CL1.2

The Last Interglacial (LIG) period (~129,000–116,000 years BP) and the mid-Holocene (MH) (~6,000 years BP) are the two most recent intervals with temperatures comparable to low emissions scenarios for the end of the 21^st century. During the LIG and the MH differences in the seasonal and latitudinal distribution of insolation led to enhanced northern hemisphere high-latitude warmth relative to the pre-industrial, despite similar greenhouse gas concentrations, marking these intervals as potentially useful analogs for future change in regions like North America. Further, the inclusion of both LIG (127 ka) and MH (6 ka) experiments in the CMIP6-PMIP4 effort provides an opportunity to better understand the regional hydroclimate responses to radiative forcing during these two intervals. The dense coverage of paleoclimate proxy records for North America during the MH (N=260 sites) reveals a pattern of relative aridity in the Pacific Northwest and Western Canada and wetness in the southern Great Basin and Mexico. However, the seasonality and driving mechanisms of rainfall patterns across the continent remain poorly understood. Our understanding of terrestrial hydroclimate in North America during the LIG is more limited (N=39 sites), largely because the LIG is beyond the range of radiocarbon dating.

Here we present spatial comparisons between output from 14 PMIP4 global circulation models and LIG and MH networks of moisture-sensitive proxies compiled for the North American continent. We utilize two statistical measures of agreement – weighted Cohen’s Kappa and Gwet’s AC2 – to assess the degree of categorical agreement between moisture patterns produced by the models and the proxy networks for each time-slice. PMIP4 models produce variable precipitation anomalies relative to the pre-industrial for both the LIG and MH experiments, often disagreeing on both the sign and magnitude of precipitation changes across much of North America. The models showing the best agreement with the proxy network are similar but not identical for the two measures, with Gwet’s AC2 values tending to be larger than Cohen’s Kappa values for all models. This pattern is enhanced for the much larger MH proxy network and is likely related to the fact that Gwet’s AC2 is a more predictable statistic in the presence of high agreement. Overall agreement is lower for the mid-Holocene than for the LIG, reflecting smaller MH rainfall anomalies in the models. The models with the highest agreement scores during the LIG produce aridity in the Rocky Mountains and Pacific Northwest and wetness in Alaska, the Yukon, the Great Basin, and parts of the Mid-West and Eastern US, although spatial coverage of the proxies in these latter two regions is poor. The models with the highest agreement score for the mid-Holocene tend to produce aridity across Canada and the northern US with dry conditions extending down the US Pacific coast and increased wetness in the American Southeast and across the North American Monsoon region. Our analyses help elucidate the driving mechanisms of rainfall patterns during past warm states and can inform which models may be the most useful for predictions of near-future hydroclimate change across North America.

How to cite: de Wet, C., Oster, J., Ibarra, D., and Belanger, B.: North American rainfall patterns during past warm states: A proxy network-model comparison for the Last Interglacial and the mid-Holocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6574, https://doi.org/10.5194/egusphere-egu21-6574, 2021.

The Atacama Desert in Northern Chile is considered to be the driest desert on Earth. At present, the annual rainfall amount is less than 1mm for parts of the hyper-arid core of the desert. The processes controlling this hyper-aridity are known, but the mean state and variability of the regional climate on geological time scales is not well understood. In this study, we aim to analyse climate conditions in the Atacama Desert from PMIP4 simulations. Our focus is on the Last Glacial Maximum (LGM), when climate records from the Central Atacama point to a substantially different climate with wetter conditions than at present (Diederich et al., 2020). We statistically analyse and evaluate PMIP4 historical simulations with respect to circulation patterns over the Southeast Pacific and Western South America which are associated with rare rainfall events in the Atacama Desert. For the evaluation, PMIP4 simulations for the historical period are compared to Reanalysis data, and we will focus on troughs and cutoff lows over the subtropical Southeast Pacific, and on the Bolivian High (Reyers et al., 2020). We then assess changes of the characteristics, e.g., the frequency of occurrence, of such circulation patterns for Paleo-climate conditions compared to the present. In the framework of our study, we perform km-scale simulations with the regional climate model WRF, using results from PMIP4 experiments for the historical period and for the LGM as boundary conditions. In the future, these simulations will be used to better understand the meso-scale processes, e.g., involved in local wind systems, that contribute to changes in the hydrological cycle and potentially impact the dust-emission activity of the desert. This study is part of the Collaborative Research Centre 1211 “Earth- Evolution at the dry Limit” (https://sfb1211.uni-koeln.de/).

Diederich, J,  Wennrich, V, Bao, R, and co-authors (2020). A 68 ka precipitation record from the hyperarid core of the Atacama Desert in northern Chile. Global and Planetary Change, 184, 103054. DOI:10.1016/j.gloplacha.2019.103054.

Reyers, M, Boehm, C, Knarr, L, Shao, Y, and Crewell, S (2020). Synoptic-to-Regional-Scale Analysis of Rainfall in the Atacama Desert (18°–26°S) Using a Long-Term Simulation with WRF. Monthly Weather Review, 149, 91-112. DOI:10.1175/MWR-D-20-0038.1.

How to cite: Reyers, M., Fiedler, S., and Shao, Y.: Paleo-climate shifts in the Atacama Desert from PMIP4 simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3185, https://doi.org/10.5194/egusphere-egu21-3185, 2021.

During the last million years, northern Africa has alternated between arid and humid conditions, as recorded by different kinds of climate archives, including fossil pollen, lake sediments, marine sediments and archaeological remains. Variations occur at millennial scale, with dry phases being similar to the current desert state in the region, and with wet phases, known as African Humid Periods (AHPs), characterised by a strong summer monsoon which can carry enough moisture inland to support rivers, lakes and lush vegetation further north than seen today. Recent sediment records from the Mediterranean Sea revealed that the previous five AHPs had different intensities, in relation to rainfall and vegetation extent. Motivated by these findings, our work focuses on explaining what caused such differences in intensity. To this end, we use the CLIMBER-2 climate model to study the AHP response to changes in three drivers of atmospheric dynamics: Earth's orbit variations, atmospheric concentration of CO₂ and inland ice extent. Global transient simulations of the last 190,000 years are used in new factorisation analyses, which allow us to separate the individual contributions of the forcings to the AHP intensity, as well as those of their synergies. We confirm the predominant role of the orbital forcing in the strength of the last five AHPs, and our simulations agree with previous estimates of a threshold in orbital forcing above which an AHP develops. Moreover, we show that atmospheric CO₂ and the extent of ice sheets can also add up to be as important as the orbital parameters. High values of CO₂, past a 205 ppm threshold, and low values of ice sheets extent, below an 8 % of global land surface threshold, yield the AHPs with the most precipitation and vegetation. Additionally, our results show that AHPs differ not only in amplitude, but also in their speed of change, and we find that the non-linear vegetation response of AHPs does not correlate with a single forcing and that the vegetation growth response is faster than its subsequent decline. In regards to future change, an extension of the simulations until the next 50,000 years, shows CO₂ to be the main driver of AHPs, with orbital forcing only setting the pace and their intensities being scenario-dependent.

How to cite: Duque-Villegas, M., Claussen, M., Brovkin, V., and Kleinen, T.: External and internal forcing of African Humid Periods from MIS 6 to MIS 1, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12167, https://doi.org/10.5194/egusphere-egu21-12167, 2021.

During the mid-Holocene, an expansion of vegetation, lakes and wetlands over North Africa reinforced the West African monsoon precipitation increase that was initiated by changes in the orbital forcing. Sedimentary records reflect these surface changes, however, they provide only limited spatial and temporal information about the size and distribution of mid-Holocene lakes and wetlands. Previous simulation studies that investigated the influence of mid-Holocene lakes and wetlands on the West African monsoon precipitation, prescribed either a small lake and wetland extent or focusing on mega-lakes only. In contrast to these simulation studies, we investigate the range of simulated West African monsoon precipitation changes caused by a small and a potential maximum lake and wetland extent during the mid-Holocene.

