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Heinrich-type ice-sheet surges are one of the dominant signals of glacial climate variability. They are characterised as abrupt, quasi-periodic episodes of ice-sheet instabilities during which large amounts of ice are discharged from ice sheets into the ocean. The occurence of ice-sheet surges strongly influences the global climate evolution by altering the ocean circulation through the addition of freshwater and the atmospheric circulation through changes in ice-sheet height. The mechanisms controlling the timing and occurence of Heinrich-type ice-sheet surges remain poorly constrained to this day. Here, we use a coupled ice sheet-solid earth model to identify and quantify the importance of boundary forcing for the surge cycle length of Heinrich-type ice-sheet surges for two prominent ice streams of the Laurentide ice sheet – the land-terminating Mackenzie ice stream and the marine-terminating Hudson ice stream. We show that surface mass balance perturbations have the largest effect on the surge cycle length. Pertubations to the ice surface temperature and geothermal heatflux also influence the surge cycle length, but to a lesser degree. Ocean and sea-level forcing as well as different frequencies of the same forcing have a negligible effect on the surge cycle length. The simulations also highlight that only a certain parameter space exists under which ice-sheet oscillations can be maintained. Transitioning from an oscillatory state to a persistent ice streaming state, can result in an ice volume loss of up to 26 % for the respective ice stream drainage basin under otherwise constant climate conditions. We show that Mackenzie ice stream is susceptible to undergoing such a transition in response to all tested positive climate perturbations. This underlines the potential of the Mackenzie region to have contributed to prominent abrupt climate change events of the last deglaciation.

The last deglaciation was accompanied by a gradual warming with superimposed abrupt climate changes. In transient simulations of the last deglaciation with the comprehensive Max Planck Institute Earth System Model (MPI-ESM) we show that the timing and occurrence of abrupt climate changes are highly dependent on the utilized ice-sheet boundary condition. Simulations with different ice-sheet reconstructions show that the variability of North Atlantic surface temperatures are dominated by the timing and amplitude of meltwater fluxes from ice sheets, as derived from reconstructions. While some abrupt climate events (e.g. the Younger Dryas) only occur under certain boundary conditions in the transient simulations, other climate events such as the Bølling Allerød warming (about 14.7-14.2 ka BP) cannot be simulated with any of the applied and widely used reconstructions. However, in a sensitivity experiment with changing ice sheets but no addition of meltwater into the ocean, the North Atlantic experiences a warming during the time of the Bølling-Allerød. This warming is associated with a reorganization of the ocean circulation and deep-water formation sites. Prior to this reorganization, during the glacial and early part of the deglaciation, a rather zonal jet stream maintains a strong subpolar gyre in the North Atlantic. In addition, salty and dense water masses form in the Arctic. Until about 16.5 ka BP the Arctic freshens significantly and the surface elevation over the Laurentide ice sheet reduces. The latter leads to a shift in the atmospheric circulation at around 14.2 ka BP. The resulting changes in wind stress strongly reduce the eastward extent of the North Atlantic subpolar gyre. Here, we examine the physical mechanisms behind the reorganization and explore additional simulations with fixed deglacial key parameters (e.g. CO2, insolation, ice sheets) to identify the key drivers of the climate changes during the early deglaciation and Bølling Allerød.

In previous model-data comparisons, the centennial to millennial scale variance of local climate (e.g., SST) reconstructed from proxies was significantly higher than that simulated by climate models. One possible explanation is the lack of long-term feedback mechanisms, e.g. from the representation of changes in ice-sheets in models. Additionally, proxy records are short, and sparse, and the climate signal is significantly modified during the processes of encoding, archiving, and recovery.Here we introduce new methods to infer the climate variability of the past from proxy data and compare them to new transient model simulations of the last deglaciation. This will allow us to estimate the amplitude of climate variability and to evaluate whether climate models are capable of capturing changes in climate variability between different climate states (e.g. glacial vs. interglacial periods), which is also relevant for the accuracy of future projections. We compare the variability of marine d18O reconstructed from proxies with that simulated by a state-of-the-art Earth System Model.From the proxy side, our analysis is based on a new dataset of marine oxygen isotope data from planktonic foraminifera compiled for the PALMOD project. We use new methods to first calculate power-spectra for the LGM, transition and Holocene and then to correct these spectra by fitting a Bayesian model describing the effects of bioturbation and measurement error on the reconstructed climate signal. From the model side we use marine d18O variability calculated using temperature and salinity from transient model simulations of the last deglaciation, performed within the PALMOD project, that include changes in the ice sheets.This combination of new data and methods will allow us to investigate the effect of different ice-sheet configurations and physical parametrizations in the model on their ability to characterise long-timescale climate variability and its dependence on climate state.

Heinrich events are one of the prominent signals of glacial climate variability. They are characterised as abrupt, quasi-periodic episodes of ice-sheet instabilities during which large numbers of icebergs are released from the Laurentide ice sheet. These events affect the evolution of the global climate by modifying the ocean circulation through the addition of freshwater and the atmospheric circulation through changes in ice-sheet height. However, the mechanisms controlling the timing and occurrence of Heinrich events remain enigmatic to this day. Here, we present simulations with a coupled ice-sheet solid Earth model that aim to quantify the importance of different boundary forcings for the timing of Heinrich events. We focus the analysis on two prominent ice streams of the Laurentide ice sheet with the land-terminating Mackenzie ice stream and the marine-terminating Hudson ice stream. Our simulations identify different surge characteristics for the Mackenzie ice stream and the Hudson ice stream. Despite their different glaciological and climatic settings, both ice streams exhibit responses of similar magnitude to perturbations to the surface mass balance and the geothermal heat flux. However, Mackenzie ice stream is more sensitive to changes in the surface temperature. Changes to the ocean temperature and the global sea level have a negligible effect on the timing of Heinrich events in our simulations for both ice streams. We also show that Heinrich events for both ice streams only occur in a certain parameter space. Transitioning from an oscillatory Heinrich event state to a persistent streaming state can lead to an ice volume loss of up to 30%. Mackenzie ice stream is situated in a climate that is particularly close to this transition point, underlining the potential of the ice stream to have contributed to prominent abrupt climate events during glacial-interglacial transitions.

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.

