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The Greenland ice sheet is considered as one of the main causes of sea level rise (SLR) at the end of the 21st century. But what if it is already too late to reverse the loss of ice from the Greenland ice sheet? The mass balance (MB) resulting from the coupling between the Regional Atmospheric Model (MAR, ULiège) and the Parallel Ice Sheet Model (PISM, PIK) over Greenland following the CESM2 ssp585 climate indicates that even if we stop the CESM2 warming in 2100 and continue with a +7°C climate until 2200 with respect to the reference period (1961-1990), the GrIS continues to lose mass up to a contribution equivalent to 60 cm of SLR in 2200. From this coupling experiment, we ran several coupled simulations by stabilizing the warming at different thresholds (+ 1, 2, 3, ... °C) with respect to our reference period in order to highlight a kind of tipping point of the ice sheet with respect to atmospheric warming. Other experiments have been launched by reversing the climate imposed by CESM2 from 2100 to 2000, for example, with the aim of identifying whether the GrIS could gain ice mass again with a climate as warm as the present one.

Global warming has already substantially altered the Arctic cryosphere. Due to the Arctic warming amplification, the temperature is increasing more strongly leading to pervasive changes in this area. Recent years were notably marked by melt records over the Greenland Ice Sheet while other regions such as Svalbard seem to remain less influenced. This raises the question of the current state of the Greenland Ice Sheet and the various ice caps in the Arctic for which few studies are available. We here run the Regional Climate Model (RCM) Modèle Atmosphérique Régional (MAR) at a resolution of 6 km over 4 different domains covering all the Arctic grounded cryosphere to produce a unified Surface Mass Balance product from 1950 to present day. We also compare our results to large-scale indices to better understand the heterogeneity of the evolutions across the Arctic and their links to recent climate change. We find a sharp decrease of SMB over the western Arctic (Canada and Greenland), in relationship with the atmospheric blocking situations that have become more frequent in summer, resulting in a 41 % increase of the melt rate since 1950. This increase is not seen over the Russian Arctic and Svalbard permanent ice areas, where melt rates have increased by only 9 % on average, illustrating a heterogeneity in the Arctic SMB response to global warming.

The Greenland ice sheet (GrIS) is a key contributor to see level rise. By melting in surface, ice sheet is thinning and reaches higher temperature which accelerate the melting processes coming from Global Warming. The main goal of our research is to improve the representation of melt-elevation feedback, which is crucial to determine how and when GrIS will melt and will involve in a near future, by coupling two kind of numerical models. The difficulty to model this feedback relies on the fact that ice-sheet models (ISMs) can reproduce the dynamic of the ice sheet and thus provide an evolution of the surface elevation, whereas (regional) climate models (RCMs) can represent the ice/snow and atmosphere interactions trough the surface mass balance (SMB). A coupling between these models appears as a solution and has already been accomplished. However, ISMs responses to a same forcing field may be quite different, while SMB from different RCMs are relatively more similar with the same forcing. Coupling could therefore be dependent of which ISMs are used. To avoid a coupling, costly in computing time, SMB vertical gradient as a function of local elevation variations could be used by ISMs to correct SMB. Nonetheless, these SMB gradients are computed with a RCM using a fixed topography, which could introduce biases if the surface elevation vary significantly. Here we decide to full couple the RCM MAR, specifically developed for polar climate and forced at his lateral boundaries by CESM2 (a CMIP6 model, scenario ssp585), with the ISM PISM. The coupling means that, each year, we exchange ice thickness from PISM to update the topography and ice mask of MAR, and SMB from MAR to update forcing fields of PISM. First of all the aim is to analyze what became the GrIS in 2100 with this extreme scenario. Then we want to define a coupling time threshold to determine after how much years an update of the topography in MAR is needed by varying the time step (from 1 to 5, 10, 20, 30 and 50 years) of the coupling. The final aim is to determine until when the MAR based SMB gradients are valid for a same topography in MAR.

