The area of floating ice that each Antarctic ice shelf can lose without its grounded portion sliding into the ocean is mapped in a paper published online in Nature Climate Change this week. The study suggests that although some ice sheets can lose a large area without any immediate effect on the rest of the ice shelf, others have limited or almost no area that can be lost without major consequences.

Melt of the Antarctic ice sheets has important implications for global sea-level rise. The floating portion of the ice shelves supports the portion that lies on land, preventing it from flowing into the ocean. However, the exact area that is essential for this support had not yet been quantified.

For each Antarctic ice shelf, Johannes Furst and colleagues calculate and map the area that has little or no supporting effect on the upstream ice and can be lost with little impact, which they refer to as passive shelf ice (PSI). They find that, overall, around 13% of the ice sheets is PSI, with ice shelves in the Amundsen (7% PSI) and Bellingshausen (5% PSI) seas being the most susceptible to further ice loss. The authors conclude that monitoring these shelves for any fractures outside the safety band is urgently required, as they have also been thinning for the past two decades and their upstream grounded ice sits on a backward sloping bed, a precondition for marine ice-sheet instability.