A long simulation over the period 1901–2010 with an eddy-permitting ocean circulation model is used to study the variability of the upper layer circulation in the North Ionian Gyre (NIG) in the Eastern Mediterranean Sea (EMed). The model is driven by the atmospheric forcing from the twentieth century reanalysis data set ERA-20C, ensuring a consistent performance of the model over the entire simulation period. The main modes of variability known in the EMed, in particular the decadal reversals of the NIG upper layer circulation observed since the late 1980s are well reproduced. We find that the simulated NIG upper layer circulation prior to the observational period is characterized by long-lasting cyclonic phases with weak variability during years 1910–1940 and 1960–1985, while in the in-between period (1940–1960) quasi-decadal NIG circulation reversals occur with similar characteristics to those observed in the recent decades. Our simulation indicates that the NIG upper layer circulation is rather prone to the cyclonic mode with occasional kicks to the anticyclonic mode. The coherent variability of the NIG upper layer circulation mode and of the Adriatic Deep Water (AdDW) outflow implies that atmospheric forcing triggering strong AdDW formation is required to kick the NIG into an anticyclonic circulation 1–2 years later. A sensitivity experiment mimicking a cold winter event over the Adriatic Sea supports this hypothesis. Our simulation shows that it is the multi-decadal variability of the salinity in the Adriatic Sea that leads to periods where low salinity prevents strong AdDW formation events. This explains the absence of quasi-decadal NIG reversals during 1910–1940 and 1960–1985.

New IRSL ages of eolianites close to Muñique (Lanzarote) demonstrate the influence of millennial scale climatic variability on the sedimentary dynamics on the Canary Islands during the last glacial cycle. The repetitive succession of interstadial and stadial climatic conditions formed multiple sequences of eolian deposits, each in general comprising three depositional types. DepoType 1 and DepoType 2 consist mainly of marine biogenic carbonate detritus with small amounts of dust from the Sahara representing interstadial conditions. DepoType 2 compared to DepoType 1 is characterized by larger amounts of land snails and calcified brood cells. A DepoType 3 rich in dust from African subtropical/tropical Latisols terminates a sequence. IRSL dating on DepoType 3 type deposits clearly shows that these were deposited during Heinrich Events under stadial conditions. The stadial cooling of the North Atlantic Ocean caused a southern shift of climate zones that culminated during Heinrich Events when the arctic climate reaches its most southerly extent. As a consequence, atmospheric changes led to massive dust supply from the then-dry Sahel. The increase in dust and precipitation from the dry DepoTypes 1 to the more humid DepoTypes 3 originates from a modified atmospheric dynamic during a millennial cycle.

Projections of anthropogenic climate change suggest possible surface temperature increases similar to those during past major shifts of the mean climate like the Last Deglaciation. Such shifts do not only affect the mean but rather the full probability distributions of climatic variables such as temperature and precipitation. Changes to their distributions can thus be expected for the future as well and need to be constrained. To this end, we examine transient simulations of the Last Deglaciation from a hierarchy of climate models, ranging from an energy balance model to state-of-the-art Earth System Models. Besides the mean, we use the higher moments of variability – variance, skewness, and kurtosis – to characterize changes of the distribution. The analysis covers annual to millennial timescales and examines how patterns vary with timescale and region in response to warming. Furthermore, we evaluate how the changes of the distributions affect the occurrence of extremes. To analyze the influence of forcings on the distributions, we compare the patterns of the fully-forced simulations to those in sensitivity experiments that isolate the effects of individual forcings. In particular, the effect of volcanism is examined across the hierarchy, as well as changes in ice cover, freshwater input, CO2, and orbit. While large-scale global patterns can be found, there are significant regional differences as well as differences between simulations, relating for example to differences in the modelling of ice cover changes and freshwater input. Finally, we investigate whether climate model projections show the same trends with respect to the change in moments as those found in the deglacial simulations and thus whether the patterns found might hold for future climate.

Present day Mediterranean Sea (MedSea) is an oligotrophic semi-enclosed basin. Nutrient supply by rivers, net nutrient export through the Strait of the Gibraltar and the basin-wide circulation create a biological desert. Sediment core records, however, indicate periods of higher production during paleo times.To gain insight into biogeochemical conditions during the Last Glacial Maximum (LGM), we apply a regional ocean-biogeochemistry model of the Mediterranean Sea. A consistent forcing is available from a transient simulation with an Earth System Model (ESM) over 22,000 years based, among others, on an ice sheet reconstruction. The ESM run provides atmospheric forcing fields, being downscaled to the regional setup, temperature and salinity conditions at the open western boundary in the Atlantic, and river runoff. Nutrient concentrations of river discharge and at the Atlantic boundary are set to present-day estimates. The automatic bathymetry adjustment to account for sea level variations due to meltwater fluxes is adopted from the ESM simulation. The LGM simulation starts at 22 ka to allow for a 1000 yr spinup run with transient forcing. The LGM period results are compared to a present-day simulation based on the same consistent ESM forcing. Colder temperatures and thus lower basin-wide evaporation, as well as a shallower sill depth at the Strait of Gibraltar, lead to lower baroclinic watermass exchange between the MedSea and the Atlantic. The zonal overturning circulation is more sluggish during the LGM than present day. River discharge to the MedSea increases by 35% during the LGM, causing an increased net primary production near the river mouths. Despite a higher nutrient inventory of the MedSea at the LGM, net primary production of the entire MedSea is lower than present day. Colder LGM temperatures reduce phytoplankton growth rates and increase the remineralisation length scale.More characteristics of LGM biogeochemistry are presented and their drivers will be disentangled, also including additional sensitivity studies on changes in bathymetry.

The elevation feedback on melt has been identified as a key process to explain (Gregoire et al, Nature, 2012) and project (Aschwanden et al., Sci Advances, 2019; Ridley et al, J Clim, 2005; Vizcaino et al, Clim Dyn, 2008) long-term deglaciation. It is also central to the theory of ice sheet evolution hysteresis, deglaciation thresholds/tipping points, and the problem of reversibility (Garbe et al, Nature, 2020; Gregory et al, TC, 2020; Gregory et al, Nature, 2004, Robinson et al, Nature Clim. Change, 2012). Ice-sheet-model-only estimates of this feedback rely on a largely unexplored parameter, the so-called “lapse rate”. This parameter is defined as the rate at which the near-surface atmospheric temperature changes strictly due to surface elevation change.In this work, we use coupled an uncoupled ice sheet and climate simulations with two different General Circulation/Earth System Models (Vizcaino et al, GRL, 2015; Muntjewerf et al, JAMES, 2019) to estimate the temperature lapse rate over the Greenland ice sheet as it deglaciates. We find that this lapse rate is highly variable over seasons, with much reduced lapse rates during summer over melting surfaces. We propose that uncoupled state-of-the-art projections are likely overestimating deglaciation rates due to too high summer lapse rates over the ablation area.