The Greenland Ice Sheet (GrIS) will be losing mass at an accelerating pace throughout the 21st century, with a direct link between anthropogenic greenhouse gas emissions and the magnitude of Greenland mass loss. Currently, approximately 60 % of the mass loss contribution comes from surface melt and subsequent meltwater runoff, while 40 % are due to ice calving. Where most of the surface melt occurs (in the ablation zone), most of the energy for the surface melt is provided by absorbed shortwave fluxes, which could be reduced by solar geoengineering measures. However, so far very little is known about the potential impacts of an artificial reduction of the incoming solar radiation on the GrIS surface energy budget and the subsequent change in meltwater production. By forcing the regional climate model MAR with the latest CMIP6 future scenarios ssp245, ssp585 and associated G6solar experiment from the Earth System Model CNRM-ESM2-1, we evaluate the local changes due to the reduction of the solar constant on the projected GrIS surface mass balance (SMB) decrease. Overall, our results show that even in case of low mitigation greenhouse gas emissions scenario (ssp585), the Greenland surface mass loss can be brought in line with the medium mitigation emissions scenario (ssp245) by reducing the solar downward flux at the top of the atmosphere by ~40 W/m2 or ~1.5 % (using the G6solar experiment). In addition to reduce Global Warming in line with ssp245, G6solar also decreases the efficiency of surface meltwater production over the Greenland ice sheet by damping the well-known positive melt-albedo feedback which mitigates the projected Greenland ice sheet surface melt increase by 6 %. However, only more constraining geoengineering experiments than G6solar allows to maintain positive SMB till the end of this century without any reduction in our greenhouse gas emissions.

The surface mass balance (SMB) of the Antarctic ice sheet is often considered as a negative contributor to the sea level rise as present snowfall accumulation largely compensates for ablation through wind erosion, sublimation and runoff. The latter is even almost negligible since current Antarctic surface melting is limited to relatively scarce events over generally peripheral areas and refreezes almost entirely into the snowpack. However, melting can significantly affect the stability of ice shelves through hydrofracturing, potentially leading to their disintegration, acceleration of grounded ice and increased sea level rise. Although a large increase in snowfall is expected in a warmer climate, more numerous and stronger melting events could conversely lead to a larger risk of ice shelf collapse. In this study, we provide an estimation of the SMB of the Antarctic ice sheet for the end of the 21st century by forcing the state-of-the-art regional climate model MAR with three different global climate models. We chose the models (from both the Coupled Model Intercomparison Project Phase 5 and 6 - CMIP5 and CMIP6) providing the best metrics for representing the current Antarctic climate. While the increase in snowfall largely compensates snow ablation through runoff in CMIP5-forced projections, CMIP6-forced simulations reveal that runoff cannot be neglected in the future as it accounts for a maximum of 50% of snowfall and becomes the main ablation component over the ice sheet. Furthermore, we identify a tipping point (ie., a warming of 4°C) at which the Antarctic SMB starts to decrease as a result of enhanced runoff particularly over ice shelves. Our results highlight the importance of taking into account meltwater production and runoff and indicate that previous model studies neglecting these processes yield overestimated SMB estimates, ultimately leading to underestimated Antarctic contribution to sea level rise. Finally, melt rates over each ice shelf are higher than those that led to the collapse of the Larsen A and B ice shelves, suggesting a high probability of ice shelf collapses all over peripheral Antarctica by 2100.

In the past years, Greenland has been affected by several extreme melt events, particularly in the years 2010, 2012 and most recently, during this year's spring/summer. With progressing climate change, extreme melt events can be expected to occur more frequently and become more severe/persistent. So far, however, longer-term projections of ice loss from Greenland typically rely on scenarios that only take account of gradual changes in the climate, for instance, based on the Representative Concentration Pathways. Extreme melt events have generally been underestimated and their potential effect on future surface mass balance shows already serve impacts for sea-level rise. Here we investigate the total impact of future extreme melt events on the Greenland Ice Sheet. We force the thermodynamically coupled parallel ice sheet model PISM with idealized surface-mass-balance scenarios that include extreme melt events. Thereby, we investigate the dynamical response of the ice sheet model to changes in frequency and intensity of extreme melt events and quantify their impacts with respect to sea-level rise.

