Christian Stepanek

Atlantic Meridional Overturning Circulation (AMOC) is a major feature in the climate system, moving heat and carbon globally. Climate models suggest AMOC’s strength will decrease in the coming decades, with profound implications for regional and global climate patterns. Biogeochemical proxies from benthic foraminifera tests can act as water mass tracers and provide information on deep ocean circulation during past periods of global warmth. The strength of AMOC during the Late Pliocene warm period (∼3.0–3.3 Ma), a pseudo-analogue for future climate change, is poorly constrained, with benthic foraminiferal δ¹³C showing a spatially expanded North Atlantic Deep Water (NADW), whereas models often simulate a spatially retracted NADW relative to Pre-Industrial. Here we compare δ¹³C and δ¹⁸O proxies in the Atlantic for the warm mid-Piacenzian KM5c interval (3.205 ± 0.02 Ma) with four isotope-enabled climate model simulations allowing for a direct comparison between proxy data and climate models. Proxy data generally show a more expansive NADW with Antarctic Bottom Water limited to the greatest depths. Although climate models resolve key intermediate and deep water masses, differences in spatial patterns between the models are attributed to model parameterisations and boundary conditions. The models and proxy data show different deep ocean structures, which limits our ability to understand deep ocean circulation during the warm mid-Piacenzian KM5c interval and its relevance to future warming scenarios.

The long-term evolution of the Greenland Ice Sheet (GrIS) remains a major source of uncertainty in projections of future sea-level rise. Recent advances in coupled climate–ice sheet models provide new opportunities to investigate the role of feedbacks between the ice sheet and the climate system, yet substantial divergence persists across climate-ice sheet model projections. Here, we present results from the Greenland Ice sheet Coupled Model Intercomparison Project (GrICMIP), using three coupled climate–ice sheet models to simulate GrIS evolution under multiple emission scenarios to the year 4000 CE. While projected sea-level contributions remain modest by 2100 (0.03–0.11 m), they diverge strongly on longer timescales, reaching up to 3.5 m by 2500. Under the high-emission scenario, full GrIS disintegration is projected as early as 3000 CE. By means of targeted sensitivity experiments, we identify the dominant sources of diverging GrIS trajectories. Across all models, changes in surface mass balance (SMB), and in particular net surface melt, control the long-term ice-sheet retreat. Differences in SMB formulation, especially in the underlying energy balance models, together with differences in the simulated climate, outweigh the influence of initial ice-sheet geometry. This establishes a hierarchy of uncertainties in which atmospheric processes and their representation within the model systems propagate non-linearly into ice-sheet evolution. Our results demonstrate that reliable long-term projections of the GrIS critically depend on improving the representation of climate and SMB, rather than on refining initial conditions alone.

Atlantic Meridional Overturning Circulation (AMOC) is a major feature in the climate system, moving heat and carbon globally. Climate models suggest AMOC’s strength will decrease in the coming decades, with profound implications for regional and global climate patterns. Biogeochemical proxies from benthic foraminifera tests can act as water mass tracers and provide information on deep ocean circulation during past periods of global warmth. The strength of AMOC during the Late Pliocene warm period (∼3.0–3.3 Ma), a pseudo-analogue for future climate change, is poorly constrained, with benthic foraminiferal δ13C showing a spatially expanded North Atlantic Deep Water (NADW), whereas models often simulate a spatially retracted NADW relative to Pre-Industrial. Here we compare δ13C and δ18O proxies in the Atlantic for the warm mid-Piacenzian KM5c interval (3.205 ± 0.02 Ma) with four isotope-enabled climate model simulations allowing for a direct comparison between proxy data and climate models. Proxy data generally show a more expansive NADW with Antarctic Bottom Water limited to the greatest depths. Although climate models resolve key intermediate and deep water masses, differences in spatial patterns between the models are attributed to model parameterisations and boundary conditions. The models and proxy data show different deep ocean structures, which limits our ability to understand deep ocean circulation during the warm mid-Piacenzian KM5c interval and its relevance to future warming scenarios.

