Brice Nöel, University of Utrecht
The Greenland ice sheet (GrIS) is the world’s second largest ice mass, storing about one tenth of the Earth’s freshwater. Totally melted, global sea level would rise by 7.4 m, affecting low-lying regions worldwide. Since the mid-1990s, increased atmospheric and oceanic temperatures have accelerated GrIS mass loss through increased meltwater runoff and icebergs discharge from marine-terminating glaciers. To study the recent GrIS mass loss, we use a climate model that simulates the GrIS surface mass balance (SMB), i.e. the difference between winter snowfall and summer runoff.
Using the model, we identify 1997 as a tipping point for the mass balance of Greenland’s peripheral ice caps (GICs), i.e. detached from the main ice sheet. These ice caps are covered by porous snow that acts as a ”sponge” absorbing a large amount of meltwater, which subsequently refreezes in winter. The excess meltwater runs off to the ocean, contributing to mass loss. Until 1997, the snow could actively sponge and refreeze meltwater. Around 1997, decades of increased melt saturated the snow, so that additional melt is now forced to run off to the ocean, tripling the mass loss. Likewise, ice caps of the Canadian Arctic underwent uninterrupted mass loss during the last six decades. Consequently, these ice caps will likely undergo irreversible mass loss in the future. The Greenland ice sheet is still safe, as porous snow absorbs most of the summer melt. However, the tipping point crossed by Arctic ice caps must be regarded as an alarm-signal for the ice sheet.