conducted sensitivity studies regarding LGM dust deposition as the latter is subject to considerable uncertainties.

We find that for the deep LGM AMOC, the difference between the impact of the two particle sinking schemes on the ocean biogeochemical tracers is small. On the contrary, for shallow LGM AMOC, the M4AGO scheme yields more remerineralised carbon in the deep ocean and, therefore, better agreement with δ¹³C data, suggesting the quantitative impact of particle sinking schemes strongly depends on the background LGM circulation state. For the default sinking scheme, increased glacial dust deposition increases iron fertilisation and thus leads to a rise in both primary production and export production. For the M4AGO scheme, however, the iron fertilisation effect is surpassed by the ballasting effect that reduces the surface nutrient concentration, and LGM primary production decreases with dust deposition. This preliminary result shows that the new marine aggregate sinking scheme adds further complexities to the marine biological carbon pump response to the climate states. Our further analysis will encompass the other nutrients and dissolved oxygen, as well as the comparison to corresponding proxy data. 

How to cite: Liu, B., Maerz, J., and Ilyina, T.: Sensitivity of the glacial marine biological pump to particle sinking and dust deposition in MPI-ESM, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12139, https://doi.org/10.5194/egusphere-egu23-12139, 2023.

Paleoclimate proxy records from the North Atlantic region reveal substantially greater multi-centennial temperature variability during the Last Glacial Maximum (LGM) compared to the current interglacial. As there was no obvious change in external forcing, causes for the increased variability remain unknown. Here we provide a mechanism for enhanced multi-centennial North Atlantic climate variability during the LGM based on experiments with the coupled climate model CESM. The model simulates an internal mode of multi-centennial variability, which is associated with variations in the Atlantic meridional overturning circulation. In accordance with high-resolution proxy records from the glacial North Atlantic, this mode induces highest surface temperature variability in subpolar and mid latitudes and almost no variance in low latitudes. Greenland surface air temperature varies by up to 4°C, which is in line with multi-centennial variability reconstructed from ice cores. We show that this mode is based on a salt-oscillator mechanism and emerges only under full LGM climate forcing. Moderate deviations from full-glacial boundary conditions lead to its disappearance. We further argue that the multi-centennial mode has to be distinguished from millennial-scale Dansgaard-Oeschger oscillations.

How to cite: Prange, M., Jonkers, L., Merkel, U., Schulz, M., and Bakker, P.: A multi-centennial mode of North Atlantic climate variability throughout the Last Glacial Maximum, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13276, https://doi.org/10.5194/egusphere-egu23-13276, 2023.

The AMOC has undergone abrupt and quasi-periodic changes during the MIS-3. The prevailing background climatic conditions that produce such behavior in AMOC have yet to be fully understood. Previous studies have demonstrated that some climate models tend to have an oscillatory behavior in their AMOC under specific conditions that vary from model to model. A systematic study that compares these conditions across models is missing. Moreover, the relative impact of greenhouse gas and icesheet forcings on the mean strength of AMOC remain unresolved.

Here, we present our results from CMIP/PMIP style simulations with MIS-3 boundary conditions. This study has been carried out under the PalMOD project. Based on the minimum and maximum ice sheet extent and greenhouse gas radiative forcing, we carried out a set of 4 experiments. These experiments are the LGM, 38ka, LGM_38kaghg (LGM topography with 38ka greenhouse gas concentrations), and 38ka_LGMghg (38ka topography with LGM greenhouse gas concentrations). We have used two Earth system models (ESM), Viz. the MPI-ESM and the CESM. The experiments in MPI-ESM were carried out with two versions of the river run-off directions - one in which run-off directions are compatible with the topography and the other where run-off directions are set to that of the modern-day. Thus, we have three sets of simulations for each experiment.

A robust feature across these simulations is that during the MIS-3, the mean strength of AMOC is sensitive to changes in greenhouse gases, and the changes in ice sheets do not significantly affect the AMOC. The density of water in the North Atlantic Deep-Water formation (NADW) region does not change significantly in response to these forcings. However, the variations in the density in the Arctic and Southern Ocean deep-water formation region drive variations in AMOC strength. The AMOC in CESM undergoes Dansgaard-Oeschger (DO) like oscillations in the 38ka LGMghg simulation. No oscillations are found in any MPI-ESM experiments with the run-off adapted for topography. However, Bo-like oscillations appear in the LGM simulation with modern run-off. This highlights the importance of model parameters and the location of freshwater input into the ocean in determining the conditions that lead to oscillatory behavior in AMOC.

How to cite: Jalihal, C., Merkel, U., Prange, M., and Mikolajewicz, U.: On the sensitivity of the ocean response to LGM and MIS3-forcings, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12646, https://doi.org/10.5194/egusphere-egu23-12646, 2023.