An increasing number of climate model simulations is becoming available for the transition from the Last Glacial Maximum to the Holocene. Assessing the simulations’ reliability requires benchmarking against environmental proxy records. To date, no established method exists to compare these two data sources in space and time over a period with changing background conditions. Here, we develop a new algorithm to rank simulations according to their deviation from reconstructed magnitudes and temporal patterns of orbital- as well as millennial-scale temperature variations. The use of proxy forward modeling avoids the need to reconstruct gridded or regional mean temperatures from sparse and uncertain proxy data. First, we test the reliability and robustness of our algorithm in idealized experiments with prescribed deglacial temperature histories. We quantify the influence of limited temporal resolution, chronological uncertainties, and non-climatic processes by constructing noisy pseudo-proxies. While model-data comparison results become less reliable with increasing uncertainties, we find that the algorithm discriminates well between simulations under realistic non-climatic noise levels. To obtain reliable and robust rankings, we advise spatial averaging of the results for individual proxy records. Second, we demonstrate our method by quantifying the deviations between an ensemble of transient deglacial simulations and a global compilation of sea surface temperature reconstructions. The ranking of the simulations differs substantially between the considered regions and timescales. We attribute this diversity in the rankings to more regionally confined temperature variations in reconstructions than in simulations, which could be the result of uncertainties in boundary conditions, shortcomings in models, or regionally varying characteristics of reconstructions such as recording seasons and depths. Future work towards disentangling these potential reasons can leverage the flexible design of our algorithm and its demonstrated ability to identify varying levels of model-data agreement.

Transitions from a stable, periodically oscillating ice-sheet system to a perpetual ice stream has potentially far-reaching implications for the timing of the onset of the last deglaciation as well as for climate transitions such as the Younger Dryas. These periodical ice-sheet oscillations known as Heinrich-type ice sheet surges are among the most dominant signals of glacial climate variability. They are quasi-periodic events during which large amounts of ice are discharged from ice sheets into the ocean. The addition of freshwater strongly affects the ocean circulation, resulting in a pronounced cooling in the North Atlantic region. In addition, changes in the ice sheet geometry also have significant effects on the climate. Here, we use a coupled ice sheet-solid earth model that is driven with forcing from a comprehensive Earth System Model that includes interactive ice sheets to identify key drivers controlling the surge cycle length of Heinrich-type ice-sheet surges from two main outlet glaciers of the Laurentide ice sheet. Our simulations show different surge initiation behaviour for the land-terminating Mackenzie ice stream and marine-terminating Hudson ice stream. For both ice streams, the surface mass balance has the largest effect on the surge cycle length. Ice surface temperature and geothermal heat flux also influence the surge cycle length, but to a lesser degree. Ocean forcing and different frequencies of the same forcing have a negligible effect on the surge cycle length. The simulations also highlight that a certain parameter space exists under which stable surge oscillations can be maintained. This parameter range is much narrower for the Mackenzie ice stream than for the Hudson ice stream. Leaving the stable regime results in a dynamical switch that turns the system from periodically oscillating system into a perpetual ice stream system. This transition can lead to a volume loss of up to 36% for the respective ice stream drainage basin under otherwise glacial climate conditions.

The intensity and frequency of extreme weather events in Europe are one of the most dangerous consequences of a warming climate. Some regions suffer more under heat waves and droughts, while others are experiencing extreme rainfalls. Thus, for example, a severe flood in July 2021 in several European countries caused widespread damages particularly in Belgium and Germany.Which extreme weather events are to be expected in the future? How can the damage of irretrievable historical sites be avoided or, at least, limited or dealt with? All these questions are addressed in the three-year research project KERES, which is funded by the German Federal Ministry of Education and Research (BMBF) and is coordinated by the Fraunhofer ISC together with the Fraunhofer EU Office in Brussels.As first step the regional relevance of future extreme weather events in Germany will be investigated. This information will be further used to estimate the potential damages to buildings and outdoor facilities. The precautionary and responsive measures to manage potential or acute damage situations will be investigated as well. The designed methodologies will be tested for five case studies including World Heritage Sites (historical buildings and historical gardens) in Germany.The major tools of KERES include building component and indoor climate simulations and high-resolution urban climate models. The necessary input will be taken from the most recent ensemble of regional climate change projections of the EURO-CORDEX Initiative (www.euro-cordex.net). As a result, an ontology-based information system will be designed for managing the expected damage situations.We will present first results from the KERES project. The discussion will be focused on how to access and visualize the robustness of projected changes of extreme weather events in a way oriented to individual cultural heritage sites.

Warming of the North Atlantic region in climate history often was associated with massive melting of the Greenland Ice Sheet. To identify the meltwater's impacts and isolate these from internal variability and other global warming factors, we run single‐forcing simulations including small ensembles using three complex climate models differing only in their ocean components. In 200‐year‐long preindustrial climate simulations, we identify robust consequences of abruptly increasing Greenland runoff by 0.05 Sv: sea level rise of 44 ± 10 cm, subpolar North Atlantic surface cooling of 0.7°C, and a moderate AMOC decline of 1.1–2.0 Sv. The latter two emerge in under three decades—and reverse on the same timescale after the perturbation ends in year 100. The ocean translates the step‐change perturbation into a multidecadal‐to‐centennial signature in the deep overturning circulation. In all simulations, internal variability creates notable uncertainty in estimating trends, time of emergence, and duration of the response., Plain Language Summary: Enhanced melting of Greenland's glaciers is considered to be a major player in past rapid climate transitions and anticipated to soon impact ocean circulation under current global warming. Global warming triggers complex processes and feedbacks, of which greater amounts of meltwater slowing the large‐scale ocean circulation is only one. To better understand the sensitivity of the real but also the model ocean to just this meltwater, we run idealized experiments with up‐to‐date climate models, which use the same atmosphere and land but different ocean components. We find that sea level rise, cooling of the North Atlantic region, and slowing of the ocean circulation are responses common to all models while regional magnitudes of these responses differ considerably. Once we stop adding freshwater, all three models show that surface temperature and ocean circulation recover as quickly (or slowly) as they changed at the beginning of the experiment. Sea level rise is a lasting impact though., Key Points: Sudden increase in Greenland freshwater release is turned into century scale change by deep ocean dynamics. Upper ocean responses to moderately enhanced freshwater release from Greenland reverse on the same timescale once release ceases. Ocean model formulation affects regional expressions but basin‐scale responses are robust, so is the timing on decadal to centennial scales., Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347

The last deglaciation was characterized by drastic climate changes, most prominently melting ice sheets. Melting ice sheets have a significant impact on the atmospheric and oceanic circulation, due to changes in the topography and meltwater release into the ocean. In a set of transient simulations of the last deglaciation with the Max Planck Institute for Meteorology Earth System Model we explore differences in the climate response that arise from different boundary conditions and implementations suggested within the Paleoclimate Modeling Intercomparison Project - Phase 4 (PMIP4) deglaciation protocol. The underlying ice-sheet reconstruction dominates the simulated deglacial millennial-scale climate variability in terms of timing and occurrence of observed climate events. Sensitivity experiments indicate that the location and timing of meltwater release from the ice sheets into the ocean are crucial for the ocean response. The results will allow a better interpretation of inter-model differences that arise from different implementations proposed within the PMIP4 protocol.