Therefore, four mid-Holocene sensitivity experiments are conducted using the atmosphere model ICON-A and the land model JSBACH4 at 160 km resolution. The simulations have a 30-year evaluation period and only differ in their lake and wetland extent over North Africa: (1) pre-industrial lakes, (2) small lake extent, (3) maximum lake extent and (4) maximum wetland extent. The small lake extent is given by the reconstruction map of Hoelzmann et al. (1998) and the potential maximum lake and wetland extent is given by a model derived map of Tegen et al. (2002).

The simulation results reveal that the maximum lake extent shifts the Sahel precipitation threshold (> 200 mm/year) about 3 ° further northward than the small lake extent. The major precipitation differences between the small and maximum lake extent results from the lakes over the West Sahara. Additionally, the maximum wetland extent causes a stronger West African monsoon precipitation increase than the equally large maximum lake extent, particularly at higher latitudes.

How to cite: Specht, N., Claußen, M., and Kleinen, T.: Influence of a small and maximum lake and wetland extent on the simulated West African monsoon precipitation during the mid-Holocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15050, https://doi.org/10.5194/egusphere-egu21-15050, 2021.

We give an overview on the global change in mid-to late Holocene vegetation pattern derived from a transient MPI-ESM1.2 simulation and discuss the vegetation trend in the context of the simulated Holocene climate change. The model captures the main trends found in reconstructions. Most prominent are the southward retreat of the northern treeline, coinciding the strong reduction of forest cover in the high northern latitudes during the Holocene, and the vast increase of the Sahara desert that is embedded in a general decrease and equator-ward retreat of the vegetation in the northern hemispheric monsoon margin regions. In contrast, large parts of the extratropical North American continent experience a greening during the Holocene, caused by an increase in forest and grass cover.

While the broad forest decline in the high northern latitudes can mainly be explained by the cooling of the warm season climate, precipitation is the driving factor for the tropical and extratropical vegetation trends on the northern hemisphere south of 60°N. The model indicates that most of the changes in rainfall can be related to the weakening of the northern hemispheric monsoon systems and the response of the global atmospheric circulation to this weakening.

The southern hemisphere is less affected by changes in total vegetation cover during the last 8000 years, but the monsoon related increase in precipitation and the insolation-induced cooling of the winter climate lead to shifts in the vegetation composition, mainly in between the woody plant functional types (PFTs).

The simulated large-scale global vegetation pattern almost linearly follow the subtle, approximately linear orbital forcing. Non-linear and more rapid changes in vegetation cover occur only on a regional level. The most striking area is the western Sahel-Sahara domain that experiences a rapid vegetation decline to a rather desertic state, in line with a strong decrease in moisture availability. The model also indicates rapid shifts in the vegetation composition in some regions in the high northern latitudes, in South Asia and in the monsoon margins of the southern hemisphere. These rapid transitions are mainly triggered by changes in the winter temperatures, which go into, or move out of, the bioclimatic tolerance range of the individual PFTs defined in the model and therefore have to be interpreted differently.

In summary, our model results identify the global monsoon system as the key player in Holocene climate and vegetation history and point to a far greater importance of the monsoon systems on the extra-monsoonal regions than previously assumed.

How to cite: Dallmeyer, A., Claussen, M., and Herzschuh, U.: Dominant role of the global monsoon intensity on large-scale Holocene vegetation transitions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10640, https://doi.org/10.5194/egusphere-egu21-10640, 2021.

We carry out three sets of last interglacial (LIG) experiments, named lig127k, and of pre-industrial experiments, named piControl, both as part of PMIP4/CMIP6 using three versions of the MIROC model: MIROC4m, MIROC4m-LPJ, and MIROC-ES2L. The results are compared with reconstructions from climate proxy data. All models show summer warming over northern high-latitude land, reflecting the differences between the distributions of the LIG and present-day solar irradiance. Globally averaged temperature changes are −0.94 K (MIROC4m), −0.39 K (MIROC4m-LPJ), and −0.43 K (MIROC-ES2L).
Only MIROC4m-LPJ, which includes dynamical vegetation feedback from the change in vegetation distribution, shows annual mean warming signals at northern high latitudes, as indicated by proxy data. In contrast, the latest Earth system model (ESM) of MIROC, MIROC-ES2L, which considers only a partial vegetation effect through the leaf area index, shows no change or even annual cooling over large parts of the Northern Hemisphere. Results from the series of experiments show that the inclusion of full vegetation feedback is necessary for the reproduction of the strong annual warming over land at northern high latitudes. The LIG experimental results show that the warming predicted by models is still underestimated, even with dynamical vegetation, compared to reconstructions from proxy data, suggesting that further investigation and improvement to the climate feedback mechanism are needed.

How to cite: O'ishi, R., Chan, W.-L., Abe-Ouchi, A., Sherriff-Tadano, S., Ohgaito, R., and Yoshimori, M.: PMIP4/CMIP6 last interglacial simulations using three different versions of MIROC: importance of vegetation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3792, https://doi.org/10.5194/egusphere-egu21-3792, 2021.

The climate evolution of the past few thousand years is essential for understanding the context in which civilisation arose and for understanding the natural background of anthropogenic influence. Proxy-inferred records show a complex picture of earlier warming and later cooling during the Holocene depending on region and reconstruction method. In contrast climate model simulations almost uniformly show warming throughout the past 10,000 years and for example also fail to reproduce a major advance of rainbelt over the Sahara.  These discrepancies raise questions about the reliability of climate models on longer-time scales.

We present a suite of four new transient Holocene simulations covering the last 8500 years using the HadCM3B-M21aD coupled general circulation. We use an optimised version of this model which is able to replicate the greening of the Sahara through changes to the atmospheric convection and vegetation schemes. We apply transient changes in Earth’s orbit, ice-sheets and sea-level and greenhouse gases, and optionally solar output, volcanic eruptions and anthropogenic land-use change.  The simulations without land-use show a warming throughout the Holocene, albeit with significantly higher variability once volcanic eruptions are included. With the inclusion of land-use change temperature trends in Northern Hemisphere are reversed from around 4000 years before present.

We explore the contribution of different forcings to the regional trends in the model ensemble and we compare the simulations against the Holocene reconstructions to evaluate the relative importance of each forcing. We also use the model ensemble to quantify the terrestrial coverage of proxy locations that is required to reliably infer global mean temperature variations.

How to cite: Hopcroft, P. and Valdes, P.: Contribution of forcings to Holocene climate evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4304, https://doi.org/10.5194/egusphere-egu21-4304, 2021.

How to cite: Wirths, C., Ziegler, E., Toohey, M., Schindlbeck-Belo, J. C., Kutterolf, S., Anders, H., and Rehfeld, K.: Comparing temperature trends and variability over the Holocene in climate models of low and high complexity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1842, https://doi.org/10.5194/egusphere-egu21-1842, 2021.

Multiple evidences from the analysis of satellite, in-situ and proxy data show that the climate is already changing toward a warmer Earth System due to our emissions of CO2 into the atmosphere. However, the magnitude and the extent of changes remain difficult to predict. A change in the ocean thermohaline circulation and its consequences for climate, such as drought, regional sea-level and ocean carbon uptake remain under debate as this circulation has been long thought to be stable during warm Earth periods – Interglacials. However, recent high-resolution reconstructions of carbon isotopes (δ¹³C) from the deep North Atlantic challenge this idea of stability and point toward abrupt modifications in the ocean interior biogeochemistry and/or ocean thermohaline circulation during the Last Interglacial (LIG, 125ka – 115ka).

Our model simulation of the LIG reproduces the observed magnitude and timescale of the reconstructed variations of δ¹³C, highlighting crucial dynamical changes in two regions of the North Atlantic deep-water formation (south of Greenland and south of Svalbard). These regions are found to drive the variations in the strength of the Atlantic Overturning Circulation (AMOC) when the Arctic sea-ice extent is perturbed.