Dansgaard-Oeschger (DO) events are the most iconic mode of millennial-scale variability during the last glacial period. The manifestation of DO events outside the North Atlantic region and mechanisms responsible for the propagation of the North Atlantic signal across the globe are still little understood. Propagation of DO events to the Southern Hemisphere (SH) has first been explained by oceanic processes, that result in a muted and delayed signal in the Antarctic ice core record, known as Antarctic Isotope Maxima (AIM). Recent ice core-based reconstructions found an additional short-timescale response (years-to-decades, compared to centuries for the oceanic processes) in phase with the climate changes in Greenland. This fast response has been interpreted as the result of atmospheric transport processes. Shifts in the intertropical convergence zone and SH mid-latitude westerlies are seen as mediators of this response.

Here, we investigate the propagation of abrupt climate changes in the North Atlantic region to the SH in general circulation model simulations with spontaneous DO-like oscillations under glacial conditions. We study the relative timing of changes in temperature, hydroclimate, and atmospheric circulation and compare our results with ice core and speleothem based reconstructions. In the simulations, the timing of changes in different elements of the climate system varies on a continuum of timescales from months to centuries. This indicates the existence of more complex propagation mechanisms than the simple separation into an atmospheric and an oceanic mode. Our work emphasizes that future analysis of simulations of DO-like events should focus not just on the mechanisms responsible for the spontaneous oscillations but also on the spatio-temporal fingerprint of the oscillations across the globe.

How to cite: Trombini, I., Weitzel, N., Racky, M., Valdes, P., and Rehfeld, K.: Atmosphere-mediated response of the Southern Hemisphere hydroclimate in simulations of spontaneous Dansgaard-Oeschger-like oscillations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2885, https://doi.org/10.5194/egusphere-egu23-2885, 2023.

Reliable projections of future climate change are vital for mitigation and adaptation efforts. Such efforts require not only projections of mean changes but of changes in variability, too, since those directly affect the occurrence of extremes. The evaluation of climate models regarding their ability to simulate expected changes in variability of temperature and precipitation relies on the comparison of observations with simulations of past and present-day climate. As such, studying past periods of warming furthers the understanding of the climate system and its projected changes. However, the response of the climate system to forcings depends on the background state. Thus, understanding how insights from studies of the past transfer to future projections and the limitations of this transfer is vital.

Here, we present an analysis of temperature and precipitation variability in transient simulations of the Last Deglaciation and projected future climate. To this end, we analyze how the distributions of temperature and precipitation change as exemplified by the moments of the distribution, i.e. variance, skewness and kurtosis. We identify trends in the projections and compare them to results for the Last Deglaciation and present commonalities and differences between the responses in these climate states. We further present how these changes relate to differences in the background state, forcings, and the timescales on which these forcings act as well as the limitations imposed by these differences. Based on this analysis of the state-dependency of variability and its change with a warming mean state, we present conclusions on how past climates can inform and support studies of future climate change.

How to cite: Ziegler, E. and Rehfeld, K.: Past and future changes of temperature and precipitation variability in climate model projections and transient simulations of the Last Deglaciation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9813, https://doi.org/10.5194/egusphere-egu23-9813, 2023.

Understanding climate variability from millennial to glacial-interglacial timescales remains challenging due to the complex and non-linear feedbacks between ice, ocean, and atmosphere. Although the ever-increasing number of reconstructions has helped to form compelling hypotheses for the evolution of ocean and atmosphere circulation or ice sheet extent over the last glacial cycle, climate models, required for systematically testing these hypotheses, struggle to dynamically and comprehensively simulate such long time periods as a result of the large computational costs. Here, we therefore coupled a dynamical ice sheet model to the Bern3D Earth system model of intermediate complexity, that allows for simulating multiple glacial-interglacial cycles in reasonable time. To test the fully-coupled model, we explore the climate evolution over the entire last glacial cycle in a transient simulation forced by the orbital configuration and greenhouse gas and aerosol concentrations. We are able to simulate Global Mean Surface Temperature (GMST) in fair agreement with reconstructions exhibiting a gradual cooling trend since the last interglacial that is interrupted by two more rapid cooling events during the early Marine Isotope Stage (MIS) 4 and Last Glacial Maximum (LGM). The glacial-interglacial GMST and mean ocean temperature differences are 5 °C and 1.6 °C, respectively. Ice volume shows pronounced variability on orbital timescales mirroring northern hemispheric summer insolation. From early MIS3 to the LGM ice volume roughly doubles in good agreement with recent sea-level reconstructions. The Atlantic overturning circulation shows larger variability during the relatively warm MIS5 than during the cooler MIS3, however we note that Dansgaard-Oeschger events are not intrinsically simulated in our setup. At the LGM the Atlantic overturning has a strength of about 14 Sv, which is a reduction by about one quarter compared to the pre-industrial. We thus demonstrate that the new coupled model is able to realistically simulate glacial-interglacial cycles, which allows as to systematically investigate the sensitivities to parameters such as equilibrium climate sensitivity or aerosol radiative forcing during the last glacial cycle.

How to cite: Pöppelmeier, F., Joos, F., and Stocker, T. F.: The last glacial cycle transiently simulated with a coupled climate-ice sheet model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2586, https://doi.org/10.5194/egusphere-egu23-2586, 2023.

