By Sarah Wauthy

The ice sheet mass balance (MB) measures the changes in the total mass of the ice sheet. It is the difference between the ice accumulation due to snow fall and ice mass losses mainly due to local melting (and sublimation) or ice transport toward the ocean, where ice ultimately melts too.

More precisely, the surface mass balance is computed as the difference between surface mass balance (SMB), and solid ice discharge (D, i.e the formation of icebergs) and ice shelf bottom melting (M) (the ice sheet part that starts to float on the ocean – by virtue of Archimedes’ principle – in the coastal regions of Antarctica), i.e. MB = SMB – D – M.

The Antarctic ice sheet mass balance is the principal indicator of the “health state” of the ice sheet and it directly affects the sea level: the Antarctic contribution to sea level rise depends on the evolution of the mass balance in the near future under anthropogenic warming. While solid ice discharge and ice shelf bottom melting are relatively well constrained by observations, surface mass balance is poorly evaluated.

The surface mass balance is the difference between incoming (accumulation) and outgoing mass (ablation) at the ice sheet interface: accumulation is due to snowfall while ablation is due to melting processes (fusion and sublimation), to meltwater runoff and transport/erosion by wind.

This surface mass balance is characterized by strong spatial and temporal variations: snowfall depends on the local topography but also on large scale phenomena (e.g. atmospheric circulation and sea ice cover of the ocean).

It is therefore essential to better assess the variability of this surface mass balance using data obtained in the field and after lab analyzes, in order to correctly model the different contributions to the ice sheet mass balance and thus be able to project sea level changes due to human activities.