Our study suggests that the AMOC may have experienced great instability phase during some parts of the LIG. The water mass geometry reorganization from the warm onset at 125ka to the glacial inception at 115ka could also have greatly impacted the distribution of carbon in the interior Ocean. Changes in sea-ice cover either south of Svalbard or in the Southern Ocean seem to play a determining role. However, in our global warming context, our study suggests that the mechanisms responsible for the LIG AMOC instability of the LIG may not occur by the end of the century if the Arctic sea-ice retreats from the high latitudes of the North Atlantic as projected by climate models.

How to cite: Kessler, A., Roche, D., Galaasen, E., Tjiputra, J., Bouttes, N., and Ninnemann, U.: AMOC instability during the Last Inerglacial, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12364, https://doi.org/10.5194/egusphere-egu21-12364, 2021.

To better understand the processes contributing to future climate change, palaeoclimate model simulations are an important tool because they allow testing of the models’ ability to simulate very different climates than that of today.  As part of CMIP6/PMIP4, the latest version of the UK’s physical climate model, HadGEM3-GC31-LL (hereafter, for brevity, HadGEM3), was recently used to simulate the mid-Holocene (~6 ka) and Last Interglacial (~127 ka) simulations and the results were compared to the preindustrial era, previous versions of the same model and proxy data (see Williams et al. 2020, Climate of the Past).  Here, we use the same model to go further back in time, presenting the results from the mid-Pliocene Warm Period (~3.3 to 3 ma, hereafter the “Pliocene” for brevity).  This period is of particular interest when it comes to projections of future climate change under various scenarios of CO₂ emissions, because it is the most recent time in Earth’s history when CO₂ levels were roughly equivalent to today.  In response, albeit due to slower mechanisms than today’s anthropogenic fossil fuel driven-change, during the Pliocene global mean temperatures were 2-3°C higher than today, more so at the poles.

Here, we present results from the HadGEM3 Pliocene simulation.  The model is responding to the Pliocene boundary conditions in a manner consistent with current understanding and existing literature.  When compared to the preindustrial era, global mean temperatures are currently ~5°C higher, with the majority of warming coming from high latitudes due to polar amplification from a lack of sea ice.  Relative to other models within the Pliocene Modelling Intercomparison Project (PlioMIP), this is the 2^nd warmest model, with the majority of others only showing up to a 4.5°C increase and many a lot less.  This is consistent with the relatively high sensitivity of HadGEM3, relative to other CMIP6-class models.  When compared to a previous generation of the same UK model, HadCM3, similar patterns of both surface temperature and precipitation changes are shown (relative to preindustrial).  Moreover, when the simulations are compared to proxy data, the results suggest that the HadGEM3 Pliocene simulation is closer to the reconstructions than its predecessor.

How to cite: Williams, C., Lunt, D., Sellar, A., Roberts, W., Smith, R., Hopcroft, P., and Stone, E.: Simulation of the mid-Pliocene Warm Period using HadGEM3-GC31-LL: Pliocene climate relative to the pre-industrial era, previous model versions, other climate models and proxy data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7707, https://doi.org/10.5194/egusphere-egu21-7707, 2021.

Marine Isotope Stage (MIS) 13, ~500 ky ago, represents a Quaternary interglacial of primary interest due to the unexpected enhancement of monsoon systems under a cool climate characterised by low atmospheric CO₂ and larger ice volume than the present interglacial. Yet, key questions remain about its regional expression (intensity, climate variability, length) and underlying forcing factors. Here we examine the SW Iberian vegetation and terrestrial climate during MIS 13 directly compared with the sea surface temperatures using sediments from IODP Site U1385, and combine those terrestrial-marine profiles with climate-model experiments. We show for the first time that MIS 13 stands out for its large forest expansions with a reduced Mediterranean character alternating with muted forest contractions, indicating that this stage is marked by a cool-temperate climate regime with high levels of humidity. Results of our data-model approach reveal that that the dominant effect of MIS 13 insolation forcing on the regional vegetation and precipitation regime in SW Iberia is amplified by the relatively large extent of the ice-sheets in high northern latitudes. In qualitative agreement with the pollen-based evidence, model results show that ice-sheet forcing triggers an increase in the SW Iberian tree fraction along with both intensified winter and summer rainfall. We propose that the interactions between ice-sheets and major atmospheric circulation systems may have resulted in the persistent influence of the mid-latitude cells over the SW Iberian region, which led to intensified moisture availability and reduced seasonality, and, in turn, to a pronounced expansion of the temperate forest.

How to cite: Oliveira, D., Desprat, S., Yin, Q., Rodrigues, T., Naughton, F., Trigo, R., Su, Q., Grimalt, J. O., Alonso-Garcia, M., Voelker, A. H. L., Abrantes, F., and Sánchez Goñi, M. F.: Enhanced humidity in SW Iberia driven by the combination of insolation and ice-sheet forcing during MIS 13 interglacial, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11408, https://doi.org/10.5194/egusphere-egu21-11408, 2021.

Earth System Models (ESMs) project drying of the northern subtropics by the end of the 21^st century. However, geologic evidence from intervals with elevated concentrations of atmospheric carbon dioxide (pCO₂), like the mid-Pliocene, suggest mesic subtropical conditions. Several hypotheses, including an El Niño-like SST pattern and weaker Hadley circulation, have been proposed to explain this mismatch. Here, we show that PlioMIP2 ensemble broadly capture the pattern of proxy reconstructed Pliocene hydroclimate, notably a wetter Sahel and southeast Asia. Sensitivity simulations reveal that this pattern is driven by summertime rainfall increases as a result of lowered albedo and a distinct surface warming pattern, generated by prescribed vegetation and ice sheet changes. The resultant tropospheric moistening and stationary wave pattern enhance moisture convergence into the northern subtropics. Our results suggest that mid-Pliocene hydroclimate is part of the Earth system feedback to sustained CO₂ concentrations similar to today.

How to cite: Feng, R., Bhattacharya, T., Otto-bliesner, B., and Brady, E. and the PlioMIP2: Mid-Pliocene mesic subtropical hydroclimate over continents driven by land surface changes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13907, https://doi.org/10.5194/egusphere-egu21-13907, 2021.

The mid-Pliocene climate is the most recent geological period with a greenhouse gas concentration of approximately 400 ppmv, similar to the present day. Proxy reconstructions indicate enhanced warming in the high North Atlantic in the mid-Pliocene, which has been suggested to be a response to a stronger Atlantic Meridional Overturning Circulation (AMOC). PlioMIP2 ensemble results show a stronger AMOC and simulated North Atlantic sea surface temperatures (SSTs) match reconstructions better than PlioMIP1. A major difference between PlioMIP1 and PlioMIP2 is the closure of the Bering Strait and Canadian Archipelago in the Pliocene. Previous studies have shown that closure of these Arctic gateways leads to an enhanced AMOC due to altered freshwater fluxes in the Arctic.

Analysis of our Community Earth System Model (CESM1) simulations shows that the simulated increase in North Atlantic SSTs and strengthened AMOC in the Pliocene is a result of Pliocene boundary conditions rather than CO₂ concentration increase. Here we compare results from two runs with pre-industrial boundary conditions and 280 and 560 ppmv CO₂ concentrations and three runs with PlioMIP2 boundary conditions and 280, 400 and 560 ppmv CO₂ concentrations. Results show a 10-15% stronger AMOC in the Pliocene simulations as well as enhanced warming and saltening of the North Atlantic sea surface. While there is a stronger AMOC, the Atlantic northward ocean heat transport (OHT) in the Pliocene simulations only increases 0-3% with respect to the pre-industrial. Analysis indicates there is an altered relationship between the AMOC and OHT in the Pliocene, pointing to fundamentally different behavior of the AMOC in the Pliocene simulations. This is supported by a specific spatial pattern of deep water formation (DWF) areas in the Pliocene simulations that is significantly different from that of the pre-industrial. In the Pliocene simulations, DWF areas adjacent to south Greenland disappear and new DWF areas appear further southwards in the Labrador Sea off the coast of Newfounland. These results indicate that insight into the effect of the palaeogeographic boundary conditions is crucial to understanding the Pliocene climate and its potential as a geological equivalent to a future greenhouse climate.