Paleo records indicate significant variation in sea level and temperature proxies between different glacial cycles. What is unclear is the extent to which these differences are due to noise in the physical system versus a robust response to external forcings. When one considers what is happening with each individual ice sheet, variations between glacial cycles are largely unknown, given the few relevant records available to constrain ice sheet extent before the Eemian. 

To explore both the controls on past ice sheet and climate evolution and explore bounds on what the evolution might actually have looked like, we are running ensemble simulations of the last two glacial cycles with the fully coupled ice/climate model LCice. LCice is a coupled version of the Loveclim EMIC and GSM glacial systems model with hybrid shallow shelf and shallow ice flow and global visco-elastic glacio-isostatic adjustment. The current configuration includes all 4 ice sheet complexes and is subject to only orbital and greenhouse gas forcing.

To answer the above questions, we present ensemble results for the last two glacial inceptions, focusing on what key ice sheet and climate characteristics are robust across the ensemble and what are not. The role of key forcings and feedbacks are also isolated through a set of sensitivity experiments.  

How to cite: Geng, M. and Tarasov, L.: A comparison of the last two glacial inceptions via fully coupled transient ice and climate modelling., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16335, https://doi.org/10.5194/egusphere-egu23-16335, 2023.

We use the new Earth system model of intermediate complexity CLIMBER-X to investigate pathways of Northern Hemisphere (NH) glaciation. We perform experiments in which different combinations of orbital forcing and atmospheric CO₂ concentration are maintained constant in time. Each model simulation is run for 300 thousand years (kyr) starting from present-day conditions, and using an acceleration technique with asynchronous coupling between the climate and ice sheet model components.

We find that in the pathway to a NH glaciation, several bifurcations might occur. The bifurcations separate a diversity of stable configurations, which have different spatial and temporal prints. We identify four different bifurcations, separating five different equilibrium states: (i) completely ice-free conditions, (ii) present-day (ice only over Greenland), weak glaciation (with ice coverage north and west of Hudson Bay, Greenland and Scandinavia), (iv) Last Glacial Maximum – type of glaciation (with large North American and medium-size Eurasian ice sheets) and (v) mega-glaciation (full ice coverage over both North America and Eurasia).

The transitions are also clustered in terms of differential timescales. While the North-American continent full glaciation has a development timescale of ~ 100 kyr, an extensive ice coverage of the Eurasian continent involves a much longer time-frame of ~ 250 kyr. This could explain why a complete glaciation of the Eurasian continent was never observed. This result is also consistent with previous studies in the sense that one glaciation cycle is not long enough for the Eurasian ice sheet to fully grow.

How to cite: Talento, S., Ganopolski, A., and Willeit, M.: Transitions in the Northern Hemisphere glaciation process, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8546, https://doi.org/10.5194/egusphere-egu23-8546, 2023.

The Arctic is warming at a rate greater than the global average. End-of-summer minimum sea ice extent is declining and reaching new minimums for the historical record of the last 4 decades. The Greenland ice sheet is now losing more mass than it is gaining, with increased surface melting. Earth System Models suggest that these trends will continue in the future. The geologic past can be used to inform what could happen in the future. Emiliani in his 1972 Science paper commented on the relevance of paleoclimate for understanding our future Earth.

Interglacials of the last 800,000 years, including the present (Holocene) period, were warm with low land ice extent. In contrast to the current observed global warming trend, which is attributed primarily to anthropogenic increases in atmospheric greenhouse gases, regional warming during these interglacials was driven by changes in Earth’s orbital configuration. Although the circumstances are different, understanding the behavior, processes, and feedbacks in the Arctic provides insights relevant to what we might expect during future global warming.

Data compilations suggest that despite spatial heterogeneity, Marine Isotope Stages (MIS) 5e (Last Interglacial, ~129 to 116 ka) was globally strong. The Last Interglacial (LIG) is characterized by large positive solar insolation anomalies in the Arctic during boreal summer associated with the large eccentricity of the orbit and perihelion occurring close to the boreal summer solstice. The atmospheric carbon dioxide concentration was similar to the preindustrial period.

Geological proxy data for the LIG indicates that Arctic latitudes were warmer than present, boreal forests extended to the Arctic Ocean in vast regions, summer sea ice in the Arctic was much reduced, and Greenland ice sheet retreat contributed to the higher global mean sea level. Model simulations provide critical complements to this data as the they can quantify the sensitivity of the climate system to the forcings, and the processes and interplay of the different parts of the Arctic system on defining these responses. As John Kutzbach explained in a briefing for science writers, "climate forecasts suffer from lack of accountability. Their moment of truth is decades in the future. But when those same computer programs are used to hindcast the past, scientists know what the correct answer to the test should be."

Significant attention and progress have been made in modeling the LIG in the last 2 decades. Earth System Models now capture more realism of processes in the atmosphere, ocean, and sea ice, can couple to models of the Greenland ice sheet, and include representations of the response of Arctic vegetation to the NH high-latitude summer warming. Increases in computing power has allowed these models to be run at higher spatial resolution and to perform transient simulations to examine the evolving orbital forcing during the LIG.  The international PMIP4 simulations for 127 ka illustrated the importance of positive cryosphere and ocean feedbacks for a warmer Arctic. A CESM2-Greenland ice sheet, transient LIG simulation from 127 ka to 119 ka, established a key role of vegetation feedbacks on Arctic climate change.