The Arctic Ocean rapidly transitions towards ice-free summers, a "blue" (i.e., summer sea ice-free) state driven by Arctic amplification of abrupt global warming. To improve climate projections and better understand the consequences of reduced Arctic sea ice in a warming climate, it is essential to examine past warm intervals such as the Early Pliocene (~5–4 Ma), when diminished Arctic sea ice coincided with higher global temperatures. Proxy records from the Arctic-Atlantic Gateway (AAG) (~70–80° N), combined with climate model simulations, indicate periodically ice-free summers during this time. Yet, model-data comparisons for the central Arctic Ocean (>80° N) reveal winter sea ice as a persistent feature. Here, we show that proxy evidence for a "blue" Arctic is best reproduced when enhanced vertical ocean mixing is integrated into climate models. Under enhanced mixing, Arctic warming occurs through different mechanisms depending on the model geography, whereas simulations with standard mixing show substantially weaker agreement with proxy evidence. We do not interpret enhanced mixing as the unique solution but rather as a physically motivated sensitivity test that represents unresolved ocean processes. Our results demonstrate that modifying a single ocean parameterization within physically plausible boundaries substantially improves proxy-model agreement for a warmer than modern Early Pliocene, both at the AAG and for the wider Arctic. Combined with published proxy evidence for a warmer Arctic Ocean interior, our findings underscore the need for improved representation of ocean dynamics in paleoclimate simulations. Next-generation climate models should evaluate the sensitivity of simulated "blue" Arctic climates to ocean mixing alongside other key processes, including cloud microphysics and radiative feedbacks, to increase confidence in projections of seasonally ice-free Arctic conditions

The long-term evolution of the Greenland Ice Sheet (GrIS) remains a major source of uncertainty in projections of future sea-level rise. Recent advances in coupled climate–ice sheet models provide new opportunities to investigate the role of feedbacks between the ice sheet and the climate system, yet substantial divergence persists across climate-ice sheet model projections. Here, we present results from the Greenland Ice sheet Coupled Model Intercomparison Project (GrICMIP), using three coupled climate–ice sheet models to simulate GrIS evolution under multiple emission scenarios to the year 4000 CE. While projected sea-level contributions remain modest by 2100 (0.03–0.11 m), they diverge strongly on longer timescales, reaching up to 3.5 m by 2500. Under the high-emission scenario, full GrIS disintegration is projected as early as 3000 CE. By means of targeted sensitivity experiments, we identify the dominant sources of diverging GrIS trajectories. Across all models, changes in surface mass balance (SMB), and in particular net surface melt, control the long-term ice-sheet retreat. Differences in SMB formulation, especially in the underlying energy balance models, together with differences in the simulated climate, outweigh the influence of initial ice-sheet geometry. This establishes a hierarchy of uncertainties in which atmospheric processes and their representation within the model systems propagate non-linearly into ice-sheet evolution. Our results demonstrate that reliable long-term projections of the GrIS critically depend on improving the representation of climate and SMB, rather than on refining initial conditions alone.

The long-term evolution of the Greenland Ice Sheet (GrIS) remains a major source of uncertainty in projections of future sea-level rise. Recent advances in coupled climate–ice sheet models provide new opportunities to investigate the role of feedbacks between the ice sheet and the climate system, yet substantial divergence persists across climate-ice sheet model projections. Here, we present results from the Greenland Ice sheet Coupled Model Intercomparison Project (GrICMIP), using three coupled climate–ice sheet models to simulate GrIS evolution under multiple emission scenarios to the year 4000 CE. While projected sea-level contributions remain modest by 2100 (0.03–0.11 m), they diverge strongly on longer timescales, reaching up to 3.5 m by 2500. Under the high-emission scenario, full GrIS disintegration is projected as early as 3000 CE. By means of targeted sensitivity experiments, we identify the dominant sources of diverging GrIS trajectories. Across all models, changes in surface mass balance (SMB), and in particular net surface melt, control the long-term ice-sheet retreat. Differences in SMB formulation, especially in the underlying energy balance models, together with differences in the simulated climate, outweigh the influence of initial ice-sheet geometry. This establishes a hierarchy of uncertainties in which atmospheric processes and their representation within the model systems propagate non-linearly into ice-sheet evolution. Our results demonstrate that reliable long-term projections of the GrIS critically depend on improving the representation of climate and SMB, rather than on refining initial conditions alone.

The long-term evolution of the Greenland Ice Sheet (GrIS) remains a major source of uncertainty in projections of future sea-level rise. Recent advances in coupled climate–ice sheet models provide new opportunities to investigate the role of feedbacks between the ice sheet and the climate system, yet substantial divergence persists across climate-ice sheet model projections. Here, we present results from the Greenland Ice sheet Coupled Model Intercomparison Project (GrICMIP), using three coupled climate–ice sheet models to simulate GrIS evolution under multiple emission scenarios to the year 4000 CE. While projected sea-level contributions remain modest by 2100 (0.03–0.11 m), they diverge strongly on longer timescales, reaching up to 3.5 m by 2500. Under the high-emission scenario, full GrIS disintegration is projected as early as 3000 CE. By means of targeted sensitivity experiments, we identify the dominant sources of diverging GrIS trajectories. Across all models, changes in surface mass balance (SMB), and in particular net surface melt, control the long-term ice-sheet retreat. Differences in SMB formulation, especially in the underlying energy balance models, together with differences in the simulated climate, outweigh the influence of initial ice-sheet geometry. This establishes a hierarchy of uncertainties in which atmospheric processes and their representation within the model systems propagate non-linearly into ice-sheet evolution. Our results demonstrate that reliable long-term projections of the GrIS critically depend on improving the representation of climate and SMB, rather than on refining initial conditions alone.