How to cite: Weiffenbach, J., Baatsen, M., and von der Heydt, A.: Impact of Arctic gateways closure on the Atlantic Meridional Overturning Circulation in the Pliocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9760, https://doi.org/10.5194/egusphere-egu21-9760, 2021.

Simulations of the geological past using General Circulation Models (GCMs) are computationally expensive. Mainly because of the long equilibration time scales, most of these GCMs have ocean components with a horizontal resolution of 1° or coarser. Such models are non-eddying and the effects of mesoscale ocean eddies on the transport of heat and salt are parameterized. However, from present-day ocean modeling studies, it is known that eddying ocean models better represent regional and time-mean ocean flows compared to non-eddying models. At the same time, proxy data from sediment sample sites represent climate at specific locations. Hence, the coarse ocean resolution of typical palaeo-GCMs lead to a challenge for model-data comparison in past climates.

Here we present the first simulations of a global eddying Eocene ocean with a 0.1° (horizontal) resolution model, which are initialized and forced with data from a coarser resolution (1° horizontally) equilibrated coupled ocean-atmosphere GCM. We investigate the response of the model equilibrium state to the change in ocean resolution and the consequences this has for model-data comparison in the middle-late Eocene (38Ma). We find that, compared to the non-eddying model, the eddying ocean resolution of palaeomodels reduce the biases in both sea surface temperatures and biogeographic patterns which are derived from proxy data.

How to cite: Nooteboom, P., Baatsen, M., Bijl, P., van Sebille, E., Sluijs, A., Dijkstra, H., and von der Heydt, A.: Model-data comparison in a strongly eddying Eocene ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8383, https://doi.org/10.5194/egusphere-egu21-8383, 2021.

Early Eocene Climatic Optimum (EECO, ~53-51 million years) is one of the past warm periods, associated with high CO₂ concentrations (~900-2500 ppmv), which can serve as an analogue for our possible future, high C0₂ climate. One notable feature of this hothouse climate state is the weaker meridional temperature gradient relative to pre-industrial values. This have been confirmed by both proxies and models, but the extent of the temperature gradient still requires more research. Models are challenged to reproduce the stronger than present day polar amplification signal, and it is also shown that high latitude proxy data are often influenced by seasonal bias. Thus, there is an uncertainty regarding both the observed and modelled meridional gradient and the mentioned issues complicate also the comparison between modeled and proxy data.

In our work we aim to investigate the EECO period with a simple energy balance box model and apply the maximum entropy production principle to explore the possible scenarios of meridional temperature gradients. We find that the maximum entropy production principle could be beneficial in the paleoclimate context since it has the utility to give an accurate prediction for non-equilibrium systems with the minimal amount of information. We also assess the heat transport signaled by proxy data and by state-of-the-art model outputs in accordance to our theoretical constrains based on the idealized test case.

How to cite: Kelemen, F. D. and Ahrens, B.: Exploring the possible meridional temperature gradient of Early Eocene Climatic Optimum with an energy balance model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7365, https://doi.org/10.5194/egusphere-egu21-7365, 2021.

Past climates contain precious information about the workings of the climate system, and about what can be expected in a changed climate. The Last Interglacial (LIG; ca. 125,000 years ago) is the most recent period of climate warmer than modern, at least in the Northern Hemisphere. Because of this, it has been often proposed that the LIG holds a partial analogy with a future warmer climate forced by enhanced greenhouse effect. Still, such analogy has never been examined in a quantitative manner. Here we address the question: for which scenario, time horizon, regions and season is the climate of the LIG a useful analogue of the future? We use the results of 13 climate models that performed the standard experiments of PMIP4 and CMIP6, and present a comparison of hemispheric temperature and precipitation between the LIG and SSP scenarios of the future. We also two independent assessments of models performance, by comparing their temperature and precipitation to climate reanalysis of the last decades and to proxies of the LIG. Insights gained from this comparison can inform studies in disciplines beyond climate studies, such as hydrology and ecology.

How to cite: Scussolini, P., Bakker, P., De Luca, P., Coumou, D., Bosmans, J., Lohmann, G., Thomas, Z., Turney, C., Menviel, L., Obase, T., Abe-Ouchi, A., Braconnot, P., Otto-Bliesner, B., Yin, Q., Prange, M., Tzedakis, C., Capron, E., Renssen, H., Ward, P., and Aerts, J.: Global temperature and hydroclimate in warmer climates of the past and future: the Last Interglacial versus greenhouse scenarios, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15417, https://doi.org/10.5194/egusphere-egu21-15417, 2021.

We present new insights from the project PalMod, which started in 2016 and is envisioned to run for a decade. The modelling initiative PalMod aims at filling the long-standing scientific gaps in our understanding of the dynamics and variability of the climate system during the last glacial-interglacial cycle. One of the grand challenges in this context is to quantify the processes that determine the spectrum of climate variability on timescales that range from seasons to millennia. Climatic processes are intimately coupled across these timescales. Understanding variability at any one timescale requires understanding of the whole spectrum. If we could successfully simulate the spectrum of climate variability during the last glacial cycle in Earth system models, would this enable us to more reliably assess the future climate change? Such simulations are necessary to deduce, for example, if a regime shift in climate variability could occur during the next centuries and millennia in response to global warming. PalMod is specifically designed to enhance our understanding of the Earth system dynamics and its variability on timescales up to the multimillennial with complex Earth System Models.

The following major goals were achieved up to now:

• Full coupling of atmosphere, ocean and ice-sheet models, enabling investigation of Heinrich Events and bi-stability of the AMOC, and millennial-scale transient climate-ice sheet simulations.
• Implementation of a coupled ocean and land biogeochemistry enabling simulations with prognostic atmospheric CO₂ concentrations and including improved representation of methane (CH₄) in transient deglaciation runs.
• Systematic comparison of newly compiled proxy data with model simulations.

The major goal for the next two years is to set up the fully coupled physical-biogeochemical model which will be tested for three time periods: deglaciation, glacial inception and Marine Isotope Stage 3 (MIS3). This fully coupled model will be eventually used to simulate the complete glacial cycle and project the climate over the next few millennia.

How to cite: Fieg, K., Latif, M., Schulz, M., and Ilyina, T.: From the last interglacial to the future – new insights from modeling the last glacial-interglacial cycle in PalMod, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11083, https://doi.org/10.5194/egusphere-egu21-11083, 2021.

We propose a reduced-complexity process-based model for the long-term evolution of the global ice volume, atmospheric CO₂ concentration and global mean temperature. The model only external forcings are the orbital forcing and anthropogenic CO₂ cumulative emissions. The model consists of a system of three coupled non-linear differential equations, representing physical mechanisms relevant for the evolution of the climate – ice sheets – Carbon cycle system in timescales longer than thousands of years. The model is successful in reproducing the glacial-interglacial fluctuations of the last 800 kyr, in good agreement with paleorecords both in terms of timing and amplitude, with a correlation between modelled and paleo global ice volume of up to 0.86.

Using different model realisations, we generate a probabilistic forecast of the evolution of the Earth system over the next 1 million years under natural and several fossil-fuel CO₂ release scenarios. In the natural scenario, the model assigns high probability of occurrence of long interglacials in the periods between present and 50 kyr after present, and between 400 kyr and 500 kyr after present. The next full glacial conditions are most likely to occur 90 kyr after present. The model shows that even already achieved cumulative CO₂ anthropogenic emissions (500 PgC) are capable of affecting the climate evolution for up to half million years, indicating that the beginning of the next glaciation is highly unlikely in the next 150 kyr. If cumulative fossil-fuel CO₂ emissions reach 3000 PgC, or higher, the model predicts with high probability ice-free Northern Hemisphere landmass conditions will prevail in the next half million years, postponing the natural occurrence of the next glacial inception to 600 kyr after present.

How to cite: Talento, S. and Ganopolski, A.: Evolution of the climate in the next million years: A reduced-complexity model for glacial cycles and impact of fossil fuel CO2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8512, https://doi.org/10.5194/egusphere-egu21-8512, 2021.