We investigate driving forces of the biogeochemistry of the Benguela upwelling system (BUS) and their temporal changes over the 20th century. For this purpose, we developed a global ocean-only model in a stretched grid configuration, which resolves meso-scale circulation structures in the area of interest. The biogeochemical module of this model is extended by a more comprehensive nitrogen cycle to account for the specific nitrogen loss processes common in eastern upwelling systems. The model is forced by 110 years of atmospheric reanalysis data. To assess the impact of meso-scale circulation structures on local biogeochemical processes we compare our results to a set-up with a coarser horizontal resolution, comparable to the ocean component of Earth system models used for anthropogenic climate projections. In the higher spatial resolution we find enhanced intermediate depth ventilation (200–1000 m) with concurrent reduced loss of bioavailable nitrogen and a high shelfbound biological production, in line with observations. Moreover, only in the high resolution setup do multi-decadal trends of deoxygenation match observation-based estimates. Our study supports the view that the presence of meso-scale circulation structures exerts a major influence on biogeochemical patterns, especially on mid-depth oxygen concentrations. Furthermore, we show for the first time that by including this high spatio-temporal variability of the circulation, the regional anthropogenic carbon uptake of the BUS over the 20th century is lower than in the coarse resolution model. This indicates that, at least for some regions, the pathway of changes in the marine biogeochemistry as projected by state-of-the-art coarse resolution Earth system models is associated with high uncertainty.

The Last Glacial Maximum (LGM, ∼ 21 000 years ago) has been a major focus for evaluating how well state-of-the-art climate models simulate climate changes as large as those expected in the future using paleoclimate reconstructions. A new generation of climate models has been used to generate LGM simulations as part of the Paleoclimate Modelling Intercomparison Project (PMIP) contribution to the Coupled Model Intercomparison Project (CMIP). Here, we provide a preliminary analysis and evaluation of the results of these LGM experiments (PMIP4, most of which are PMIP4-CMIP6) and compare them with the previous generation of simulations (PMIP3, most of which are PMIP3-CMIP5). We show that the global averages of the PMIP4 simulations span a larger range in terms of mean annual surface air temperature and mean annual precipitation compared to the PMIP3-CMIP5 simulations, with some PMIP4 simulations reaching a globally colder and drier state. However, the multi-model global cooling average is similar for the PMIP4 and PMIP3 ensembles, while the multi-model PMIP4 mean annual precipitation average is drier than the PMIP3 one. There are important differences in both atmospheric and oceanic circulations between the two sets of experiments, with the northern and southern jet streams being more poleward and the changes in the Atlantic Meridional Overturning Circulation being less pronounced in the PMIP4-CMIP6 simulations than in the PMIP3-CMIP5 simulations. Changes in simulated precipitation patterns are influenced by both temperature and circulation changes. Differences in simulated climate between individual models remain large. Therefore, although there are differences in the average behaviour across the two ensembles, the new simulation results are not fundamentally different from the PMIP3-CMIP5 results. Evaluation of large-scale climate features, such as land–sea contrast and polar amplification, confirms that the models capture these well and within the uncertainty of the paleoclimate reconstructions. Nevertheless, regional climate changes are less well simulated: the models underestimate extratropical cooling, particularly in winter, and precipitation changes. These results point to the utility of using paleoclimate simulations to understand the mechanisms of climate change and evaluate model performance.

Downscaling simulations for emission scenario RCP8.5 with a regionally coupled ocean-atmosphere climate system model indicate a regime shift in the dynamics of Atlantic nutrient supply to the Northwest European Shelf. Reduced winter convection in the Northeast Atlantic leads to a substantial decrease in upper ocean nutrient concentrations. Yet, vertical mixing processes at the shelf break maintain a connection to nutrient-enriched subpycnocline water masses and decouple the nutrient supply to the shelf from upper ocean conditions in the North Atlantic. As a consequence, primary production on the shelf does not weaken as strongly as it was expected from declining nutrient concentrations in the open ocean. Furthermore, nutrient fluxes to the shelf become increasingly variable due to a positive feedback between the winter mixed layer depth near the shelf break and the density of the upper water column, making the productivity on the shelf more sensitive to the major modes of variability in the North Atlantic.

Heinrich events are among the dominant modes of glacial climate variability. During these events, massive iceberg armadas were released by the Laurentide Ice Sheet and sailed across the Atlantic where they melted and released freshwater, as well as detritus, that formed characteristic layers on the seafloor. Heinrich events are known for cold climates in the North Atlantic region and global climate changes. We study these events in a fully coupled complex ice sheet–climate model with synchronous coupling between ice sheets and oceans. The ice discharges occur as an internal variability of the model with a recurrence period of 5 kyr, an event duration of 1–1.5 kyr, and a peak discharge rate of about 50 mSv, roughly consistent with reconstructions. The climate response shows a two-stage behavior, with freshwater release effects dominating the surge phase and ice sheet elevation effects dominating the post-surge phase. As a direct response to the freshwater discharge during the surge phase, deepwater formation in the North Atlantic decreases and the North Atlantic deepwater cell weakens by 3.5 Sv. With the reduced oceanic heat transport, the surface temperatures across the North Atlantic decrease, and the associated reduction in evaporation causes a drying in Europe. The ice discharge lowers the surface elevation in the Hudson Bay area and thus leads to increased precipitation and accelerated ice sheet regrowth in the post-surge phase. Furthermore, the jet stream widens to the north, which contributes to a weakening of the subpolar gyre and a continued cooling over Europe even after the ice discharge. This two-stage behavior can explain previously contradicting model results and understandings of Heinrich events.

