Which ice sheet matters more for the sea, Antarctica or Greenland?

Right now, Greenland is the bigger contributor, but Antarctica holds far more ice and is expected to overtake it later this century. The two ice sheets are very different beasts, and that difference decides how much each one could add to the sea.

An ice sheet is a glacier on a continental scale, and only two exist today. The Antarctic ice sheet covers 98 percent of its continent, averages more than 2 kilometres thick and holds 61 percent of all the fresh water on Earth. The Greenland ice sheet is the second-largest body of ice in the world, averaging 1,673 metres thick. If all of Greenland's ice melted it would raise global sea level by about 7.4 metres; Antarctica holds far more: its eastern sheet alone contains about 53 metres of sea level.

Why does Greenland lose more today? It sits in the Arctic, where warming is amplified. Its yearly share of sea level rise grew from 0.07 millimetres a year between 1992 and 1997 to 0.68 millimetres a year between 2012 and 2017. Antarctica is less simple. Its huge eastern part is higher and more stable, and was still gaining mass in the early 2020s because more snow was falling on top, while the smaller western part is losing ice fast.

The western part is the worry, because much of its bed lies below sea level, in places more than 2,500 metres below. That makes it more vulnerable, and glaciers such as Thwaites and Pine Island are where the losses are concentrated.

So the honest answer has two halves. For the sea level you see this decade, Greenland dominates. For centuries to come, the big unknown is Antarctica, mainly its western glaciers. Antarctica's losses are measured with several satellite systems, and the estimates have shifted as methods changed.