How to cite: Otto-Bliesner, B. L.: Milankovitch cycles and the Arctic: insights from past interglacials, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9748, https://doi.org/10.5194/egusphere-egu23-9748, 2023.

We present transient simulations of the last glacial inception using the Earth system model CLIMBER-X with interactive ice sheets and visco-elastic solid-Earth response. The simulations are initialized at the Eemian interglacial (125 ka) and run until 100 ka, driven by prescribed changes in orbital configuration and greenhouse gas concentrations from ice core data.
CLIMBER-X simulates a robust ice sheet expansion over North America and Scandinavia through MIS5d, in accordance with proxy data. However, we show that the crossing of a bifurcation point in the ice-covered area, which leads to a rapid (~7 million square km over a few centuries) expansion of ice sheets over North America, is critical to get a large enough ice volume to match the sea level drop of ~40 m indicated by reconstructions during the last glacial inception. As a consequence of the presence of this bifurcation point, the model results are highly sensitive to climate model biases. We also show that in the model the vegetation feedback plays an important role during glacial inception.
Further results suggest that, as long as the system responds almost linearly to insolation changes during the last glacial inception, the model results are not very sensitive to changes in the ice sheet model resolution and the acceleration factor used to speed-up the climate component. This is not valid, however, when the system response is characterized by strongly-nonlinear processes, such as a rapid increase in ice-covered area.

How to cite: Willeit, M., Talento, S., and Ganopolski, A.: Rapid expansion of ice sheet area in transient simulations of the last glacial inception, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11127, https://doi.org/10.5194/egusphere-egu23-11127, 2023.

The wide range of Effective Climate Sensitivity (ECS) values in climate models are driven by inter-model spread in cloud feedbacks. The most recent generation of models (CMIP6) show an increase in both average ECS values as well as the appearance of very high ECS values (> 4.5 K) compared to the previous generation which has been attributed to an increase in the strength of total cloud feedbacks in CMIP6. Constraining ECS and in particular the high range of ECS values is paramount for reliable predictions of future climate change. The Last Glacial Maximum (LGM) is an out-of-sample climate for modern models and thus provides a valuable evaluation test for these models. This work explores whether models with high ECS values are compatible with the Last Glacial Maximum (LGM) climate and whether we can use the LGM to constrain a plausible upper boundary of ECS. We create a single model ensemble with a wide range of ECS values by modifying cloud feedbacks in the MPI-ESM1.2 model. We simulate the LGM with this ensemble and compare it with four different paleo-reconstructions. Our results indicate models with an ECS > 4 K are incompatible with the existing LGM climate reconstructions: global surface air temperature (SAT) anomalies are too cold compared to reconstructions and ultimately become unstable due to sea ice dynamics in the model. Our study indicates that models with large total cloud feedbacks and high ECS values are not plausible during the LGM. This study highlights the value of using paleoclimates to benchmark models particularly in areas where existing validation techniques are not yet sufficient i.e. constraining cloud feedbacks.

How to cite: Sagoo, N. and Mauritsen, T.: Are high sensitivity models compatible with the Last Glacial Maximum?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8827, https://doi.org/10.5194/egusphere-egu23-8827, 2023.

Stable water isotopes in polar ice cores are widely used to reconstruct past temperature variations over several orbital climatic cycles. One way to calibrate the isotope-temperature relationship is to apply the present-day spatial relationship as a surrogate for the temporal one. However, this method leads to large uncertainties because several factors like the sea surface conditions or the origin and the transport of water vapor influence the isotope-temperature temporal slope. In this study, we investigate how the sea surface temperature (SST), the sea ice extent and the strength of the Atlantic Meridional Overturning Circulation (AMOC) affect these temporal slopes in Greenland and Antarctica for Last Glacial Maximum (LGM, ~21 000 years ago) to preindustrial climate change. For that, we use the isotope-enabled atmosphere climate model ECHAM6-wiso [1, 2], forced with a set of sea surface boundary condition datasets based on reconstructions (GLOMAP [3] and Tierney et al. (2020) [4]) or MIROC 4m simulation outputs [5]. We found that the isotope-temperature temporal slopes in East Antarctic coastal areas are mainly controlled by the sea ice extent, while the sea surface temperature cooling affects more the temporal slope values inland. Mixed effects on isotope-temperature temporal slopes are simulated in West Antarctica with sea surface boundary conditions changes, because the transport of water vapor from the Southern Ocean to this area can dampen the influence of temperature on the changes of the isotopic composition of precipitation and snow. In the Greenland area, the isotope-temperature temporal slopes are influenced by the sea surface temperatures very near the coasts of the continent. The greater the LGM cooling off the coast of southeast Greenland, the larger the temporal slopes. The presence or absence of sea ice very near the coast has a large influence in Baffin Bay and the Greenland Sea and influences the slopes at some inland ice cores stations. We emphasize that the extent far south of the sea ice is not so important. On the other hand, the seasonal variations of sea ice distribution, especially its retreat in summer, influence the water vapor transport in this region and the modeled isotope-temperature temporal slopes in the eastern part of Greenland. A stronger LGM AMOC decreases LGM to preindustrial isotopic anomalies in precipitation in Greenland, degrading the isotopic model-data agreement. The AMOC strength does not modify the temporal slopes over inner Greenland, and only a little on the coasts along the Greenland Sea where the changes in surface temperature and sea ice distribution due to the AMOC strength mainly occur.