The impact of tides on the spreading of the Mediterranean Outflow Waters (MOW) in the Gulf of Cadiz is investigated through a series of targeted numerical experiments using an ocean general circulation model. The full ephimeridic luni-solar tidal potential is included as forcing. The model grid is global with a strong zoom around the Iberian Peninsula. Thus, the interaction of processes of different space and time scales, which are involved in the MOW spreading, is enabled. This is of particular importance in the Strait of Gibraltar and the Gulf of Cadiz, where the width of the MOW plume is a few tens of km. The experiment with enabled tides successfully simulates the main tidal features of the North Atlantic and in the Gulf of Cadiz and the Strait of Gibraltar. The comparison of the fields from simulations with and without tidal forcing shows drastically different MOW pathways in the Gulf of Cadiz: The experiment without tides shows an excessive southwestward spreading of Mediterranean Waters along the North African slope, whereas the run with tides is closer to climatology. A detailed analysis indicates that tidal residual currents in the Gulf of Cadiz are the main cause for these differences.

As ice sheets grow or decay, the net flux of freshwater into the ocean changes and the bedrock adjusts due to isostatic adjustments, leading to variations in the bottom topography and the oceanic boundaries. This process was particularly intense during the last deglaciation due to the high rates of ice-sheet melting. It is, therefore, necessary to consider transient ocean bathymetry and coastlines when attempting to simulate the last deglaciation with Earth system models (ESMs). However, in most standard ESMs the land-sea mask is fixed throughout simulations because the generation of a new ocean model bathymetry implies several levels of manual corrections, a procedure that is hardly doable very often for long runs. This is one of the main technical problems towards simulating a complete glacial cycle with general circulation models. For the first time, we present a tool allowing for an automatic computation of bathymetry and land-sea mask changes in the Max Planck Institute Earth System Model (MPI-ESM). The algorithms developed in this paper can easily be adapted to any free-surface ocean model that uses the Arakawa-C grid in the horizontal and z-grid in the vertical including partial bottom cells. The strategy applied is described in detail and the algorithms are tested in a long-term simulation demonstrating the reliable behaviour. Our approach guarantees the conservation of mass and tracers at global and regional scales; that is, changes in a single grid point are only propagated regionally. The procedures presented here are an important contribution to the development of a fully coupled ice sheet–solid Earth–climate model system with time-varying topography and will allow for transient simulations of the last deglaciation considering interactive bathymetry and land-sea mask.

This study analyzes the response of the Atlantic meridional overturning circulation (AMOC) to different CO2 concentrations and two ice sheet configurations in simulations with the coupled climate model MPI-ESM. With preindustrial (PI) ice sheets, there are two different AMOC states within the studied CO2 range: one state with a strong and deep upper overturning cell at high CO2 concentrations and one state with a weak and shallow upper cell at low CO2 concentrations. Changes in AMOC variability with decreasing CO2 indicate two stability thresholds. The strong state is stable above the first threshold near 217 ppm, and the weak state is stable below the second threshold near 190 ppm. Between the two thresholds, both states are marginally unstable, and the AMOC oscillates between them on millennial time scales. The weak AMOC state is stable when Antarctic Bottom Water becomes dense and salty enough to replace North Atlantic Deep Water (NADW) in the deep North Atlantic and when the density gain over the North Atlantic becomes too weak to sustain continuous NADW formation. With Last Glacial Maximum (LGM) ice sheets, the density gain over the North Atlantic and the northward salt transport are enhanced with respect to the PI ice sheet case. This enables active NADW formation and a strong AMOC for the entire range of studied CO2 concentrations. The AMOC variability indicates that the simulated AMOC is far away from a stability threshold with LGM ice sheets. The nonlinear relationship among AMOC, CO2, and prescribed ice sheets provides an explanation for the large intermodel spread of AMOC states found in previous coupled LGM simulations.

Much about the response of temperature variability to a change in the climate's mean state, as the one projected for the current century, remains uncertain. These uncertainties include spatiotemporal patterns, the magnitude, and, in some cases, even the sign. For the last Deglaciation, - the last change in global mean temperature of a similar degree to that expected in projections - variability analyses of climate model simulations and temperature proxies produce conflicting results. Here, we build a hierarchy of transient simulations covering the period since the Last Glacial Maximum about 26k years ago. We include a range of climate models, from conceptual to complex Earth System Models. The simulations cover a variety of temporal and spatial resolutions, parameterizations, and modeled processes. For annual to multi-millennial temporal as well as regional to global spatial scales, we compare variability patterns and power spectra and analyze how they relate to model properties and the background state of Earth's climate. This allows for the examination of regional temperature differences between low, middle, and high latitudes and at locations of available paleoclimate proxy records. For sets of sensitivity experiments, we investigate effects of changes to ice sheets, sea ice, and in volcanic, solar, greenhouse, and orbital forcing on modeled climate variability. Thus, our analysis provides insights into when and how models disagree with each other and with proxies, and what differences arise due to specific models, simulation setups, and boundary conditions. Based on these results, we can then gauge the degree of complexity which is required to reproduce past temperature variability and predict its changes in the future.

Sediment archives of the Eastern Mediterranean Sea (EMed) indicate very different physical and biogeochemical conditions during the LGM and the early Holocene than for present day. The ultimate goal of the here presented project is to disentangle the controlling processes of the circulation in the EMed over the last deglacial period by applying a regional ocean model including biogeochemistry covering the entire Mediterranean Sea. This model setup will be driven by downscaled forcing fields from a simulation with the paleo version of Max Planck Institute Earth System Model (pMPI-ESM) spanning from 26 to 0 kaBP. pMPI-ESM has unique features like automatic bathymetry adjustment due to sea level rise and transient river routing. Despite its coarse model resolution, pMPI-ESM simulations catch the humid period of the early Holocene with corresponding increased Nile river discharge, a relevant driver for the conditions in the EMed. Thus, we are convinced that pMPI-ESM can provide a long-term transient and consistent forcing which is appropriate for our aims.Here we present first results to evaluate the performance of our regional model driven by the downscaled forcing from pMPI-ESM. Main characteristics of the present day Mediterranean circulation are well captured such as locations of deep water formation, Mediterranean and Black Sea fresh water budgets, and the baroclinic transports through the Strait of Gibraltar. We test our model framework for different time slices of deglaciation.