The Arctic Ocean rapidly transitions towards ice-free summers, a "blue" (i.e., summer sea ice-free) state driven by Arctic amplification of abrupt global warming. To improve climate projections and better understand the consequences of reduced Arctic sea ice in a warming climate, it is essential to examine past warm intervals such as the Early Pliocene (~5–4 Ma), when diminished Arctic sea ice coincided with higher global temperatures. Proxy records from the Arctic-Atlantic Gateway (AAG) (~70–80° N), combined with climate model simulations, indicate periodically ice-free summers during this time. Yet, model-data comparisons for the central Arctic Ocean (>80° N) reveal winter sea ice as a persistent feature. Here, we show that proxy evidence for a "blue" Arctic is best reproduced when enhanced vertical ocean mixing is integrated into climate models. Under enhanced mixing, Arctic warming occurs through different mechanisms depending on the model geography, whereas simulations with standard mixing show substantially weaker agreement with proxy evidence. We do not interpret enhanced mixing as the unique solution but rather as a physically motivated sensitivity test that represents unresolved ocean processes. Our results demonstrate that modifying a single ocean parameterization within physically plausible boundaries substantially improves proxy-model agreement for a warmer than modern Early Pliocene, both at the AAG and for the wider Arctic. Combined with published proxy evidence for a warmer Arctic Ocean interior, our findings underscore the need for improved representation of ocean dynamics in paleoclimate simulations. Next-generation climate models should evaluate the sensitivity of simulated "blue" Arctic climates to ocean mixing alongside other key processes, including cloud microphysics and radiative feedbacks, to increase confidence in projections of seasonally ice-free Arctic conditions

The ocean and sea ice are central to Earth's climate system, influencing global heat and carbon cycles, weather patterns, and sea level rise. Recent decades have seen rapid advances in Earth System Models (ESMs), but limitations remain in simulating and comparing key oceanic and cryospheric processes across models. A recurring challenge in model intercomparison efforts like the Coupled Model Intercomparison Project (CMIP) is determining the output variables that best represent essential mechanisms while remaining manageable in volume and complexity. Here we present the CMIP7 ocean and sea ice data request, developed through an international, community-based process to prioritize variables for model output. We identify seven opportunities – science-based use cases spanning ocean and cryosphere drivers and responses, paleoclimate, polar amplification, extremes, wind waves, and rapid model evaluation – to guide variable selection and temporal resolution. To address these opportunities, we request new high-frequency and depth-integrated variables, support improved diagnostics of ocean heat uptake, sea ice processes, and model-observation comparison, and build on lessons from CMIP6. Our approach enables targeted, efficient, and transparent data curation to support a wide range of users, from model developers to policymakers. This effort reflects a growing need for more sophisticated, integrative model outputs that address pressing climate questions, including regional extremes and tipping points, while laying the groundwork for future modeling developments.

Given that the Arctic could be ice-free in summer within the next ten to 20 years, accurately predicting low-ice states is of crucial importance. Paleo-evidence shows that the strong orbitally-induced high latitude insolation anomaly at 127 000 years ago (127 ky), of around +70 W m ⁻² in the Arctic during spring-summer, led to warm conditions and an Arctic that was occasionally or often ice-free during summer. Building on two Coupled Model Intercomparison Projects (CMIPs): the Sea-Ice Model Intercomparison Project and the Paleoclimate Modelling Intercomparison Project, we propose an Assessment Fast Track experiment, abrupt-127k , focusing on this seasonally ice-free, or near ice-free, Arctic at 127 ky. The abrupt-127k experiment is initialised from a piControl simulation and abruptly imposes observed values for the insolation distribution and greenhouse gas forcing at 127 ky. It provides a new opportunity to evaluate models used to compute climate projections, both against paleo-evidence and each other, during a known low Arctic sea ice state. As CMIP models are not usually tuned to paleo observations, abrupt-127k represents a true “out-of-sample” test. The abrupt-127k experiment has four key scientific objectives, to: ascertain the simulated Arctic sea ice state, including the presence and characteristics of last-ice areas; evaluate the simulated climates using Arctic paleo-evidence; characterise the central Arctic surface energy budget; and analyse the ice budget including ice melt, growth, and transport. We show that a large Arctic ice response will manifest within the first 30 years of the simulation, thus a single 100-year long run is sufficient for these objectives. Modelling groups are requested to follow standard CMIP output protocol for analysis, including the use of standard “fixed-length” output. Given abrupt-127k is similar in setup to abrupt-2xCO2 and abrupt-4xCO2 CMIP7 experiments, combined analysis of these abrupt-experiments will facilitate understanding of the impacts of instantaneous radiative forcing in the Arctic.