[1] Cauquoin and Werner, J. Adv. Model. Earth Syst., 13, https://doi.org/10.1029/2021MS002532, 2021.

[2] Cauquoin et al., Clim. Past, 15, 1913–1937, https://doi.org/10.5194/cp-15-1913-2019, 2019.

[3] Paul et al., Clim. Past, 17, 805–824, https://doi.org/10.5194/cp-17-805-2021, 2021.

[4] Tierney et al., Nature, 584, 569–573, https://doi.org/10.1038/s41586-020-2617-x, 2020.

[5] Obase and Abe-Ouchi, Geophys. Res. Lett., 46, 11 397–11 405, https://doi.org/10.1029/2019GL084675, 2019.

How to cite: Cauquoin, A., Abe-Ouchi, A., Obase, T., Chan, W.-L., Paul, A., and Werner, M.: Effects of LGM sea surface temperature and sea ice extent on the isotope-temperature slope at polar ice core sites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6063, https://doi.org/10.5194/egusphere-egu23-6063, 2023.

At the onset of the last deglaciation, beginning ~19 thousand years ago, ice sheets that covered the Northern Hemisphere at the Last Glacial Maximum started to melt, Earth began to warm, and sea levels rose. This time period is defined by major long-term, millennial-scale, climate transitions from the cold glacial to warm interglacial state, as well as many short-term, centennial- to decadal-scale warmings and coolings of more than 5 °C, sudden reorganisations of basin-wide circulations, and jumps in sea level of tons of meters. Long transient simulations of the deglaciation have been increasingly performed to better understand the long and short term processes, examine different possible scenarios, and compare model output to observable records. The Paleoclimate Modelling Intercomparison Project (PMIP) has provided a framework for an international coordinated effort in simulating the last deglaciation whilst encompassing a broad range of models and model complexities. This study is a multi-model intercomparison of 17 simulations of the last deglaciation from nine different climate models. Unlike other multi-model intercomparison projects, these simulations do not follow one particular experimental design but follow an intentionally flexible protocol suitable for all participants. The design of the protocol provides the opportunity to compare results from models using different forcings and examine a variety of scenarios, hence, representing the range of uncertainty in climate predictions of the time period. One particularly challenging choice to make in the experimental design is how to incorporate the resultant freshwater flux from the melting ice sheets. This research focusses on the divergence between climate trajectories in the simulations as a result of the meltwater scenario preferred by the modelling groups as well as other experimental design choices and their impact on the onset of the deglaciation. These results provide a better understanding of modelling this time period as well as model biases and uncertainty with respect to deglacial forcings and the observable proxy records. 

How to cite: Snoll, B., Ivanovic, R., Gregoire, L., and Sherriff-Tadano, S. and the PMIP4 Working Group: A multi-model assessment of the early last deglaciation (PMIP4 LDv1), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7448, https://doi.org/10.5194/egusphere-egu23-7448, 2023.

The global vegetation cover underwent strong changes during the past glacial cycle. These have been driven by climatic fluctuations but also by spatiotemporal vegetation dynamics, including migration to new climatologically suitable areas and interactions with other species. However, how much migration lag contributed to the vegetation change after the Last Glacial Maximum (LGM) is often not clear. We used the newly-implemented model LPJ-GM 2.0 to simulate the vegetation change of southern and central Europe from the end of the LGM (18.5 ka) to the preindustrial era (1.5 ka). The model couples a migration module to the dynamic global vegetation model LPJ-GUESS, thus allowing species to migrate simultaneously while interacting with each other. We compared two dispersal settings (free dispersal and dispersal limitation) against pollen data to test the reliability of the migration module to provide realistic paleo-vegetation reconstructions for biome and species distributions. Furthermore, we calculated range shifts of the leading edges and centroids to detect potential species-specific migration lags and range filling delays across simulation time. Our results show that the setting with dispersal limitation is better at capturing the initial post-glacial expansion of non-boreal forests in southern and central Europe than the scenario assuming free dispersal. Range shift analysis shows significant migration lags for most tree species at times of sudden temperature rise (start of the Bølling–Allerød warming event and following the Younger Dryas). Overall, our study suggests that it is necessary to include migration processes when simulating vegetation range expansion under rapid climate change, with implications for future vegetation projections.

How to cite: Zani, D., Lischke, H., and Lehsten, V.: The role of dispersal limitation in the post-glacial forest expansion of southern and central Europe, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1311, https://doi.org/10.5194/egusphere-egu23-1311, 2023.

