CoAuthors

Andrew Shepherd (Centre for Polar Observation and Modelling, School of Earth and Environment, University of Leeds, Leeds, UK, United Kingdom); Malcolm McMillan (Lancaster Environment Centre, Lancaster University, Lancaster, UK, United Kingdom); Amber Leeson (Lancaster Environment Centre, Lancaster University, Lancaster, UK, United Kingdom); Lin Gilbert (Mullard Space Science Laboratory, Department of Space & Climate Physics, University College London, London, UK, United Kingdom); Alan Muir (Mullard Space Science Laboratory, Department of Space & Climate Physics, University College London, London, UK, United Kingdom); Peter Kuipers Munneke (Institute for Marine and Atmospheric research Utrecht, Utrecht University, Utrecht, the Netherlands, Netherlands); Brice Noël (Institute for Marine and Atmospheric research Utrecht, Utrecht University, Utrecht, the Netherlands, Netherlands); Xavier Fettweis (SPHERES Research Unit, Department of Geography, University of Liège, Liège, Belgium, Belgium); Michiel van den Broeke (Institute for Marine and Atmospheric research Utrecht, Utrecht University, Utrecht, the Netherlands, Netherlands); Kate Briggs (Centre for Polar Observation and Modelling, School of Earth and Environment, University of Leeds, Leeds, UK, United Kingdom)

Runoff from the Greenland Ice Sheet has increased over recent decades affecting global sea level, regional ocean circulation, and coastal marine ecosystems. Runoff now accounts for most of Greenland’s contemporary mass imbalance, driving a decline in its net surface mass balance as the regional climate has warmed. Although automatic weather stations provide point measurements of surface mass balance components, and satellite observations have been used to monitor trends in the extent of surface melting, regional climate models have been the principal source of ice sheet wide estimates of runoff. To date however, the potential of satellite altimetry to directly monitor ice sheet surface mass balance has yet to be exploited. Here, we explore the feasibility of measuring ice sheet surface mass balance from space by using 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. In total, the ice sheet lost 3571 ± 182 Gt of ice through runoff over the 10-year survey period, with record-breaking losses of 527 ± 56 Gt/yr first in 2012 and then 496 ± 53 Gt/yr in 2019. 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.