When rain fell for the first time at the highest point of Greenland’s ice sheet , it marked a significant climate change milestone. But researchers have now revealed in unprecedented detail how an accompanying plume of hot air had a far bigger impact by melting snow, with the rain only playing a minor role.
The snow melt created a feedback loop by exposing darker ice, which absorbs more heat and causes more snow to melt in turn. Events like this will also play a small role in causing more global warming, a “climate feedback” like and the darker water that is exposed absorbs more of the sun’s energy rather than reflecting it.
The rain, which fell more than 3000 metres up at the Arctic’s only year-round research station, was triggered by an “atmospheric river” of unusually warm air. Together, the heat from the air and rain pushed the area’s snow line – the boundary of the area covered by snow at the peak – more than 700 metres uphill and swelled a river to record volumes.
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The episode extended the ice sheet’s melt season just when it would have otherwise been drawing to a close. Greenland’s ice sheet loss has been accelerating in recent years and has the potential to raise sea level by 6 metres.
“The rain really got our attention. [But] it was ironic that it was the heat associated with the atmospheric river, the heat surrounding the rain, if you will, that was the bigger impact,” says Jason Box at the Geological Survey of Denmark and Greenland, who led the research combing through weather instrument and satellite data.
He had expected most of the heat to come from the rain, so he was surprised to find it only accounted for about 2 per cent of the heat, most of which was delivered by condensation and other mechanisms, including the darkening effect of the meltwater. A huge area of ice, effectively the whole southern ice sheet of Greenland, had its reflectivity reduced by about 10 per cent. “In that regard, it was quite impactful,” says Box.
Future rainfall events at the sheet’s peak could have an even more severe impact. If there were enough rain to percolate deep through the snow before refreezing, that would deliver a lot of heat, says Box. Fortunately, in this case, there was relatively little rain and that didn’t come to pass.
Box says the study shows that investigating the changing frequency of atmospheric rivers – which were also responsible for extreme heat in Antarctica three months ago – is “arguably a more important research question than how much rain is falling”.
Geophysical Research Letters
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