Future climate projections show a marked increase in Greenland Ice Sheet (GrIS) runoffduring the 21st century, a direct consequence of the Polar Amplification signal. Regionalclimate models (RCMs) are a widely used tool to downscale ensembles of projections fromglobal climate models (GCMs) to assess the impact of global warming on GrIS melt andsea level rise contribution. Initial results of the CMIP6 GCM model intercomparisonproject have revealed a greater 21st century temperature rise than in CMIP5 models.However, so far very little is known about the subsequent impacts on the future GrISsurface melt and therefore sea level rise contribution. Here, we show that the total GrISmelt during the 21st century almost doubles when using CMIP6 forcing compared to theprevious CMIP5 model ensemble, despite an equal global radiative forcing of +8.5 W/m2in 2100 in both RCP8.5 and SSP58.5 scenarios. The total GrIS sea level rise contributionfrom surface melt in our high-resolution (15 km) projections is 17.8 cm in SSP58.5, 7.9 cmmore than in our RCP8.5 simulations, despite the same radiative forcing. We identify a+1.7°C greater Arctic amplification in the CMIP6 ensemble as the main driver behind thepresented doubling of future GrIS sea level rise contribution

We have compared the surface mass (SMB) and energy balance of the Earth System model (ESM) CESM (Community Earth System Model) with those of the regional climate model (RCM) MAR (Modèle Atmosphérique Régional) forced by CESM over the present era (1981 — 2010) and the future (2011 — 2100 with SSP585 scenario).Until now, global climate models (GCM) and ESMs forcing RCMs such as MAR didn’t include a module able to simulate snow and energy balance at the surface of a snow pack like the SISVAT module of MAR and were therefore not able to simulate the SMB of an ice sheet. Evaluating the added value of an RCM compared to a GCM could only be done by comparing atmospheric outputs (temperature, wind, precipitation …) in both models. CESM is the first ESM including a land model capable of simulating the surface of an ice sheet and thus to directly compare the SMB of an RCM and an ESM the first time.Our results show that, if the SMB and is components are very similar in CESM and MAR over the present era, they quickly start to diverge in our future projection, the SMB of MAR decreasing more than that of CESM. This difference in SMB evolution is almost exclusively explained by a much larger increase of the melter runoff in MAR compared to CESM whereas the temporal evolution of snowfall, rainfall and sublimation is comparable in both runs.

The Greenland blocking index (GBI), an indicator of the synoptic-scale circulation over Greenland, has been anomalously positive during summers since the late 1990s. Such changes in atmospheric circulation have led to an increase in Greenland summer temperatures, a decrease in cloud cover and greater surface melt. The GBI is therefore a key indicator of melting and surface mass balance variability over the Greenland ice sheet. However, the fifth phase of the Coupled Model Intercomparison Project (CMIP5) models do not represent any increase in GBI as suggested by observations. Until 2100, no significant long-term trend in the GBI, and therefore no circulation changes, are projected. In this study the new generation of CMIP6 Earth-system models is evaluated in order to analyze the evolution of the future GBI. All CMIP5 and CMIP6 projections reveal the same trend towards a decrease of the GBI until 2100 and no model reproduces the strong increase in GBI observed over the last few decades. Significant melting events related to a highly positive GBI, as observed this summer 2019, are still not considered by CMIP6 models and therefore the projected surface melt increase of the ice sheet is likely to be underestimated if such circulation changes persist in the next decades.

Estimates for the recent period and projections of the Antarctic surface mass balance (SMB) often rely on high-resolution polar-oriented regional climate models (RCMs). However, RCMs require large-scale boundary forcing fields provided by reanalyses or general circulation models (GCMs). Since the recent variability of sea surface conditions (SSC, namely sea ice concentration (SIC) and sea surface temperature (SST)) over the Southern Ocean are not reproduced by most GCMs from the 5th phase of the Coupled Model Intercomparison Project (CMIP5) for the last decades, RCMs are then subject to potential biases. We investigate here the direct sensitivity of the Antarctic SMB to SSC perturbations around the Antarctic. With the RCM MAR, different sensitivity experiments are performed over 1979–2015 by altering the ERA-Interim SSC with (i) homogeneous perturbations and (ii) mean anomalies estimated from all CMIP5 models and two extreme ones, while atmospheric lateral boundary conditions remained unchanged. Results show increased (resp. decreased) precipitation due to perturbations inducing warmer (resp. colder) SSC than ERA-Interim significantly altering the SMB of coastal areas, as precipitation is mainly related to cyclones that do not penetrate far into the continent. At the continental scale, significant SMB anomalies (i.e, greater than the interannual variability) are found for the largest combined SST/SIC perturbations. Sensitivity experiments with warmer SSC reveal integrated SMB anomalies (+5 %–+13 %) over the present climate (1979–2015) in the lower range of the SMB increase projected for the end of the 21st century.