Between the last glacial maximum (LGM) and preindustrial times (PI), the atmospheric concentration of CH₄, as shown by reconstructions from ice cores, roughly doubled. It then doubled again from PI to the present. Ice cores, however, cannot tell us how that development will continue in the future, and ice cores also cannot shed light on the causes of the rise in methane, as well as the rapid fluctuations during periods such as the Bolling-Allerod and Younger Dryas.

We use a methane-enabled version of MPI-ESM, the Max Planck Institute for Meteorology Earth System Model, to investigate changes in methane cycling in a transient ESM experiment from the LGM to the present, continuing onwards into the future for the next millennium. The model is driven by prescribed orbit, greenhouse gases and ice sheets, with all other changes to the climate system determined internally. Methane cycling is modelled by modules representing the atmospheric transport and sink of methane, as well as terrestrial sources and sinks from soils, termites, and fires. Thus, the full natural methane cycle – with the exception of geological and animal emissions – is represented in the model. For historical and future climate, anthropogenic emissions of methane are considered, too.

We show that the methane increase since the LGM is largely driven by source changes, with LGM emissions substantially reduced in comparison to the early Holocene and preindustrial states due to lower temperature, CO₂, and soil carbon. Depending on the future climate scenario, these dependencies then lead to further increases in CH₄, with a further doubling of atmospheric CH₄ easily possible if one of the higher radiative forcing scenarios is followed. Furthermore, the future increases in CH₄ will persist for a long time, as CH₄ only decreases when the climate system cools again.

How to cite: Kleinen, T., Gromov, S., Steil, B., and Brovkin, V.: Methane in the climate system -- from the last glacial to the future, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12440, https://doi.org/10.5194/egusphere-egu21-12440, 2021.

Much of the inter-model spread in equilibrium climate sensitivity (ECS) estimates is attributed to cloud and convective parameterizations which model cloud and water vapor feedbacks. These parameterizations also directly influence water isotopes, which may be retrieved not only from modern observations, but also a plethora of paleoclimate archives that represent a much broader range of variability than is available in modern measurements. And thus, these water isotope tracers can be used to constrain ECS by flagging unrealistic parts of the parameterization phase space via model biases in a perturbed parameterization ensemble (PPE) of paleoclimate simulations. In this proof-of-concept study, we evaluate a suite of isotope-enabled atmosphere-only GISS-E2.1 simulations, each with varying cloud and convective perturbations, against speleothem and ice core δ¹⁸O for the Last Glacial Maximum (LGM, 21000 years ago), mid-Holocene (MH, 6000 years ago) and pre-Industrial periods. The first-order spatial pattern of δ¹⁸O of precipitation (δ¹⁸O_p) is in excellent agreement between proxy data and all parameterizations across all time periods. While the simulations generally capture large scale δ¹⁸O_p patterns, the magnitude of change is consistently smaller in all simulations than those of the proxies, highlighting uncertainties in both models and proxies. Not a single set of parameterizations worked well in all climate states, indicating that improving future simulations requires determining all plausible parameter combinations critical in refining ECS. Further, it may be that certain parameterization choices represent certain types of variability better than others, and there may be a non-unique solution to ideal clouds/convection parameterization choices that is modulated by the question asked.

How to cite: Ramos, R. D., LeGrande, A. N., Griffiths, M. L., Elsaesser, G. S., Litchmore, D. T., Tierney, J. E., Pausata, F. S. R., and Nusbaumer, J.: Using paleoclimate data to constrain cloud parameterizations in GISS-E2.1, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14136, https://doi.org/10.5194/egusphere-egu21-14136, 2021.

In simulations of the climate during the Last Glacial Maximum (LGM), we employ two different isotope-enabled atmospheric general circulation models (NCAR iCAM3 and MPI ECHAM6-wiso) and use simulated (by coupled climate models) as well as reconstructed (from a new global climatology of the ocean surface duing the LGM, GLOMAP) surface conditions.

The resulting atmospheric fields reflect the more pronounced structure and gradients in the reconstructions, for example, the precipitation is more depleted in oxygen-18 in the high latitudes and more enriched in low latitudes, especially in the tropical convective regions over the maritime continent in the equatorial Pacific and Indian Oceans and over the equatorial Atlantic Ocean. Furthermore, at the sites of ice cores and speleothems, the model-data fit improves in terms of the coefficients of determination and root-mean square errors.

In additional sensitivity experiments, we also use the climatologies by Annan and Hargreaves (2013) and Tierney et al. (2020) and consider the impact of changes in reconstructed sea-ice extent and the global-mean sea-surface temperature.

Our findings imply that the correct simulation or reconstruction of patterns and gradients in sea-surface conditions are crucial for a successful comparison to oxygen-isotope data from ice cores and speleothems.

How to cite: Paul, A., Cauquoin, A., Mulitza, S., Tharammal, T., and Werner, M.: Sensitivity of simulated oxygen isotopes in ice cores and speleothems to Last Glacial Maximum surface conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2305, https://doi.org/10.5194/egusphere-egu21-2305, 2021.

We present new reconstructions of global climatological temperature fields for the Last Glacial Maximum and the mid-Pliocene Warm Period.

The method is based on an Ensemble Kalman Smoother which combines globally complete modelled temperature fields, with sparse proxy-based estimates of local temperature anomalies. This ensures spatially coherent fields which respect physical principles and which are also tied closely to observational estimates. 

For the Last Glacial Maximum, we use the full set of PMIP2/3/4 model simulations, and we combine this with a wide range of proxy-based SST and SAT estimates of local temperature to ensure the best possible global coverage. Our reconstruction has a global mean surface air temperature anomaly of -5.3 +- 0.9C relative to the pre-industrial climate, and thus lies roughly half-way between the estimates of Annan and Hargreaves (2013) and Tierney et al (2020). We examine the reasons for these differences and discuss their implications.

For the mid-Pliocene Warm Period, we use the PlioMIP 1 and 2 model simulations and the PRISM proxy estimates for the 3.2 Ma time slice. These data are considerably more sparse and uncertain than for the LGM and our reconstruction is correspondingly more uncertain. We obtain an estimate of 5.6 +- 1.6C which is considerably warmer than most previous estimates, suggesting a significant discrepancy between the models and the data. We investigate the reasons for this and discuss the implications.

How to cite: Annan, J., Hargreaves, J., and Mauritsen, T.: Reconstructing the surface temperature fields of the Last Glacial Maximum and mid-Pliocene Warm Period using climate models and data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11047, https://doi.org/10.5194/egusphere-egu21-11047, 2021.

Comparing simulations and data from paleoclimate archives such as speleothems can test the capability of climate models to capture past climate changes. In past, present, and future, the hydrologic response to radiative forcing changes is far less understood and more uncertain than thermal changes.
 
Speleothems store terrestrial climate information in the form of isotopic oxygen in mineral and are found mostly in the low-to mid-latitudes of the landmasses. Their usually well preserved (semi-)continuous time series of oxygen isotope ratio δ¹⁸O can cover full Glacial-Interglacial cycles and are used for past climate reconstructions. However, the measured δ¹⁸O in the mineral is influenced by multiple climate and cave-related variables and does, therefore, not directly represent past temperature or precipitation. 

We assess the capability of the isotope-enabled models HadCM3 and ECHAM5-MPI/OM to simulate decadal to centennial climate variability beyond the instrumental period. In particular, we investigate the relationship between simulated δ¹⁸O and precipitation variability under different background conditions. By comparing simulated δ¹⁸O values at cave locations to the large global speleothem database SISALv2 (Comas-Bru et al. 2020), we also examine the consistency between modeled and archived temporal changes in δ¹⁸O in the mean state and variability. Our strategy involves forward-modeling of cave processes such as temperature-dependent fractionation and transit times to constrain a simple speleothem proxy model for the simulation output. For the late Holocene, we observe a strongly underestimated simulated isotopic variability on decadal to centennial timescales. We further test how much this underestimation depends on the background radiative forcing conditions by comparing the Last Glacial Maximum, the mid-Holocene, and the late Holocene. This provides deeper insight on low to mid-latitude state-dependent climate variability on decadal to centennial time scales. 

