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

Abstract

Abstract. 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. 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.

Surface Melt and Runoff on Antarctic Ice Shelves at 1.5°C, 2°C, and 4°C of Future Warming

“…(2021) and the four MAR simulations are evaluated in Kittel et al. (2021). For this reason, the historical period is not considered further.…”

“…Ice shelf surface melting and runoff is projected to increase with warming (Kittel et al., 2021; Trusel et al., 2015). However, the relationship between temperature and melting is highly non‐linear, and melt rates associated with different future scenarios diverge considerably around mid‐century, resulting in a wide range of values for 2100 (Trusel et al., 2015).…”

Downscaled surface mass balance in Antarctica: impacts of subsurface processes and large-scale atmospheric circulation

“…Although these differences are currently only a few percent of the total SMB, as the climate warms and melt becomes more widespread in Antarctica e.g. (Boberg et al, 2020;Kittel et al, 2020), accounting for these processes will become more important. Moreover, on a local and regional scale, the differences are more important when determining mass balance in basins or outlet glacier/ice shelves.…”

Sensitivity of the surface energy budget to drifting snow as simulated by MAR in coastal Adelie Land, Antarctica

“…MAR is a hydrostatic regional climate model solving primitive equations as originally described in Gallée and Schayes (1994), and extensively used for decade-long climate simulation over high-latitude regions (e.g., Agosta et al, 2019;Fettweis et al, 2017Fettweis et al, , 2020Mottram et al, 2020;Kittel et al, 2020). Five atmospheric water species are represented in the model: specific humidity, cloud droplets, rain drops, cloud ice crystals, and snow particles (Gallée and Schayes, 1994 through the atmosphere is calculated according to Morcrette (2002), and cloud radiative properties are calculated according to Ebert and Curry (1992) based on water species concentrations.…”

“…In this paper we use the regional climate model MAR to quantify the influence of drifting snow on the near-surface climate and surface energy budget in Adelie Land, a coastal region of East Antarctica particularly prone to erosive winds and where drifting-snow equipment deployed over the past few years provide observational support for model evaluation near the surface (Trouvilliez et al, 2014;Amory et al, 2020b). MAR has been widely used to simulate the climate and surface mass balance of polar ice sheets (e.g., Fettweis et al, 2013Fettweis et al, , 2017Fettweis et al, , 2020Hofer et al, 2017Hofer et al, , 2019Kittel et al, 2018Kittel et al, , 2020Agosta et al, 2019;Mottram et al, 2020), and includes a detailed representation of drifting-snow processes already applied to study snow mass transport and wind-driven ablation in coastal East Antarctica (Gallée et al, 2005;Gallée et al, 2013;Amory et al, 2015Amory et al, , 2020b. The explicit coupling of the drifting-snow scheme with the atmospheric component of the model enables a vertical discretization of drifting-snow profiles and related sublimation within the atmospheric boundary layer and takes into account the radiative contribution of drifting-snow particles.…”