How tropical cyclones respond to climate change remains an open question. Due to recent increases in computing power and climate model resolution, it is now possible to explicitly simulate tropical cyclone genesis and life cycle over long temporal and spatial scales. So far, most high-resolution simulations have explored tropical cyclones under present-day and future climate conditions. There has been little work on tropical cyclone activity in past climates. Here, we help fill in this gap with high resolution simulations of the last deglaciation including the Last Glacial Maximum (LGM; 21-ka), Heinrich Stadial 1 (HS1; 16-ka), and Preindustrial (PI; 1850 CE). We use the water isotope tracer enabled version of the Community Earth System Model version 1.3 (iCESM1.3) at ~0.25° horizontal resolution to simulate climate and the TempestExtremes algorithm to track tropical cyclone features. Our preliminary results show intriguing spatial changes in tropical cyclone activity at the LGM relative to PI. The Atlantic and Indian basins produce less tropical cyclones while the Western Pacific produces more tropical cyclones at the LGM. Furthermore, tropical cyclone frequency decreases in the southern hemisphere but remains similar in the northern hemisphere. The LGM simulation also produces fewer strong storms (greater than 49 m/s). Further investigation will explore the physical mechanisms for the simulated tropical cyclone responses during the deglaciation as well as the effects of freshwater flux into the North Atlantic on tropical cyclone activity.

How to cite: Tabor, C., Lofverstrom, M., Montañez, I., Oster, J., and Zarzycki, C.: Simulating Changes in Tropical Cyclone Activity During the Deglaciation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17590, https://doi.org/10.5194/egusphere-egu23-17590, 2023.

Extratropical cyclones are a major source of natural hazards in the mid latitudes as wind and precipitation extremes are associated to this weather phenomenon. Still the response of extratropical cyclones and their characteristics to strong external forcing changes is not yet fully understood. In particular, the impact of the orbital forcing as well as variations of the major ice sheets during glacial times on extratropical cyclones have not been investigated so far.  

Thus, the aim of this study is to fill this gap and to assess the impact of orbital forcing and northern hemispheric ice sheet height variations on extratropical cyclones and their characteristics during winter and summer. The main research tool is the Community Earth System Model CESM1.2. We performed a set of time slice sensitivity simulations under preindustrial (PI) conditions and for the following different glacial periods: Last Glacial Maximum (LGM), Marine Isotopic stage 4 (MIS4), MIS6, and MIS8. Additionally, we vary the northern hemispheric ice sheet height for all the different glacial periods by 33%, 66%, 100% and 125% of the ice sheet reconstructed for the LGM. For each of the simulations the extratropical cyclones are identified with a Lagrangian cyclone detection and tracking algorithm, which delivers a set of different cyclone characteristics, such as, cyclone frequency maps, cyclone area, central pressure, cyclone depth, precipitation associated to the extratropical cyclones as well as extremes in cyclone depth and extratropical cyclone-related precipitation. These cyclone characteristics are investigated for the winter and the summer season separately.

Preliminary results show that the extratropical cyclone tracks are shifted southwards on the Northern Hemisphere during the winter season. This has rather strong implication for the Mediterranean, with an increase of precipitation during glacial times over the western Mediterranean. This increase is modulated when changing the ice sheet height as extratropical cyclone tracks shift further south with increasing northern hemispheric ice sheet height. The orbital forcing shows a higher impact during the summer season, where mean precipitation is further reduced over Europe when comparing MIS4 and MIS8 with LGM. The role of the cyclones for these changes in summer needs to be assessed as well as the implication in the North Pacific.

How to cite: Raible, C. C., Messmer, M., Buzan, J., and Russo, E.: Northern Hemispheric extratropical cyclones during glacial times: impact of orbital forcing and ice sheet height, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6514, https://doi.org/10.5194/egusphere-egu23-6514, 2023.

Paleoclimatic reconstructions have suggested a reduction inthe variability of the El Niño Southern Oscillation (ENSO) during the mid-Holocene (MH). Model simulations have largely failed to capture thisreduction, potentially due to the inadequate representation of the Green Sahara.The presence of a vegetated Sahara has been shown to have significant impacts on both regional and remote climate but remains inadequately addressed in Paleoclimate Modelling Intercomparison Project / Coupled Model Intercomparison Project (PMIP/CMIP) boundary conditions. Specifically, the incorporation of a Green Sahara has been shown to impact ENSO variability through perturbations to the Walker Circulation. In this study, we evaluate the MH (6,000 years BP) ENSO signatures of simulations from four models, namely —EC-Earth 3.1, iCESM 1.2, University of Toronto version of CCSM4 and GISS Model E2.1-G. Two simulations are considered for each model—a standard PMIP simulation (MH_PMIP) with the mid-Holocene orbital parameters and greenhouse gas concentrations with vegetation prescribed to preindustrial conditions, as well as a Green Sahara simulation (MH_GS) which additionally incorporates factors such as enhanced vegetation, reduced dust, presence of lakes, and land and soil feedbacks. All models show a reduction in ENSO variability due to the incorporation of Green Sahara conditions. This variability is interpreted in the context of perturbations to the Walker Circulation, triggered by the strengthening of the West African Monsoon.

