Satellite-derived estimates of slush and ponded water extent across Antarctica's ice shelves, 2013-2021

Surface meltwater on Antarctic ice shelves is comprised of slush (saturated firn), and ponded water (lakes and streams). Often, slush forms as a precursor to ponded water, and its formation leads subsequently to water collecting in basins or flowing across ice shelf surfaces. Where slush and/or ponded water refreeze at the end of a melt season, the firn air content of ice shelves may be lowered. This can increase ice shelves’ susceptibility to future meltwater ponding, making them more vulnerable to potential hydrofracture and break-up. Slush and ponded water also have a lower albedo than snow or dry firn, further increasing ice-shelf surface melt under warmer climates. To date, most satellite-derived estimates of surface water on ice shelves have identified only ponded water, potentially underestimating the extent of surface meltwater. Here, we use a previously developed random forest classifier to produce a novel, continent-wide dataset of slush and ponded water extent across all Antarctic ice shelves between 2013 and 2021. Our dataset is comprised of monthly meltwater products for the austral summers (November-March where data availability allows), from which continent-wide, regional, and individual ice-shelf trends are investigated.

The continent-wide total meltwater coverage (assessed between November and February) was greatest during January 2017, reaching 6078 km². Notably, we find that including the slush extent in total meltwater calculations increases surface water extent by a mean of 56% during the melt-season peak (January). However, we identify marked inter-regional variation, with slush accounting for 71% of January’s total surface meltwater extent in Dronning Maud Land, but only 46% in the Antarctic Peninsula. This indicates that until now, the extent of surface meltwater across Antarctica’s ice shelves has been largely under-estimated on ice shelf, regional, and continent-wide scales, which has significant repercussions for calculations of the surface and sub-surface energy and mass balance of ice shelves, the long-term storage of meltwater on ice shelves, and predictions of future ice shelf stability.