Published: 12 September 2025
Greenland ice sheet runoff reduced by meltwater refreezing in bare ice
Matthew G. Cooper, Laurence C. Smith, Åsa K. Rennermalm, Jonathan C. Ryan, Lincoln H. Pitcher, Glen E. Liston, Clément Miège, Sarah W. Cooley & Dirk van As
Nature Communications volume 16, Article number: 8273 (2025) Cite this article
Abstract
The contribution of Greenland Ice Sheet meltwater runoff to global sea-level rise is accelerating due to increased melting of its bare-ice ablation zone. There is growing evidence, however, that climate models overestimate runoff from this critical area of the ice sheet. Climate models traditionally assume that all bare-ice runoff enters the ocean, unlike porous firn, in which some meltwater is retained and/or refrozen. We used field measurements and numerical modeling to reveal that extensive retention and refreezing also occurs in bare glacier ice. We found that, from 2009 to 2018, meltwater refreezing in bare, porous glacier ice reduced runoff by an estimated 11–17 Gt a−1 in southwest Greenland alone, equivalent to 9–15% of this sector’s annual meltwater runoff simulated by climate models. This mass retention explains evidence from prior studies of runoff overestimation on bare ice by current generation climate models and may represent an overlooked buffer on projected runoff increases. Inclusion of bare-ice retention and refreezing processes in climate models therefore has immediate potential to improve forecasts of ice sheet runoff and its contribution to sea-level rise.
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Introduction
Greenland Ice Sheet (GrIS) mass loss raised global sea level approximately 10.8
0.9 mm from 1992 to 2018, and climate models forecast an additional increase of 70–130 mm by year 21001. Most of this predicted mass loss derives from increased meltwater runoff from the ice sheet’s ablation zone2,3,4,5,6. Within this critically important zone, winter snowpack melts entirely each summer, exposing dark, bare glacier ice which absorbs up to three times more sunlight than bright snow7. Warming air temperatures and reduced summer snowfall have exposed larger areas of bare ice in recent decades2,3,8, driving enhanced surface melting in the ablation zone2,3,4. Understanding the fate of meltwater from Greenland’s growing bare-ice zone is therefore critical for accurate modeling of sea levels9.
Climate models track energy and mass budgets on the ice sheet surface and are the primary tools available to predict future ice sheet runoff10,11. However, there is substantial observational evidence that these models overestimate runoff from bare ice surfaces12,13,14,15,16,17. In the ablation zone of Greenland’s melt-intensive southwest sector, proglacial and supraglacial river discharge measurements reveal up to 67% less annual meltwater release to surrounding oceans than climate model calculations12,14,15,16. Supraglacial lakes which form on the ice sheet surface fill at slower rates than predicted by climate models17, and direct measurements of supraglacial runoff are overestimated by 21–58% during peak summer melt conditions13. Ice sheet mass changes are overestimated by 21–47% relative to GRACE satellite gravity retrievals18,19, and satellite laser altimetry measurements indicate that surface melt rates are overestimated by 14–40%20. Similar discrepancies arise from comparisons with point ablation stake measurements, ranging from an underestimation of 17% at one ablation-zone site to overestimations of 10–43% at four other sites21,22. These discrepancies do not appear to be explained by errors in modeled surface energy balance components, which largely match in situ meteorological observations13,21,22.
https://www.nature.com/articles/s41467-025-62281-0