Reference:

Comas-Bru, L., et. al. SISALv2: a comprehensive speleothem isotope database with multiple age-depth models. Earth System Science Data 12, 2579-2606 (2020) https://essd.copernicus.org/articles/12/2579/2020/

How to cite: Buehler, J., Weitzel, N., Baudouin, J.-P., Werner, M., and Rehfeld, K.: Last Glacial to present-day variability of surface climate from oxygen isotope signatures in speleothems and model simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9847, https://doi.org/10.5194/egusphere-egu21-9847, 2021.

Experiment outputs are now available from the Coupled Model Intercomparison Project’s 6^th phase (CMIP6) and the past climate experiments defined in the Model Intercomparison Project’s 4^th phase (PMIP4). All of this output is freely available from the Earth System Grid Federation (ESGF). Yet there are overheads in analysing this resource that may prove complicated or prohibitive. Here we document the steps taken by ourselves to produce ensemble analyses covering past and future simulations. We outline the strategy used to curate, adjust the monthly calendar aggregation and process the information downloaded from the ESGF. The results of these steps were used to perform analysis for several of the initial publications arising from PMIP4. We provide post-processed fields for each simulation, such as climatologies and common measures of variability. Example scripts used to visualise and analyse these fields is provided for several important case studies.

How to cite: Zhao, A. and Brierley, C.: Workflow and tools to analyse the PMIP4-CMIP6 ensemble, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2476, https://doi.org/10.5194/egusphere-egu21-2476, 2021.

How to cite: Weitzel, N., Andres, H., Baudouin, J.-P., Bothe, O., Dolman, A., Jonkers, L., Kapsch, M., Kleinen, T., May, M., Mikolajewicz, U., Paul, A., and Rehfeld, K.: Towards model-data comparison of the deglacial temperature evolution in space and time, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9683, https://doi.org/10.5194/egusphere-egu21-9683, 2021.

Modelling studies are evaluated by comparing the simulation outputs to an observational reference. In climate science, the number and complexity of the models and the mass of data have led the community to develop standardised methods and automated tools, such as the Climate Variability Diagnostic Package or the ESMValTool. However, these tools are mostly designed to evaluate simulations of the instrumental period. Different methods are required to compare paleoclimate simulations to palaeodata. For example, new variables are being modelled, such as vegetation, ice sheet extent, or isotopic ratio, and are used for the evaluation. Changing boundary conditions in transient simulations further complicate the evaluation process: traditional indices that characterise circulation (e.g. monsoon) or modes of variability (e.g. NAO, ENSO) need to be adapted, while new ones are needed to investigate modes of longer timescale and abrupt events. Finally, the palaeodata also present challenges: various type of uncertainty, complex relation to climate variables, and different spatio-temporal representativeness compared to model outputs. Here, we summarise the challenges of model-data comparison in paleo-climate studies. We then review some of the different methods and tools already developed by the community, such as biome comparison and Bayesian approaches to quantify model-data deviation. We finally discuss the implementation of an evaluation framework which aims to provide both adaptable tools to the community and automated standardised analyses.

How to cite: Baudouin, J.-P., Bothe, O., Chevalier, M., Weitzel, N., Dallmeyer, A., Brierley, C., and Rehfeld, K.: Model-data comparison challenges in paleo-climate analyses: Towards an evaluation toolbox for transient climate model simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5716, https://doi.org/10.5194/egusphere-egu21-5716, 2021.

How to cite: Ziegler, E., Andres, H., Ellerhoff, B., Kapsch, M.-L., Kutterolf, S., Mikolajewicz, U., Schindlbeck-Belo, J. C., Toohey, M., Wirths, C., Weitzel, N., and Rehfeld, K.: State-dependency of temperature variability in transient simulations of the last Deglaciation from models of varying complexity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11006, https://doi.org/10.5194/egusphere-egu21-11006, 2021.

Fossil pollen datasets can help to understand the temporal and spatial distribution patterns and driving forces of the past terrestrial biomes in high northern latitudes. Here we present a global pollen dataset since the Last Glacial Maximum, synthesized from 2821 palynological records from the Neotoma Paleoecology Database and additional literature. All terrestrial pollen taxa were taxonomically harmonized on genus (woody taxa) or family level (herb taxa) and temporally standardized by using a defined parameter setting for Bayesian age-depth modeling based on 14C dating. The age-depth models were statistically compared with existing models for each record. With a biomization approach, we reconstructed biomes for several time-slices throughout the last 22000 years with a temporal resolution of roughly 500 years. The reconstructed biome distributions are compared to simulated biome distributions inferred from a transient simulation for the last 25000 years, performed in the comprehensive Earth System Model of the Max Planck Institute (MPI-ESM). The overall biome trend agrees well, but the simulation shows lower forest cover in the high northern latitudes and reaches the maximum forest cover in the Holocene much earlier than the reconstructions indicate.

How to cite: Li, C., Dallmeyer, A., Böhmer, T., Postl, A., and Herzschuh, U.: Northern hemispheric biome changes synthesized from taxonomically harmonized and temporally standardized fossil pollen record since the Last Glacial Maximum in comparison to MPI-ESM simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12224, https://doi.org/10.5194/egusphere-egu21-12224, 2021.

Southern Ocean sea ice and oceanic fronts are known to play an important role on the climate system, carbon cycles, bottom ocean circulation, and Antarctic ice sheet. However, many models of the previous Past-climate Model Intercomparison Project (PMIP) underestimated sea-ice extent (SIE) for the Last Glacial Maximum (LGM)(Roche et al., 2012; Marzocchi and Jensen, 2017), mainly because of surface bias (Flato et al., 2013) that may have an impact on mean ocean temperature (MOT). Indeed, recent studies further suggest an important link between Southern Ocean sea ice and mean ocean temperature (Ferrari et al., 2014; Bereiter et al., 2018 among others). Misrepresent the Antarctic sea-ice extent could highly impact deep ocean circulation, the heat transport and thus the MOT. In this study, we will stress the relationship between the distribution of Antarctic sea-ice extent and the MOT through the analysis of the PMIP3 and PMIP4 exercise and by using a set of MIROC models. To date, the latest version of MIROC improve its representation of the LGM Antarctic sea-ice extent, affecting the deep circulation and the MOT distribution (Sherriff-Tadano et al., under review).

Our results show that available PMIP4 models have an overall improvement in term of LGM sea-ice extent compared to PMIP3, associated to colder deep and bottom ocean temperature. Focusing on MIROC (4m) models, we show that models accounting for Southern Ocean sea-surface temperature (SST) bias correction reproduce an Antarctic sea-ice extent, 2D-distribution, and seasonal amplitude in good agreement with proxy-based data. Finally, using PMIP-MIROC analyze, we show that it exists a relationship between the maximum SIE and the MOT, modulated by the Antarctic intermediate and bottom waters.

How to cite: Vadsaria, T., Sherriff-Tadano, S., Abe-Ouchi, A., Obase, T., Chan, W.-L., and Crosta, X.: Last Glacial Maximum Antarctic sea ice linked with global mean ocean temperature: evidence from PMIP3, PMIP4 and MIROC-4m simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3550, https://doi.org/10.5194/egusphere-egu21-3550, 2021.

The last deglaciation provides as unique a framework to investigate the processes of ice sheet and climate interaction during periods of mass loss as in the current climate. Here we simulate the Last Glacial Maximum (LGM) northern hemisphere ice sheets climate, surface mass balance (SMB), and dynamics with the Community Earth System Model version 2 (CESM2, Danabasoglu et al., 2020)) and the Community Ice Sheet Model version 2 (CISM2, Lipscomb et al., 2019). This LGM simulation will be later used as starting point for coupled CESM2-CISM2 simulations of the last deglaciation.

CESM2 is run at the nominal resolution used for IPCC-type projections (approx. 1 degree for all components). The model includes an advanced snow/firn and SMB calculation (van Kampenhout et al, 2019; Sellevold et al, 2019) the land component (CLM, cite) that has been evaluated and applied to the simulation of the future Greenland melt (van Kampenhout et al, 2020, Muntjewerf et al., 2020a,b, Sellevold & Vizcaino, 2020).

