Increased Variability in Greenland Ice Sheet Runoff from Satellite Observations

Runoff from the Greenland Ice Sheet has increased over recent decades affecting global sea level, regional ocean circulation, and coastal marine ecosystems, and it now accounts for most of the contemporary mass imbalance. Estimates of runoff are typically derived from regional climate models because satellite records have been limited to assessments of melting extent. Here, we use CryoSat-2 satellite altimetry to produce direct measurements of Greenland's runoff variability, based on seasonal changes in the ice sheet's surface elevation. Between 2011 and 2020, Greenland's ablation zone thinned on average by 1.4 +/- 0.4 m each summer and thickened by 0.9 +/- 0.4 m each winter. By adjusting for the steady-state divergence of ice, we estimate that runoff was 357 +/- 58 Gt/yr on average - in close agreement with regional climate model simulations (root mean square difference of 47 to 60 Gt/yr). As well as being 21 % higher between 2011 and 2020 than over the preceding three decades, runoff is now also 60 % more variable from year-to-year as a consequence of large-scale fluctuations in atmospheric circulation. Because this variability is not captured in global climate model simulations, our satellite record of runoff should help to refine them and improve confidence in their projections. Accurate assessments of ice-sheet runoff are essential for sea-level projections. A new method using satellite altimeter observations can provide near real-time surface mass balance measurements across an entire ice sheet and reveal runoff variability not captured by global climate models.