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The land ice contribution to global mean sea level rise has not yet been predicted1 using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models2-8, but primarily used previous-generation scenarios9 and climate models10, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios11,12 using statistical emulation of the ice sheet and glacier models. We find that limiting global warming to 1.5 degrees Celsius would halve the land ice contribution to twenty-first-century sea level rise, relative to current emissions pledges. The median decreases from 25 to 13 centimetres sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The projected Antarctic contribution does not show a clear response to the emissions scenario, owing to uncertainties in the competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing the median land ice contribution to 42 centimetres SLE under current policies and pledges, with the 95th percentile projection exceeding half a metre even under 1.5 degrees Celsius warming. This would severely limit the possibility of mitigating future coastal flooding. Given this large range (between 13 centimetres SLE using the main projections under 1.5 degrees Celsius warming and 42 centimetres SLE using risk-averse projections under current pledges), adaptation planning for twenty-first-century sea level rise must account for a factor-of-three uncertainty in the land ice contribution until climate policies and the Antarctic response are further constrained.

A mixed statistical-physical approach is used to emulate the spatio-temporal variability of the Antarctic ice sheet surface mass balance and runoff of a regional climate model. We demonstrate the ability of this simple method to extend existing MAR simulations to other periods, scenarios or climate models, that were not originally processed through the regional climate model. This method is useful to quickly populate ensembles of surface mass balance and runoff which are needed to constrain ice sheet model ensembles. After correcting the distribution of equilibrium climate sensitivity of 16 climate models, we find a likely contribution of surface mass balance to sea level rise of 0.4 to 2.2 cm from 1900 to 2010, and -3.4 to -0.1 cm from 2100 to 2099 under the SSP1-2.6 scenario, versus -4.4 to -1.4 cm under SSP2-4.5 and -7.8 to -4.0 cm under SSP5-8.5. Based on a more limited and uncorrected ensemble, we find a considerable uncertainty in the contribution to sea level from 2000 to 2200: between -10 and -1 cm in SSP1-2.6 and between -33 and +6 cm in SSP5-8.5. Based on a runoff criteria in our reconstructions, we identify the emergence of surface conditions prone to hydrofracturing. A majority of ice shelves could remain safe from hydrofracturing under the SSP1-2.6 scenario, but all the Antarctic ice shelves could be prone to hydrofracturing before 2130 under the SSP5-8.5 scenario. [ABSTRACT FROM AUTHOR]

Identifying and quantifying irreducible and reducible uncertainties in the Antarctic Ice Sheet response to future climate change is essential for guiding mitigation and adaptation policy decision. However, the impact of the irreducible internal climate variability, resulting from processes intrinsic to the climate system, remains poorly understood and quantified. Here, we characterise both the atmospheric and oceanic internal climate variability in a selection of three CMIP6 models (UKESM1-0-LL, IPSL-CM6A-LR and MPI-ESM1.2-HR) and estimate their impact on the Antarctic contribution to sea level over the 21st century under the SSP2-4.5 scenario. To achieve this, we use a standalone ice-sheet model driven by the ocean through parameterised basal melting and by the atmosphere through emulated surface mass balance estimates. Internal climate variability affects the Antarctic contribution to changes in sea level until 2100 by 45 % to 93 % depending on the CMIP6 model. This may be a low estimate as the internal climate variability in the CMIP models is likely underestimated. For all the three climate models and for most Antarctic regions, the effect of atmospheric internal climate variability on the surface mass balance overwhelms the effect of oceanic internal climate variability on the dynamical ice-sheet mass loss by more than a factor of 3. The atmospheric internal climate variability is similar in the three CMIP6 models analysed in this study. Conversely, the amplitude of oceanic internal climate variability around Antarctica strongly depends on the climate model as underestimated convection, due to either biases in the sea-ice behaviour or in the ocean stratification, leads to weak mid-depth ocean variability. We then issue recommendations for future ice-sheet projections: use several members in the run and in its initialisation, favor 50-year averages to correct or weight simulations over the present-day period, and couple ice-sheet and climate models. [ABSTRACT FROM AUTHOR]

