Diverging future surface mass balance between the Antarctic ice shelves and grounded ice sheet

The future surface mass balance (SMB) will influence the ice dynamics and the contribution of the Antarctic ice sheet (AIS) to the sea-level rise. Most of recent Antarctic SMB projections were based on the 5th phase of the Coupled Model Intercomparison Project (CMIP5). However, new CMIP6 results have revealed a +1.3°C higher mean Antarctic near-surface temperature than in CMIP5 at the end of the 21st century enabling estimations of future SMB in warmer climates. Here, we investigate the AIS sensitivity to different warmings with an ensemble of four simulations performed with the polar regional climate model MAR forced by two CMIP5 and two CMIP6 models over 1981--2100. Statistical extrapolation allows us to expand our results to the whole CMIP5 and CMIP6 ensembles. Our results highlight a contrasting effect on the future grounded ice sheet and the ice shelves. The SMB over grounded ice is projected to increase as a response to stronger snowfall, only partly offset by enhanced meltwater runoff. This leads to a cumulated sea-level rise mitigation (i.e. an increase in surface mass) of the grounded Antarctic surface by 5.1 ± 1.9 cm sea-level equivalent (SLE) in CMIP5-RCP8.5 and 6.3 ± 2.0 cm SLE in CMIP6-ssp585. Additionally, the CMIP6 low-emission ssp126 and intermediate-emission ssp245 scenarios project a stabilised surface mass gain resulting in a lower mitigation to sea-level rise than in ssp585. Over the ice shelves, the strong runoff increase associated with higher temperature is projected to lower the SMB with a stronger decrease in CMIP6-ssp585 compared to CMIP5-RCP8.5. Ice shelves are however predict to have a close-to-present-equilibrium stable SMB under CMIP6 ssp126 and ssp245 scenarios. Future uncertainties are mainly due to the sensitivity to anthropogenic forcing and the timing of the projected warming. Furthermore,  we compare the MAR projected SMB to the ISMIP6-derived SMB, revealing large local and integrated differences between MAR and the respective forcing ESM highlighting the need of additional projections relying on more models including both RCMs and ESMs. While ice shelves should remain at a close-to-equilibrium stable SMB under the Paris Agreements, MAR projects strong SMB decrease for an Antarctic near-surface warming above +2.5°C limiting the warming range before potential irreversible damages on the ice-shelves. Finally, our results reveal the existence of a potential threshold (+7.5°C) that leads to a lower grounded SMB increase. This however has to be confirmed in following studies using more extreme or longer future scenarios.