The zonal-annual mean inter-hemispheric convergence zone (ITCZ) is located in the northern hemisphere in the modern climate. A transient simulation of the last deglaciation using the Max Planck Institute Earth System Model (MPI-ESM), suggests that the ITCZ was located in the southern hemisphere 14 kyrs ago. This shift is due to a substantial cooling of the northern hemisphere relative the southern hemisphere, after the release of melt water pulse 1a. The ITCZ compensates for these changes in the surface temperature by shifting south, thereby leading to a northward atmospheric heat transport away from the southern hemisphere. Along with the southward shift, the intensity of the precipitation within the ITCZ decreases. These changes in the intensity of precipitation can be explained by using a framework based on the moist static energy budget. We find that these changes are primarily related to the changes in the large-scale vertical motion of the atmosphere in the tropics. This affects the vertical transport of the moist static energy, and hence total gross moist stability (TGMS).

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.

Heinrich-type ice sheet surge events are among the most prominent signals in the paleoclimate data records. Even though these events have previously been intensely studied, it still remains an open question whether the cyclic ice sheet surges are triggered by internal ice dynamics, climate forcing, or a combination of the two. In simulations of the last deglaciation using the fully-coupled Max Planck Institute Earth System Model, surges from the European and North American ice sheets often occur in synchronicity. This model behaviour is in agreement with observations from sediment cores that find a similar pattern in the isotopic fingerprint of the deposited ice-rafted detritus. The synchronicity indicates that climate forcing is playing an important role in initiating ice sheet surges. In this study, we use the coupled ice-sheet-solid earth model PISM-VILMA in a northern hemispheric setup to investigate the modelled synchronicity of the surge events. More specifically, we perform an ensemble of simulations to study if the modelled synchronicity is a direct result of one of the surge locations causing other surge locations to be a activated as well. Moreover, we aim to investigate whether previously suggested trigger mechanisms such as regional changes in sea level or ocean temperatures are indeed key processes in controlling the synchronicity of these surge events.

It is challenging to model the last deglaciation, as it is characterized by abrupt millennial scale climate events, such as ice-sheet surges, that are superimposed on long-term climate changes, such as a global warming and the decay of a substantial part of the glacial ice sheets. Within PMIP, several groups have simulated the last deglaciation with CMIP-type models prescribing ice sheets from reconstructions. Whereas this type of simulations accounts for the effects of ice-sheet changes including meltwater release on climate, the prescribed ice sheet evolution is typically not consistent with the simulated climate evolution. Here we present a set of deglacial simulations that include fully interactive ice sheets that respond to changes in the climate. The setup also allows for feedbacks between ice sheets and climate and , hence, allows for a more realistic representation of the mechanisms of the last deglaciation, as the simulated climate and ice sheet changes are fully consistent..The model consists of the coarse resolution set-up of MPI-ESM coupled to the ice sheet model mPISM (Northern Hemisphere and Antarctica) and the solid earth model VILMA. The model includes interactive icebergs and an automated calculation of the land-sea mask and river routing directions. A set of synchronously coupled simulations were started from an asynchronously coupled spin-up at 26ky and integrated throughout the deglaciation into the Holocene. The only prescribed external forcing are atmospheric concentrations of greenhouse gases and earth orbital parameters. One goal of this ensemble was to find the optimal combination of model parameters for the simulation of the deglaciation.The model simulates the decay of the ice sheets, the rise of sea level, the flooding of shelf seas and the opening of passages. A large fraction of the ice sheet retreat is due to dynamical events (e.g. the final decay of the ice sheets on Barents Shelf or the Hudson Bay). Superimposed on the relatively slow glacial/interglacial transition are abrupt climate changes, triggered for example by recurrent ice sheet surges. These surges correspond to Heinrich Events tand result in a weakening of the AMOC. Three source regions for ice sheet surges occur during these simulations: from the Laurentide ice sheet through Hudson Strait, from the Laurentide ice sheet northward directly to the Arctic ocean, and from the Fennoscandian ice sheet into the Norwegian Sea. The characteristic climate response shows a large dependence on the surge location.The simulated changes in strength of the AMOC are except for millennial-scale reduction events only moderate. However, during glacial periods, brine release is the central process for deep water formation in both hemispheres, in contrast to the Holocene. dDuring the deglaciation the ventilation of the deep ocean is strongly reduced, leading to a strong increase of the simulated deep water ages. This effect lasts longest in the deep North Pacific and extends in some simulations into the Holocene.

The increasing number of Earth system model simulations that try to simulate the climate during the last deglaciation (ca 20 to 10 thousand years ago) creates a demand for benchmarking against environmental proxy records synthesized for the same time period. Comparing these two data sources over a period with changing background conditions requires new methods for model-data comparison that incorporate multiple types and sources of uncertainty. Natural archives of past reality are distributed sparsely and non-uniformly in space and time. Signals that can be obtained are in addition perturbed by uncertainties related to dating, the relationship between the proxy sensor and environmental fields, the archive build-up, and measurement. On the other hand, paleoclimate simulations are four-dimensional, complete, and physically consistent representations of the climate. However, they are subject to errors due to model inadequacies and sensitivity to the forcing protocol, and will not reproduce any particular history of unforced variability. We present a method for probabilistic, multivariate quantification of the deviation between paleo-data and paleoclimate simulations that draws on the strengths of both sources of information and accounts for the aforementioned uncertainties. We compare the shape and magnitude of orbital- and millennial-scale temperature fluctuations during the last deglaciation and compute metrics of regional and global model-data mismatches. We test our algorithm with an ensemble of published simulations of the deglaciation and simulations from the ongoing PalMod project, which aims at the simulation of the last glacial cycle with comprehensive Earth system models. These are evaluated against a compilation of temperature reconstructions from multiple archives. Our work aims for a standardized model-data comparison workflow that will be used in PalMod. This workflow can be extended subsequently with additional proxy data, new simulations, and improved representations of proxy uncertainties.

