The Greenland ice sheet has lost mass over recent decades, contributing an estimated 10.8 ± 0.9 mm to global sea levels since 1992. Negative Surface Mass Balance has been the principal driver of this imbalance, because surface melting has progressively increased as the regional climate has warmed and modes of atmospheric circulation have promoted periods of intense surface melting. Such behaviour is likely to continue as global temperatures rise.

Making large scale observations of ice sheet runoff is, however, extremely challenging. Field-based automatic weather stations have provided point measurements of SMB components, but these are sparse in space and time, due to the logistical challenges of deploying and maintaining such systems in harsh polar environments. Satellite observations have been able to monitor trends in the spatial extent of liquid water, however these do not provide information on the key parameter of interest – the volume of meltwater generated.

Here, we therefore test the use of CryoSat-2 radar altimetry to separate long-term and seasonal elevation changes in the ablation zone of the Greenland Ice Sheet between 2011 and 2020, and use the latter to directly measure runoff at the scale of the Greenland Ice Sheet. This allows, for the first time, an observational record of recent interannual runoff variability to be derived, and compared to equivalent simulations from two regional climate models.

This new observational record shows that runoff from the Greenland Ice Sheet averaged 357 ± 58 Gt/yr between 2011 and 2020. During this period, year-to-year variability in runoff was high, with a maximum spread of 280 Gt/yr. As a consequence of an exceptionally warm summer, runoff peaked at 527 ± 56 Gt in 2012, reducing to 285 ± 64 Gt in the following year when the North Atlantic Oscillation shifted abruptly to a positive phase and atmospheric conditions were cooler. Runoff then increased to 496 ± 53 Gt/yr in 2019, a year when the persistence of anticyclonic conditions increased air temperatures and drove record surface melting in the north. By comparison to the long-term mean derived from model simulations (1980 to 2010), runoff during the period 2011 to 2020 has been 21 % higher, on average, and the year-to-year variability has also increased markedly. These results represent an important first demonstration of the capability to observe integrated runoff at the ice sheet scale.