The Marine Ice Sheet and Ocean Model Intercomparison Project - phase 2 (MISOMIP2) is a natural progression of previous and ongoing model intercomparison exercises that have focused on the simulation of ice-sheet and ocean processes in Antarctica. The previous exercises motivate the move towards realistic configurations as well as more diverse model parameters and resolutions. The main objective of MISOMIP2 is to investigate the performance of existing ocean and coupled ice-sheet-ocean models in a range of Antarctic environments, through comparisons to observational data. We will assess the status of ice-sheet-ocean modelling as a community and identify common characteristics of models that are best able to capture observed features. As models are highly tuned based on present-day data, we will also compare their sensitivity to prescribed abrupt atmospheric perturbations leading to either very warm or slightly warmer ocean conditions compared to present-day. The approach of MISOMIP2 is to welcome contributions of models as they are, including global and regional configurations, but we request standardised variables and common grids for the outputs. We target the analysis on two specific regions, the Amundsen Sea and the Weddell Sea, since they describe two different ocean environments and have been relatively well observed compared to other areas of Antarctica. An observational "MIPkit" synthesizing existing ocean and ice sheet observations for a common period is provided to evaluate ocean and ice sheet models in these two regions. [ABSTRACT FROM AUTHOR]

The Marine Ice Sheet and Ocean Model Intercomparison Project – phase 2 (MISOMIP2) is a natural progression of previous and ongoing model intercomparison exercises that have focused on the simulation of ice-sheet and ocean processes in Antarctica. The previous exercises motivate the move towards realistic configurations as well as more diverse model parameters and resolutions. The main objective of MISOMIP2 is to investigate the performance of existing ocean and coupled ice-sheet–ocean models in a range of Antarctic environments, through comparisons to observational data. We will assess the status of ice-sheet–ocean modelling as a community and identify common characteristics of models that are best able to capture observed features. As models are highly tuned based on present-day data, we will also compare their sensitivity to prescribed abrupt atmospheric perturbations leading to either very warm or slightly warmer ocean conditions compared to present-day. The approach of MISOMIP2 is to welcome contributions of models as they are, including global and regional configurations, but we request standardised variables and common grids for the outputs. We target the analysis on two specific regions, the Amundsen Sea and the Weddell Sea, since they describe two different ocean environments and have been relatively well observed compared to other areas of Antarctica. An observational 'MIPkit' synthesizing existing ocean and ice sheet observations for a common period is provided to evaluate ocean and ice sheet models in these two regions. [ABSTRACT FROM AUTHOR]

How much Antarctic ice shelf basal melt rates can increase in response to global warming remains an open question. Here we describe the response of the Southern Ocean and Antarctic ice shelf cavities to an abrupt change to high-end atmospheric conditions typical of the late 23rd century under the SSP5-8.5 scenario. To achieve this objective, we first present and evaluate a new 0.25° global configuration of the NEMO ocean and sea ice model. Our present-day simulations demonstrate good agreement with observational data for key variables such as temperature, salinity, and ice shelf melt rates, despite remaining difficulties to simulate the thermocline and melt variability in the Amundsen Sea. The ocean response to the high-end atmospheric perturbation includes a strengthening and extension of the Ross and Weddell gyres and a quasi-disappearance of sea ice, with subsequent decrease in production of High Salinity Shelf Water and increased intrusion of warmer water onto the continental shelves. This induces a substantial increase in ice shelf basal melt rates, particularly in the coldest seas, with a total ice shelf basal mass loss rising from 1,180 to 15,700 Gt yr-1 and an Antarctica averaged ice shelf melt rate increasing from 0.80 m yr-1 to 10.64 m yr-1. In the perturbed simulation, most ice shelves around Antarctica experience conditions that are currently found in the Amundsen Sea, while the Amundsen Sea also warms by 2 °C. These projections can be used as a base to calibrate basal melt parameterisations used in long-term ice sheet projections. [ABSTRACT FROM AUTHOR]