We present a mechanism for self‐sustained ocean circulation changes that cause abrupt temperature changes over Greenland in a multimillennial climate model simulation with glacial CO2 concentrations representative of Marine Isotope Stage 3. The Atlantic meridional overturning circulation (AMOC) and the subpolar gyre (SPG) oscillate on millennial time scales. When the AMOC is strong, the SPG is weak and contracted; when the AMOC is weak, the SPG is strong and extensive. The coupling between the two systems via wind‐driven and density‐driven feedbacks is key to maintaining the oscillations. The SPG controls the transport of heat and salt into the deep‐water formation sites and thus controls the AMOC strength. The strength and location of the deep‐water formation affect the density‐driven part of the SPG and thus control the mean strength and extent of the SPG. This mechanism supports the hypothesis that coupled ocean‐ice‐atmosphere interactions could have triggered abrupt glacial climate change., Plain Language Summary: Between 57.000 and 29.000 years ago, the last glacial period was marked by several abrupt warming and cooling events over Greenland and the North Atlantic. Understanding the mechanism behind these so‐called Dansgaard‐Oeschger events increases our understanding of possible tipping points that cause abrupt change in the Earth system. The role of the ocean in causing these events is still a topic of debate. We find abrupt changes in the North Atlantic circulation that resemble Dansgaard‐Oeschger events in a simulation with a state‐of‐the‐art climate model. These simulated ocean circulation changes are generated without adding external triggers such as meltwater from glaciers. Instead, the events are generated by the interaction of the two large‐scale current systems in the North Atlantic—the Atlantic meridional overturning circulation (AMOC) and the North Atlantic subpolar gyre (SPG). Both current systems are affected by changes in surface winds and the density pattern of the North Atlantic. We find that the location where the densest water is formed controls how the SPG interacts with the AMOC. Under favorable conditions, the effects of wind and density combine in such a way that changes in the SPG cause abrupt changes in the AMOC., Key Points: Millennial‐scale, self‐sustained oscillations of the ocean circulation occur in a climate model simulation. The oscillations are driven by the interaction between the Atlantic meridional overturning circulation (AMOC) and the subpolar gyre (SPG). The AMOC‐SPG coupling is controlled by a wind‐driven and a density‐driven feedback; the coupling sign depends on the dominant feedback.

The Greenland Ice Sheet (GrIS) mass loss has been accelerating at a rate of about 20 ± 10 Gt/yr2 since the end of the 1990's, with around 60 % of this mass loss directly attributed to enhanced surface meltwater runoff. However, in the climate and glaciology communities, different approaches exist on how to model the different surface mass balance (SMB) components using: (1) complex physically-based climate models which are computationally expensive; (2) intermediate complexity energy balance models; (3) simple and fast positive degree day models which base their inferences on statistical principles and are computationally highly efficient. Additionally, many of these models compute the SMB components based on different spatial and temporal resolutions, with different forcing fields as well as different ice sheet topographies and extents, making inter-comparison difficult. In the GrIS SMB model intercomparison project (GrSMBMIP) we address these issues by forcing each model with the same data (i.e., the ERA-Interim reanalysis) except for two global models for which this forcing is limited to the oceanic conditions, and at the same time by interpolating all modelled results onto a common ice sheet mask at 1 km horizontal resolution for the common period 1980–2012. The SMB outputs from 13 models are then compared over the GrIS to (1) SMB estimates using a combination of gravimetric remote sensing data from GRACE and measured ice discharge, (2) ice cores, snow pits, in-situ SMB observations, and (3) remotely sensed bare ice extent from MODerate-resolution Imaging Spectroradiometer (MODIS). Our results reveal that the mean GrIS SMB of all 13 models has been positive between 1980 and 2012 with an average of 340 ± Gt/yr, but has decreased at an average rate of −7.3 Gt/yr2 (with a significance of 96 %), mainly driven by an increase of 8.0 Gt/yr2 (with a significance of 98 %) in meltwater runoff. Spatially, the largest spread among models can be found around the margins of the ice sheet, highlighting the need for accurate representation of the GrIS ablation zone extent and processes driving the surface melt. In addition, a higher density of in-situ SMB observations is required, especially in the south-east accumulation zone, where the model spread can reach 2 mWE/yr due to large discrepancies in modelled snowfall accumulation. Overall, polar regional climate models (RCMs) perform the best compared to observations, in particular for simulating precipitation patterns. However, other simpler and faster models have biases of same order than RCMs with observations and remain then useful tools for long-term simulations. Finally, it is interesting to note that the ensemble mean of the 13 models produces the best estimate of the present day SMB relative to observations, suggesting that biases are not systematic among models.

In this study we introduce a Bayesian framework, which is flexible and explicit about the prior assumptions, for using model ensembles and observations together to constrain future climate change. The emergent constraint approach has seen broad application in recent years, including studies constraining the equilibrium climate sensitivity (ECS) using the Last Glacial Maximum (LGM) and the mid-Pliocene Warm Period (mPWP). Most of these studies were based on Ordinary Least Squares (OLS) fits between a variable of the climate state, such as tropical temperature, and climate sensitivity. Using our Bayesian method, and considering the LGM and mPWP separately, we obtain values of ECS of 2.7 K (1.1 - 4.8, 5 - 95 percentiles) using the PMIP2, PMIP3 and PMIP4 data sets for the LGM, and 2.4 K (0.4 - 5.0) with the PlioMIP1 and PlioMIP2 data sets for the mPWP. Restricting the ensembles to include only the most recent version of each model, we obtain 2.7 K (1.1 - 4.3) using the LGM and 2.4 K (0.4 - 5.1) using the mPWP. An advantage of the Bayesian framework is that it is possible to combine the two periods assuming they are independent, whereby we obtain a slightly tighter constraint of 2.6 K (1.1 - 3.9). We have explored the sensitivity to our assumptions in the method, including considering structural uncertainty, and in the choice of models, and this leads to 95% probability of climate sensitivity mostly below 5 and never exceeding 6 K. The approach is compared with other approaches based on OLS, a Kalman filter method and an alternative Bayesian method. An interesting implication of this work is that OLS-based emergent constraints on ECS generate tighter uncertainty estimates, in particular at the lower end, suggesting a higher bound by construction in case of weaker correlation. Although some fundamental challenges related to the use of emergent constraints remain, this paper provides a step towards a better foundation of their potential use in future probabilistic estimation of climate sensitivity.

We quantify the change in extreme high sea level (ESL) statistics in the German Bight under rising $${{\mathrm {CO}}_2}$$ concentrations by downscaling a large ensemble of global climate model simulations using the regionally coupled climate system model REMO-MPIOM. While the model setup combines a regionally high resolution with the benefits of a global ocean model, the large ensemble size of 32 members allows the estimation of high return levels with much lower uncertainty. We find that ESLs increase with atmospheric $${{\mathrm {CO}}_2}$$ levels, even without considering a rise in the background sea level (BSL). Local increases of up to 0.5 m are found along the western shorelines of Germany and Denmark for ESLs of 20–50 years return periods, while higher return levels remain subject to sampling uncertainty. This ESL response is related to a cascade of an enhanced large-scale activity along the North Atlantic storm belt to a subsequent local increase in predominantly westerly wind speed extremes, while storms of the major West-Northwest track type gain importance. The response is seasonally opposite: summer ESLs and the strength of its drivers decrease in magnitude, contrasting the response of the higher winter ESLs, which governs the annual response. These results have important implications for coastal protection. ESLs do not only scale with the expected BSL rise, but become even more frequent, as preindustrial 50-year return levels could be expected to occur almost every year by the end of the century. The magnitude of the relative change in ESL statistics is hereby up to half of the expected rise in BSL, depending on the location. Changes in the highest extremes are subject to large multidecadal variations and remain uncertain, thus potentially demanding even further safety measures.