How to cite: Tiwari, S., Pausata, F. S. R., LeGrande, A. N., Griffiths, M. L., Beltrami, H., de Vernal, A., Tabor, C. R., Litchmore, D., Chandan, D., and Peltier, W. R.: Reduction in ENSO variability during the mid-Holocene: a multi-model perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4683, https://doi.org/10.5194/egusphere-egu23-4683, 2023.

The East Asian summer monsoon (EASM) is a major component of the global climate system with its variability closely associated with regional changes of rainfall, impacting the lives of over one sixth of the global population strongly. Understanding the periodicities of summer rainfall influenced by the EASM is beneficial to its future projections. However, the mechanism of the response of the EASM associated summer rainfall fluctuations to orbital-scale forcing during the late Pleistocene remains far from being well understood. Here, we provide an 800-kyr long series of EASM rainfall variations by extracting data from multiple transient simulations of CLIMBER-2 over the past 3 million years. Despite a coarse model resolution, the CLIMBER-2 captures a realistic spatial distribution and magnitude of present-day summer (June-July-August) rainfall, especially in East Asia. The CLIMBER-2 model simulates correct magnitude and timing of the last eight glacial cycles in respect to both global ice sheet volume (expressed in δ18O) and CO₂ concentration. Both the simulation and reconstructions reveal predominant 100-ky and 41-ky cycles of global ice sheet volume and CO₂ concentration, although precession (23- and 19-kyr) bands dominate high-latitude summer insolation. The EASM intensity is traditionally measured by the monsoonal circulation, i.e. the low-level southerly winds in summer over East Asia. Cross-spectral analysis confirms high coherence between model and proxy at 19-kyr and 41-kyr bands implying a strong low-latitude process modulated by precession. Unlike the EASM circulation from the CLIMBER-2, simulated boreal summer rainfall in East Asia, denoted as “EASM rainfall” shows pronounced 41- and 100-kyr cycles, resembling the loess record over the past 800 kyr. The simulation results reveal a decoupling between EASM rainfall and EASM circulation, which probably is a reasonable explanation for the conflicts in proxy records, and also reflects complicated mechanisms of the EASM system on glacial–interglacial timescales.

How to cite: Jin, L., Ganopolski, A., Willeit, M., Lu, H., Chen, F., and Zhang, X.: New insights of the East Asian summer monsoon variability over the past 800 kyr from a transient simulation with CLIMBER-2, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4963, https://doi.org/10.5194/egusphere-egu23-4963, 2023.

Modelling results from PlioMIP2 (Pliocene Model Intercomparison Project Phase 2) are in strong disagreement with terrestrial proxy data over the high latitudes for the winter season.  This disagreement is large:  models simulate winter temperatures ~20°C cooler than the data suggests.  We term this the ‘warm winter paradox’.

We have shown that the warm winter paradox cannot be easily resolved.  For example, changing model boundary conditions to account for orbital and CO₂ uncertainty have only a small effect on winter temperatures.

Here we use the Hadley Centre General Circulation Model, HadCM3, to investigate whether accounting for uncertainties in model parameterisations could improve the model data agreement for the Pliocene winter.  A new set of parameters for HadCM3, which improve model-data agreement for the Eocene, will be used to investigate the Pliocene climate.  We will show that the new parameters in HadCM3 lead to additional winter Pliocene warming at some locations, although a large model-data disagreement remains.   The new model parameters do not improve the Pliocene data-model comparison as much as they do for the Eocene.  This may indicate that finding a single set of parameters capable of producing an optimised simulation of warm climate states in general is not possible, and that further exploration of model parameter uncertainty is warranted; or that the cause of model data disagreements in the high latitudes may be time period specific.   

How to cite: Tindall, J., Haywood, A., and Valdes, P.: The Warm Winter Paradox in the mid-Pliocene Warm Period - a focus on model parameterisations., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6932, https://doi.org/10.5194/egusphere-egu23-6932, 2023.

Green Sahara and a northern limit of forest in the northern hemisphere are key characteristics of the differences between the mid Holocene and present-day climate. However, the strength of vegetation feedback and the ability of state-of-the-art climate model to properly represent it still an issue. A reason is that vegetation lies at the critical zone between land and atmosphere. Its variations depend on interconnected factors such as light, energy, water and carbon and, in turn, affect climate and environmental factors. These interconnexions makes it difficult to disentangle the factors that affect the representation of vegetation in a fully interactive model. Dynamical vegetation introduces additional degrees of freedom in climate simulations, so that a model that produces reasonable results when vegetation is prescribed might not be able to properly reproduce the full coupled system, when feedbacks that are not dominant when the system is constraint induce first order cascading effects in coupled mode. Here we investigate the climate-vegetation feedback in mid-Holocene and pre-industrial simulation with the IPSL climate models using 3 different settings of the dynamical vegetation that combining differences in the choice of representation of photosynthesis, bare soil evaporation and parameters defining the vegetation competition and distribution. We show that whatever the set up the major differences expected between the mid-Holocene and preindustrial climates remains similar, but the realisms of the simulated climate can be very different due to cascading climate-vegetation feedbacks that trigger vegetation growth and snow-ice-temperature-soil feedbacks.  Interestingly, with this IPSLCM6 version of the IPSL model (Boucher et al., 2020) all the mid-Holocene simulations produce vegetation in the Sahara-Sahel region compatible with the green Sahara period, but the representation of boreal forests is strongly affected by the different vegetation modeling choices.