Our analysis examines how the global, Arctic, and North Atlantic climate result in the simulated radiative and turbulent heat fluxes over the ice sheets, and the mass fluxes from precipitation, refreezing, runoff, and sublimation. We also examine the simulated ice streams in CISM2, which is run at 8 km under a higher-order approximation for ice flow.

How to cite: Bradley, S. L., Petrini, M., Sellevold, R., Vizcaino, M., Lipscomb, W. H., and Georgiou, S.: Simulation of LGM ice sheets with the Community Earth System Model version 2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15463, https://doi.org/10.5194/egusphere-egu21-15463, 2021.

The DO events of the last ice age represent one of the best studied abrupt climate transitions, yet we still lack a comprehensive explanation for them. There is uncertainty whether current IPCC-relevant models can effectively represent the processes that cause DO events. Current Earth system models (ESMs) seem overly stable against external perturbations and incapable of reproducing most abrupt climate changes of the past (Valdes, 2011). If this holds true, this could noticeably influence their capability to predict future abrupt transitions, with significant consequences for the delivery of precise climate change projections.  In this task, the objectives of this study are (1) to cross compare existing simulations that show spontaneous DO-type oscillations using a common set of diagnostics so we can compare the mechanisms and the characteristics of the oscillations, and (2) to formulate possible pathways to a DO PMIP protocol that could help investigate cold-period instabilities through a range of insolation-, freshwater-, GHG-, and NH ice sheet-related forcings, as well as evaluating the possibility of spontaneous internal oscillations.

Although most abrupt DO events happened during MIS3, only few studies investigate DO events in coupled general circulation models under MIS 3 conditions (e.g., Kawamura et al., 2017; Zhang and Prange, 2020). Here, we thus propose that the MIS3 period could be the focus of such a DO-event modelling protocol. More specific sensitivity experiments performed under MIS 3 boundary conditions are needed in order to (1) better understand the mechanisms behind millennial-scale climate variability, (2) explore AMOC variability under intermediate glacial conditions, and (3) help answer the question: “are models too stable?”.

How to cite: Malmierca-Vallet, I., Sime, L. C., and Valdes, P. J.: Possible pathways to a Dansgaard-Oeschger (DO) PMIP protocol, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8730, https://doi.org/10.5194/egusphere-egu21-8730, 2021.

Understanding the processes causing variations in the carbon cycle is critical to accurately simulate the future carbon cycle and climate. Paleoclimate models can provide insights about these processes since they are used under different conditions than present-day’s and evaluated against paleoproxy data. In particular, the Last Glacial Maximum (LGM) has been a focus of the Paleoclimate Modelling Intercomparison Project (PMIP) as it is well-documented thanks to numerous paleoclimate archives. Around 21,000 years ago, the LGM was a colder period with extensive ice sheets in the Northern Hemisphere and a resulting lower sea-level. Although this period has been studied for years, the causes of the lower atmospheric CO₂ concentration at the time (around 186 ppm, against 280 ppm at the pre-industrial) remain unclear, and models struggle to simulate this low CO₂ value. The ocean is thought to have played a significant role due to different processes (through changes of the biological pump efficiency, ocean circulation, sea-ice, and CO₂ solubility due to colder temperatures), but no consensus has been reached yet as to their contribution (Khatiwala et al. [2019], Yu et al. [2016], Marzocchi and Jansen [2019]).

Despite the carbon cycle being simulated by more and more climate models, it has not been systematically analysed within the framework of PMIP multimodel comparisons. In this context, the ongoing PMIP-carbon project aims at comparing climate-carbon interactions in LGM simulations, and includes results from both intermediate complexity models and general circulation models. The PMIP protocol proposes standardized forcing parameters and boundary conditions (Kageyama et al. [2017]) and specifies a few recommendations for ocean biogeochemistry models (adjustment of salinity, dissolved inorganic carbon, alkalinity, and nutrients to account for the change in ocean volume). Indeed, the bathymetric changes associated with a sea-level drop of 133 m entail a change of the reservoir size and potential technical issues concerning the conservation of carbon.

In this study, we use outputs from PMIP-carbon models and other models available on the ESGF (MIROC4m-COCO, MIROC-ES2L, CESM, IPSL-CM5A2, UVic, LOVECLIM, iLOVECLIM, CLIMBER_2P ; GISS-E2-R, MRI-CGCM3, MPI-ESM-P, CNRM-CM5, MIROC-ESM) to compute total ocean volumes and compare them to high resolution topographic data (etopo1 for the PI, GLAC-1D and ICE-6G-C for the LGM). We show that the deglacial volume change is rarely accurate. We then use the iLOVECLIM model with a new bathymetry implementation method (Lhardy et al. [in review, 2020]) to demonstrate the effect of an improved ocean volume on the simulated oceanic carbon content, resulting in an increase of the already overestimated atmospheric CO₂ concentration. We also quantify the effect of the mentioned adjustments of salinity, alkalinity, and carbon. The results reinforce the idea that a realistic ocean volume is needed, as well as consistency between models in dealing with large changes in bathymetry.

How to cite: Lhardy, F., Bouttes, N., Roche, D., Abe-Ouchi, A., Chase, Z., Ivanovic, R., Jochum, M., Kageyama, M., Kobayashi, H., Menviel, L., Muglia, J., Nuterman, R., Oka, A., Schmittner, A., Vettoretti, G., and Yamamoto, A.: The impact of bathymetry on the simulated carbon at the Last Glacial Maximum, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7297, https://doi.org/10.5194/egusphere-egu21-7297, 2021.

Coupled climate models have produced very different states of the Atlantic Meridional Overturning Circulation (AMOC) in simulations of the Last Glacial Maximum (LGM). In particular, many of them failed to capture the shoaling of the upper AMOC cell, which was indicated by reconstructions. In sensitivity simulations with the Max-Planck-Institute Earth System Model (MPI-ESM) we found that the glacial AMOC response is the sum of two large opposing effects: a strengthening and deepening of the upper cell in response to the glacial ice sheets and a weakening and shoaling of the upper cell in response to the low glacial greenhouse gas concentrations. The magnitude of the respective effects likely depends on the background climate, the ice sheet reconstruction used, and model specifics such as the representation of brine release in the Southern Ocean.

Transient simulations of the deglaciation with two differently tuned versions of MPI-ESM and two different ice-sheet reconstructions differ strongly in their respective AMOC states during the LGM. These simulations, together with selected PMIP3 and PMIP4 LGM simulations, provide a good opportunity to compare the effect of different ice sheet reconstructions on the glacial AMOC. We compare key variables such as water mass properties, salt transport and Southern Ocean sea-ice formation across this ensemble of opportunity with the aim of increasing our understanding of the role of ice sheets in the glacial AMOC response.

How to cite: Klockmann, M., Kapsch, M.-L., and Mikolajewicz, U.: Different ice sheets – different AMOC? Revisiting the ice-sheet effect on the LGM AMOC, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8652, https://doi.org/10.5194/egusphere-egu21-8652, 2021.

The last deglaciation is a time of large climate transition from a cold Last Glacial Maximum at 21,000 years BP with extensive ice sheets, to the warmer Holocene 9,000 years BP onwards with reduced ice sheets. Despite more and more proxy data documenting this transition, the evolution of climate is not fully understood and difficult to simulate. The PMIP4 protocol (Ivanovic et al., 2016) has indicated which boundary conditions to use in model simulations during this transition. The common boundary conditions should enable consistent multi model and model-data comparisons. While the greenhouse gas concentration evolution and orbital forcing are well known and easy to prescribe, the evolution of ice sheets is less well constrained and several choices can be made by modelling groups. First, two ice sheet reconstructions are available: ICE-6G (Peltier et al., 2015) and GLAC-1D (Tarasov et al., 2014). On top of topographic changes, it is left to modelling groups to decide whether to account for the associated bathymetry and land-sea mask changes, which is technically more demanding. These choices could potentially lead to differences in the climate evolution, making model comparisons more complicated.