The world's largest ice shelves are found in the Antarctic Weddell Sea and Ross Sea where complex interactions between the atmosphere, sea ice, ice shelves and ocean transform shelf waters into High Salinity Shelf Water (HSSW) and Ice Shelf Water (ISW), the parent waters of Antarctic Bottom Water (AABW). This process feeds the lower limb of the global overturning circulation as AABW, the world's densest and deepest water mass, spreads outwards from Antarctica. None of the coupled climate models contributing to CMIP6 directly simulated ocean–ice shelf interactions, thereby omitting a potentially critical piece of the climate puzzle. As a first step towards better representing these processes in a global ocean model, we run a 1 ∘ resolution Nucleus for European Modelling of the Ocean (NEMO; eORCA1) forced configuration to explicitly simulate circulation beneath the Filchner-Ronne Ice Shelf (FRIS), Larsen C Ice Shelf (LCIS) and Ross Ice Shelf (RIS). These locations are thought to supply the majority of the source waters for AABW, and so melt in all other cavities is provisionally prescribed. Results show that the grid resolution of 1 ∘ is sufficient to produce melt rate patterns and total melt fluxes of FRIS (117 ± 21 Gt yr -1), LCIS (36 ± 7 Gt yr -1) and RIS (112 ± 22 Gt yr -1) that agree well with both high-resolution models and satellite measurements. Most notably, allowing sub-ice shelf circulation reduces salinity biases (0.1 psu), produces the previously unresolved water mass ISW and re-organizes the shelf circulation to bring the regional model hydrography closer to observations. A change in AABW within the Weddell Sea and the Ross Sea towards colder, fresher values is identified, but the magnitude is limited by the absence of a realistic overflow. This study presents a NEMO configuration that can be used for climate applications with improved realism of the Antarctic continental shelf circulation and a better representation of the precursors of AABW. [ABSTRACT FROM AUTHOR]

Ocean warming around Antarctica has the potential to trigger marine ice‐sheet instabilities. It has been suggested that abrupt and irreversible cold‐to‐warm ocean tipping points may exist, with possible domino effect from ocean to ice‐sheet tipping points. A 1/4° ocean model configuration of the Amundsen Sea sector is used to investigate the existence of ocean tipping points, their drivers, and their potential impact on ice‐shelf basal melting. We apply idealized atmospheric perturbations of either heat, freshwater, or momentum fluxes, and we characterize the key physical processes at play in warm‐to‐cold and cold‐to‐warm climate transitions. Relatively weak perturbations of any of these fluxes are able to switch the Amundsen Sea to an intermittent or permanent cold state, that is, with ocean temperatures close to the surface freezing point and very low ice‐shelf melt rate. The transitions are reversible, that is, canceling the atmospheric perturbation brings the ocean system back to its unperturbed state within a few decades. All the transitions are primarily driven by changes in surface buoyancy fluxes resulting from the freshwater flux perturbation or from modified net sea‐ice production due to either heat flux or sea‐ice advection anomalies. These changes affect the vertical ocean stratification over the continental shelf and thereby the eastward undercurrent at the shelf break, which both impact ice‐shelf melting. As sea‐ice induced deep convection is already quite limited in present‐day conditions, surface buoyancy gain in a warmer climate has relatively little effect on deep ocean properties compared to colder climate conditions. Plain Language Summary: The West Antarctic Ice Sheet is under the threat of a partial collapse, which would induce rapid global sea level rise. This threat is partly related to the thinning of floating ice shelves, and the consequent retreat of the grounding line, which is a self‐sustained ice dynamics process. It is triggered by increased basal melting of the ice shelves, which results from enhanced flow of relatively warm waters onto the continental shelf. It has been suggested that self‐sustained ocean processes may lead to abrupt changes in the flow of warm water into ice‐shelf cavities, which could facilitate the tipping to a marine ice‐sheet instability. Here, we analyze whether such abrupt ocean changes can occur under cold‐to‐warm or warm‐to‐cold transitions in the Amundsen Sea, West Antarctica. We use a regional ocean model with a set of idealized local atmospheric perturbations to characterize the thresholds and reversibility of ocean abrupt changes.We find that the currently warm Amundsen Sea could switch intermittently or permanently to a cold state for relatively weak atmospheric perturbations and could be slightly warmer in the future. All transitions are reversible. The main mechanism involved on decadal scale is related to a change in the surface buoyancy fluxes. Key Points: The currently warm ice‐shelf cavities of the Amundsen sector could become or have been cold for slightly colder climatic conditionsThe transitions are reversible: canceling the atmospheric perturbation brings the ocean back to its unperturbed state within a few decadesAll the transitions are primarily driven, at multi‐decadal scale, by changes in surface buoyancy fluxes over the continental shelf [ABSTRACT FROM AUTHOR]