How to cite: Braconnot, P., Viovy, N., and Marti, O.: Mid Holocene dynamic vegetation highlights unavoidable climate feedbacks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6289, https://doi.org/10.5194/egusphere-egu23-6289, 2023.

We compare Holocene forest-cover changes in Europe derived from a transient MPI-ESM1.2 simulation with high spatial resolution time-slice simulations conducted in LPJ-GUESS and pollen-based quantitative reconstructions of forest cover based on the REVEALS model (pol-RVs). The dynamic vegetation models and pol-RVs agree with respect to the general temporal trends in forest cover for most parts of Europe, with a large forest cover during the mid-Holocene and substantially smaller forest cover closer to the present time. However, the age of the start of decrease in forest cover varies between regions, and is much older in the pol-RVs than in the models. The pol-RVs suggest much earlier anthropogenic deforestation than the prescribed land-use in the models starting 2000 years ago. While LPJ-GUESS generally overestimates forest cover compared to pol-RVs, MPI-ESM indicates lower percentages of forest cover than pol-RVs, particularly in Central Europe. A comparison of the simulated climate with chironomid-based climate reconstructions reveal that model-data mismatches in forest cover are in most cases not driven by biases in the climate. Instead, sensitivity experiments show that the model results strongly depend on the models tuning regarding natural disturbance regimes (e.g. fire and wind throw). The frequency and strength of disturbances are – like most of the parameters in the vegetation models – static and calibrated to modern conditions. However, these parameter values may not be valid during climate and vegetation states totally different from today’s. In particular, the mid-Holocene natural forests were probably more stable and less sensitive to disturbances than present day forests that are heavily altered by human interventions. Our analysis highlights the fact that such model settings are inappropriate for paleo-simulations and complicate model-data comparisons with additional challenges. Moreover, our study suggests that land-use is the main driver of forest decline in Europe during the mid- and late-Holocene.

How to cite: Dallmeyer, A., Poska, A., Marquer, L., Seim, A., and Gaillard-Lemdahl, M.-J.: Holocene forest-cover changes in Europe - a comparison of dynamic vegetation model results and pollen-based REVEALS reconstructions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11206, https://doi.org/10.5194/egusphere-egu23-11206, 2023.

Land cover, and thereby vegetation, can alter climate through biogeochemical and biogeophysical effects. Specifically, plants mediate radiative and turbulent fluxes between the surface and atmosphere and contribute to defining temperature and precipitation patterns in continental areas. In recent decades, pioneering works based both on fossil records and climate modelling have shown that vegetation parameterization is pivotal for accurately simulating past climates. Here, we focused on the Cretaceous, during which the radiation of angiosperms was accompanied by a physiological revolution characterized in the fossil record by an increase in the density of leaf veins and, ultimately, an unprecedented rise in their stomatal conductance. Emulating such an evolution of leaf traits, quantifying their consequences on plant productivity and transpiration, and identifying the associated feedbacks in the Cretaceous climate is a very challenging task. We addressed this triple problem by embedding the reconstruction of physiological paleotraits from the fossil record within the IPSL-CM5A2 earth system model, which land surface scheme allows for the interaction between stomatal conductance and carbon assimilation.

We built and evaluated vegetation parameterizations accounting for the increase in stomatal conductance during angiosperm radiation, which is consistent with the fossil record, by altering the hydraulic and photosynthetic capacities of plants in a coupled fashion. These experiments, combined with two extreme atmospheric pCO2 scenarios, show that a systematic increase in transpiration is simulated when vegetation shifts from a proto-angiosperm state to a modern-like state, and that its magnitude is related to primary productivity modulated by light, water stress, and evaporative demand. Under a high pCO2 scenario, only stomatal conductance plays a role, and the feedback of vegetation change consists mainly of more intense water recycling and rainfall over the continents. At low pCO2, the effect of the high stomatal conductance on transpiration is enhanced by the development of vegetation cover, resulting in more transpiration and higher precipitation rates at all latitudes. Enhanced turbulent fluxes lead to a surface cooling that outcompete the warming linked to the lower surface albedo. Our results suggest a larger impact of angiosperms on climate when atmospheric pCO2 is decreasing, and stresses the importance of accounting for fossil-based paleotraits in paleoclimate simulations.

How to cite: Sepulchre, P., Bres, J., Pikeroen, Q., Viovy, N., and Vuichard, N.: Angiosperms leaf evolution and the Cretaceous continental hydrological cycle : accounting for paleotraits in paleoclimate numerical simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16377, https://doi.org/10.5194/egusphere-egu23-16377, 2023.