We use the iLOVECLIM model of intermediate complexity (Goosse et al., 2010) to evaluate the impact of different ice sheet reconstructions and the effect of bathymetry changes on the global climate evolution during the Last deglaciation. We test the two ice sheet reconstructions (ICE-6G and GLAC-1D), and have implemented changes of bathymetry and land-sea mask. In addition, we also evaluate the impact of accounting for the Antarctic ice sheet evolution compared to the Northern ice sheets only.

We show that despite showing the same long-term changes, the two reconstructions lead to different evolutions. The bathymetry plays a role, although only few changes take place before ~14ka. Finally, the impact of the Antarctic ice sheet is important during the deglaciation and should not be neglected.

References

Goosse, H., et al., Description of the Earth system model of intermediate complexity LOVECLIM version 1.2, Geosci. Model Dev., 3, 603–633, https://doi.org/10.5194/gmd-3-603-2010, 2010

Ivanovic, R. F., et al., Transient climate simulations of the deglaciation 21–9 thousand years before present (version 1) – PMIP4 Core experiment design and boundary conditions, Geosci. Model Dev., 9, 2563–2587, https://doi.org/10.5194/gmd-9-2563-2016, 2016

Peltier, W. R., Argus, D. F., and Drummond, R., Space geodesy constrains ice age terminal deglaciation: The global ICE-6G_C (VM5a) model, J. Geophys. Res.-Sol. Ea., 120, 450–487, doi:10.1002/2014JB011176, 2015

Tarasov,L.,  et al., The global GLAC-1c deglaciation chronology, melwater pulse 1-a, and a question of missing ice, IGS Symposium on Contribution of Glaciers and Ice Sheets to Sea-Level Change, 2014

How to cite: Bouttes, N., Roche, D., Lhardy, F., Quiquet, A., Paillard, D., and Goosse, H.: Impact of ice sheet reconstructions on the deglacial climate in iLOVECLIM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8776, https://doi.org/10.5194/egusphere-egu21-8776, 2021.

How to cite: Romé, Y., Ivanovic, R., and Gregoire, L.: Meltwater driven abrupt climate changes in the North Atlantic simulated in a Heinrich Stadial 1 background, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8724, https://doi.org/10.5194/egusphere-egu21-8724, 2021.

The global mean sea level in the last interglacial (LIG, about 130,000 to 115,000 years before present) was very likely higher than the present level, driven mainly by mass loss of the Antarctic ice sheet. Some studies have suggested that this mass loss may have been caused by the warmer temperature over the Southern Ocean in the LIG compared with the present interglacial. However, the ultimate cause of the difference in Antarctic warming between the last and current interglacials has not been explained. Here, based on transient simulations of the last deglaciation using a fully coupled ocean–atmosphere model, we show that greater meltwater (by a factor of 1.5 relative to the last deglaciation) during the middle and later stages of the deglaciation could have produced the difference in Antarctic warmth. Northern Hemisphere ice sheet model experiments suggest that the difference in meltwater was caused by slightly smaller orbital eccentricity in our current interglacial than in the LIG, indicating that mass loss of the Antarctic ice sheet is influenced by the preceding northern summer insolation and disintegration of Northern Hemisphere ice sheets.

How to cite: Obase, T., Abe-Ouchi, A., and Saito, F.: Antarctic warmth in the last interglacial driven by Northern insolation and deglaciation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8145, https://doi.org/10.5194/egusphere-egu21-8145, 2021.

A popular hypothesis in paleoclimatology posits that the episodic discharges of glacial water from the Laurentide Ice Sheet (LIS) to the North Atlantic caused abrupt changes in ocean circulation and climate during the last (de)glacial periods. Implicit in this hypothesis is that the glacial water spread away from the coast and reached critical sites of deep water formation. Among the processes that could favour the offshore export of glacial water released along the eastern North American coast is the entrainment with the Gulf Stream near Cape Hatteras, where the Stream is observed to detach from the coast in the modern climate, or at other locations between Cape Hatteras and the Grand Banks of Newfoundland.

Here we investigate the fate of glacial water released in the western North Atlantic from the Laurentian Channel, which geologic evidence suggests to have been the main route of ice discharge from the Québec-Labrador Ice Dome of the LIS. To this end, we conduct numerical experiments with an ocean circulation model with eddy-resolving resolution and configured to represent the region north of Bermuda and west of the Grand Banks. Experiments with different regional sea levels are performed which correspond to different estimates of global sea level since the Last Glacial Maximum. In each experiment, glacial water in liquid form is discharged from the Laurentian Channel, providing a paleoceanographic analogue of the dam-break problem. As expected from the action of the Coriolis force and from the properties of the glacial water inflow, the discharged water turns to the right of the Channel and then produces a narrow buoyant current that flows along the coast to the southwest towards Cape Hatteras. Our presentation will focus on the interaction of this current with the Gulf Stream, particularly with its meanders and rings, and on the role of this interaction both in the seaward export of glacial water and in the modification of the Stream itself.

How to cite: Marchal, O. and Condron, A.: Interaction of a Glacial Water Outflow with the Gulf Stream, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6346, https://doi.org/10.5194/egusphere-egu21-6346, 2021.

Freshwater, in the form of glacial runoff, is hypothesized to play a critical role in centennial to millennial scale climate variability, eg. the Younger Dryas and Dansgaard Oeschger events. Freshwater injection, or hosing, model experiments demonstrate that freshwater has the capability to generate abrupt climate transitions.  However, in an attempt to mitigate the inability of most models to resolve the smaller-scale features relevant to freshwater transport (such as boundary currents and mesoscale eddies), these hosing experiments commonly apply the entirety of the freshwater directly to the regions of deepwater formation (DWF). Our results indicate that this can inflate the freshwater signal in those regions by as much as four times. We propose a novel method of freshwater injection for such low-resolution models that spatially distributes the freshwater in accord with the results of eddy-permitting modelling. Furthermore, this “freshwater fingerprint” method not only impacts the timing of simulated climate transitions but also can allow us to evaluate how much we are overestimating the effects of freshwater when injected directly into sites of DWF.

The freshwater fingerprints we develop are based on a suite of freshwater injection experiments performed using an eddy permitting Younger Dryas configuration of the MITGCM. Freshwater injection locations include the Mackenzie River, Gulf of St. Lawrence, Gulf of Mexico and a location off the coast of Norway, with flux amounts bounded by glacial reconstructions. These simulations indicate that freshwater from the Mackenzie River and Fennoscandia have the largest impact on salinity in most of the conventional sites of DWF (GIN and Labrador Seas, and in these simulations, predominantly the northern North Atlantic due to extensive sea ice), while freshwater from the Gulf of St. Lawrence is effective at freshening only the northern North Atlantic. The Gulf of Mexico has little impact on any DWF region we consider, mostly because the lower but continual flux in our simulations does not allow freshwater to penetrate northward past the Gulf Stream. The dilution of the freshwater signal as it is transported from the site of injection to the DWF zones leads to a reduction in the effective freshwater forcing, making hosing directly over DWF zones even with realistic freshwater amounts unrealistic. Thus, we construct freshwater fingerprints from these simulations by extracting the freshwater anomaly spatial pattern averaged over the last 5 simulation years, vertically integrating the field and normalizing it.

The freshwater fingerprint is then implemented in the COSMOS Earth Systems Model, which is run at resolutions typical for paleoclimate simulations (non-eddy permitting). Initial results show that freshwater from the Mackenzie River using our  fingerprint method leads to a more gradual cooling than if the meltwater is released directly over the hosing region (50-70N). We conclude that hosing over DWF zones, even with realistic freshwater amounts, produces an unrealistically large climate response. Additional results for the remaining injection locations and with the fingerprint implemented in a simpler climate model will be presented.

How to cite: Love, R., Andres, H., Condron, A., Zhang, X., Lohmann, G., and Tarasov, L.: Towards more physically constrained freshwater injection via eddy permitting simulations of the last glacial cycle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10357, https://doi.org/10.5194/egusphere-egu21-10357, 2021.