We investigate the changes in terrestrial natural methane emissions between the Last Glacial Maximum (LGM) and preindustrial (PI) periods by performing time-slice experiments with a methane-enabled version of MPI-ESM, the Max Planck Institute Earth System Model. We consider all natural sources of methane except for emissions from wild animals and geological sources, i.e. emissions from wetlands, fires, and termites. Changes are dominated by changes in tropical wetland emissions, with mid-to-high-latitude wetlands playing a secondary role, and all other natural sources being of minor importance. The emissions are determined by the interplay of vegetation productivity, a function of CO2 and temperature; source area size, affected by sea level and ice sheet extent; and the state of the West African monsoon, with increased emissions from northern Africa during strong monsoon phases. We show that it is possible to explain the difference in atmospheric methane between LGM and PI purely by changes in emissions. As emissions more than double between LGM and PI, changes in the atmospheric lifetime of CH4, as proposed in other studies, are not required.

The continually evolving large ice sheets present in the Northern Hemisphere during the last glacial cycle caused significant changes to river pathways both through directly blocking rivers and through glacial isostatic adjustment. Associated with these changing river pathways was the formation and evolution of large glacial lakes such as Lake Agassiz. Studies have shown this changing hydrology had a significant impact on the ocean circulation through changing the pattern of freshwater discharge into the oceans. A coupled Earth system model (ESM) simulation of the last glacial cycle thus requires a hydrological discharge and lake model that uses a set of river pathways and lakes that evolve with Earth's changing orography while being able to reproduce the known present-day river network given the present-day orography. Here, we present a method for dynamically modelling rivers and lakes by applying predefined corrections to an evolving fine-scale orography (accounting for the changing ice sheets and isostatic rebound) each time the river directions and lakes basins are recalculated. The corrected orography thus produced is then used to create a set of fine-scale river pathways and these are then upscaled to a coarser scale on which an existing present-day hydrological discharge model within the JSBACH land surface model simulates the river flow. The associated glacial lakes are delineated from the same corrected fine scale orography; lake inflow and outflow being linked to the river flow model.

Ocean circulation plays an essential role in Earth’s climate and the global carbon cycle. A prerequisite for improving confidence in future climate projections is the accurate numerical modeling of past deep ocean circulation changes. Unfortunately our understanding of such changes is impeded by ambiguities in the data-based reconstructions which heavily rely on radiocarbon dating of marine samples. Central to this method is the knowledge of the reservoir age (the age difference between the surface ocean and the atmosphere). Concomitant changes in atmospheric levels, air-sea exchange rates, and ocean circulation have the potential to drive large temporal and spatial changes of this reservoir age over the deglaciation. However these changes are not well constrained by field evidence. In consequence large uncertainties affect the dating of main climate events. Model studies allow complementing field data while also providing the means of assessing the sensitivity to different processes.Here, we investigate the sensitivity of the radiocarbon reservoir ages and reconstructed calendar ages over the last termination. For this purpose we take advantage of a set of transient simulations performed with the Max Planck Institute Earth System Model (MPI-ESM) with interactive calculation of river runoff and automatic adjustment of model topography. The experiments, starting at 26 ka BP, are constrained with prescribed time varying ice sheets and topography in addition to variations of the Earth orbital parameters and reconstructed atmospheric greenhouse gases concentrations. Changes in ice sheet volume naturally result in freshwater surges which affect the global circulation and water masses distribution. Ocean radiocarbon is included in the model. The atmospheric 14C follows the INTCAL13 reconstruction while the impacts of varying wind speed, sea-ice cover, and atmospheric CO2 on air-sea exchange rates are explicitly included.Different ice-sheets reconstructions (ICE-6G_C and GLAC-1D) and model configurations (addressing vertical mixing, bathymetry and land-sea mask) provide a range of ocean responses. The impact on reservoir ages of uncertainties related to planktonic foraminifer species-specific habitat is also considered. Together with the suite of model states this provides a range of reservoir ages over time. A calibration step allows then obtaining an estimate of the temporal evolution over the deglaciation of the time resolution of the radiocarbon dating method. Regional and global evolutions are examined and discussed.

The Last Glacial Maximum (LGM, ~ 21,000 years ago) has been a major focus for evaluating how well state-of-the-art climate models simulate climate changes as large as those expected in the future using paleoclimate reconstructions. A new generation of climate models have been used to generate LGM simulations as part of the Palaeoclimate Modelling Intercomparison Project (PMIP) contribution to the Coupled Model Intercomparison Project (CMIP). Here we provide a preliminary analysis and evaluation of the results of these LGM experiments (PMIP4-CMIP6) and compare them with the previous generation of simulations (PMIP3-CMIP5). We show that the PMIP4-CMIP6 are globally less cold and less dry than the PMIP3-CMIP5 simulations, most probably because of the use of a more realistic specification of the northern hemisphere ice sheets in the latest simulations although changes in model configuration may also contribute to this. There are important differences in both atmospheric and ocean circulation between the two sets of experiments, with the northern and southern jet streams being more poleward and the changes in the Atlantic Meridional Overturning Circulation being less pronounced in the PMIP4-CMIP6 simulations than in the PMIP3-CMIP5 simulations. Changes in simulated precipitation patterns are influenced by both temperature and circulation changes. Differences in simulated climate between individual models remain large so, although there are differences in the average behaviour across the two ensembles, the new simulation results are not fundamentally different from the PMIP3-CMIP5 results. Evaluation of large-scale climate features, such as land-sea contrast and polar amplification, confirms that the models capture these well and within the uncertainty of the palaeoclimate reconstructions. Nevertheless, regional climate changes are less well simulated: the models underestimate extratropical cooling, particularly in winter, and precipitation changes. The spatial patterns of increased precipitation associated with changes in the jet streams are also poorly captured. However, changes in the tropics are more realistic, particularly the changes in tropical temperatures over the oceans. Although these results are preliminary in nature, because of the limited number of LGM simulations currently available, they nevertheless point to the utility of using paleoclimate simulations to understand the mechanisms of climate change and evaluate model performance.