Evolution of Antarctic firn air content under three future warming scenarios

Abstract. The Antarctic firn layer provides pore space in which an estimated 94 to 96 % of the surface melt refreezes or is retained as liquid water. Future depletion of pore space in the firn layer by increased surface melt, densification rates and formation of impermeable ice slabs can potentially lead to extensive meltwater ponding, followed by ice-shelf disintegration by hydrofracturing. Here, we investigate 21st century evolution of the total firn air content (FAC) and accessible FAC (i.e. the pore space that is accessible for meltwater) across Antarctica. We use the semi-empirical firn model IMAU-FDM with an updated dynamical densification expression. The firn model is forced by general circulation model CESM2 output for three climate scenarios (SSP1-2.6, SSP2-4.5 and SSP5-8.5), dynamically downscaled to a 27 km horizontal resolution by the regional climate model RACMO2.3p2. To estimate the accessible FAC, we prescribe a relationship between ice-slab thickness and permeability. In our simulations, ice shelves in the Antarctic Peninsula and Roi Baudouin ice shelf in Dronning Maud Land are particularly vulnerable to FAC depletion (> 50 % decrease), even for strong and intermediate mitigation scenarios. Especially in the high-end warming scenario, the formation of ice slabs further reduces accessible FAC on ice shelves with low accumulation rates (current rates of < 500 mm yr-1), including many East-Antarctic ice shelves and on Filchner-Ronne, Ross, Pine Island and Larsen C ice shelves. Our results underline the different response of low- and high-accumulation ice shelves to atmospheric warming, indicating a potentially large impact of ice slab formation on the viability of low-accumulation ice shelves.