“It turns out that the rain itself wasn’t the most important factor”, says Prof. Jason Box from GEUS and lead author of
the paper reporting their results, which were just published in Geophysical Research Letters.
“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, this sudden increase of surface ice melt on Greenland could have happened without any rain ever touching the ground.
The main culprit was the heat itself, melting and completely removing the surface snow, thereby changing the surface albedo, Greek for ‘whiteness’, so that Greenland snow and ice absorbed more of the Sun’s rays.
The researchers found that, between 19 and 20 August 2021, this melt caused the altitude of the ice sheet’s snowline to retreat by a whopping 788 metres, exposing a wide area of dark bare ice.
Illustration: One of the areas of the ice sheet hit by the heat wave, causing the snowline to retreat immensely in a short period of time. This is along the PROMICE/GC-Net Kangerlussuaq transect of automatic weather stations (white dots) from where the researchers documented the very warm, wet ice and snow conditions during their field work. Usually the snow at KAN_U station is not this difficult to walk in during august. (Illustrations: GEUS, edited from Box et al. 2022 by C. Thuesen, photos Dirk van As).
Under normal circumstances, snow would cover and insulate this ice, but the snow melted suddenly and exposed the ice to heat, causing even more melting.
Since 2017, Prof. Box and colleagues have been consistently monitoring these melt dynamics with data from the Copernicus Sentinel-3 mission as part of their research supported by ESA’s Earth Observation Science for Society programme.
They found that more bare ice had been exposed on the one day of 19–20 August than any other day since their research began.
Graph: Area of bare ice measured from 2017 to 2021, where the heatwave in 2021 greatly increases bare ice extent very late in the melt season. (Illustration: Edited from Box et al. 2022 by C. Thuesen, GEUS)