On August 14, 2021, for the first time ever recorded, rain fell on the high central region of the Greenland ice sheet – an extraordinary event that was followed by rapid melting of surface snow and ice. A research team led by the Geological Survey of Denmark and Greenland (GEUS) has now investigated what caused this extreme rainfall and how did it affect the ice sheet.
By analyzing data collected by automatic weather stations placed across the ice sheet, satellite measurements of surface reflectivity (“albedo”), and information on atmospheric circulation patterns, the experts discovered that the rain had been preceded by a major heatwave at a time of year when melting is usually slowing down.
“It turns out that the rain itself wasn’t the most important factor,” said study lead author Jason Box, a professor of Glaciology at GEUS. “There is an irony. It’s not really the rain that did the damage to the snow and ice, it’s the darkening effect of the meltwater and how the heat from the event erased snow that had overlaid darker ice across the lower third of the ice sheet. Unusually warm atmospheric rivers swept along Greenland in the late summer months, bringing potent melt conditions when the melt season was drawing to a close.”
In fact, according to the scientists, this sudden increase of ice melt could have happened without any rainfall. The main culprit appeared to be the heat itself, which melted and removed the surface snow, changing the ice sheet’s albedo so that the ice absorbed more of the Sun’s rays. Between 19 and 20 August 2021, this melt caused the ice sheet’s snowline to drop by 788 meters, exposing a wide area of dark bare ice.
The researchers concluded that the heatwave which caused the rainfall is a perfect example of “melt-albedo feedback that amplifies the melt impact of the initial melt perturbation.” Thus, better understanding the nature and frequency of heatwaves appears to be more important than only analyzing the rainfall that heatwaves may or may not produce.
The study is published in the journal Geophysical Research Letters.