The Greenland ice sheet is melting year-round

As average surface air temperatures rise, it’s no surprise that ice in polar regions melts. In recent years, researchers have suspected that streams of meltwater, like the one in the photo, can bring heat to ice located elsewhere and cause further melting. But now Marilena Oltmanns (GEOMAR Helmholtz Centre for Ocean Research Kiel), Fiammetta Straneo (Scripps Institution of Oceanography), and Marco Tedesco (Lamont–Doherty Earth Observatory) have found that winter rain in Greenland also triggers ice melt. When the researchers compiled 34 years of glacial data, they found that rainy weather initiated 30% of the total melt by mass.

To distinguish snow and ice from liquid water, the authors combined high-resolution satellite imagery with data from a weather station network. For the years 1979–2012, Oltmanns and her colleagues discovered that rain from warm, moist air traveling from the southeast melts a substantial amount of the Greenland ice sheet’s surface. Even though the total precipitation stayed relatively the same over the 34-year time frame, it increasingly fell more often as rain rather than snow. The number of rainy events in winter rose from 2 in 1988 to 12 in 2012, and each event’s duration increased from two days to three. When the rain soaks into the snowy surface of the sheet, the heat it brings drives more melting. And the increased cloud cover during those weather events traps outgoing long-wave radiation that supplies yet more heat. Over the length of the time series, Oltmanns and her colleagues estimate that the ice-mass melting initiated by rain events doubled in the summer months and tripled in the winter months.

The trends in the study period were affected, in part, by an upswing in warming that the authors attribute to a temporary change in the Gulf Stream, which brings warm water to the North Atlantic Ocean. The rate of change in melting found in the study may not, therefore, be representative of future developments. Nevertheless, the best climate models available predict that temperatures will continue to rise, which is expected to amplify the melting that results from rainy weather events. (M. Oltmanns, F. Straneo, M. Tedesco, Cryosphere 13, 815, 2019 .)