Its huge icecap is thawing faster because Greenland’s winter rain means its snows are being washed away, or falling at higher altitudes.
LONDON, 8 March, 2019 − The largest body of ice in the northern hemisphere faces a problem scientists had not identified before: Greenland’s winter rain is accelerating the loss of its vast store of ice.
Two new studies have identified mechanisms for ever-faster melting of the ice. One is that the snowline keeps shifting, to alter the levels of radiation absorbed by the ice sheet that masks the Greenland bedrock.
The other is that ever more snow and ice is simply washed away by the rainfall – even in the Arctic winter. That is because global warming has raised Greenland’s summer temperatures as much as 1.8°C, and by up to 3°C in the winter months.
Reports of winter rain over an icecap large enough – if it were all washed into the ocean – to raise global sea levels by more than seven metres are a surprise: glaciologists expect some melting of the polar ice caps each summer, to be replaced each winter by snowfall that insulates the ice below and then endures for much of the following summer.
Meltwater matters more
Such icecaps are thought to shed most of their mass as glaciers deliver ice downstream to the coast, and icebergs calve and float south.
But research in the journal The Cryosphere tells a different and unexpected story: direct meltwater now running off Greenland into the sea accounts for seven-tenths of the 270 billion tonnes of ice that Greenland loses each year. And increasingly, rainy weather is the trigger that sets off the rivulets of meltwater streaming to the coast.
German and US researchers took data from 20 Greenland weather stations between 1979 and 2012, and matched this with satellite imagery that could distinguish snow from liquid water. In the data they identified more than 300 episodes of melting in which the initial trigger was the arrival of rain.
And during the 33 years of data, they found that melting associated with rainfall doubled during the summer months, and tripled in winter. Nearly a third of all the flow of water from Greenland was initiated by rainfall.
“Suddenly the snowline was just gone. In a couple of days it had moved 30 kilometres or so up the ice sheet”
Warm air can melt ice but, more potently, warming air can turn what might have been snow into rain. Liquid water carries considerable heat, to soak into the snow and melt it. And the clouds that bring the rain have a way of conserving the warmth in the air.
Some of the meltwater will refreeze as surface ice, darkened by dust and colonised by algae, to absorb solar radiation more efficiently than snow, and to melt more easily and much earlier in the summer.
“If it rains in the winter, that preconditions the ice to be more vulnerable in the summer,” said Marco Tedesco of Columbia University’s Lamont-Doherty Earth Observatory, one of the authors. “We are starting to realise you have to look at all the seasons.”
Most of the winter rainfall is in the island’s south and southwest, spilled by warm ocean winds from the south, and these may have become more common because warming has been linked to changes in the stratospheric jet stream.
Loss not gain
Marilena Oltmanns, of Germany’s Geomar Centre for Ocean Research, called the discovery “a surprise to see. The ice should be gaining mass in winter when it snows, but an increasing part of the mass gain from precipitation is lost by melt.”
But research in the journal Science Advances in the same week pinpoints another related factor in setting the rate of melting in Greenland: the snowline.
This varies significantly from year to year. Once again, snow tends to reflect radiation, and with darker ice to absorb it the new study suggests that even Greenland’s icy mountains conform to simple physics.
Researchers flew drones inland across the bare ice to identify the snowline. A pause during a few days of high winds brought a big surprise.
No specific studies
“Suddenly the snowline was just gone. In a couple of days it had moved 30 kilometres or so up the ice sheet and was now out of the range of our drones.
“That was the first moment we thought we should investigate the effects of snowline movement on melt,” said Jonathan Ryan, of Brown University in Rhode Island, who led the study.
And Laurence Smith, a researcher based at Brown University, and one of the authors, said: “People who study alpine glaciers have recognised the importance of snowlines for years, but no one has explicitly studied them in Greenland before.
“This study shows for the first time that simple partitioning between bare ice and snow matters more when it comes to melting than a whole host of other processes that receive more attention.” − Climate News Network