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Rain Is Melting Greenland’s Ice, Even in Winter
The Greenland ice sheet loses about 270 billion tons of ice each year to climate change, raising global sea levels by 7.5 millimeters (approximately 0.3 inches) between 1992 and 2011, Science Magazine explained. About half of that was due to the calving of icebergs, but recent satellite observations have revealed that 70 percent of Greenland's contribution to global sea level rise in recent years has come from meltwater running off into the ocean. Scientists wanted to understand what was driving the meltwater.
The results of one investigation, published in The Cryosphere Wednesday, show that a third of the runoff observed by the research team between 1979 and 2012 was caused by rainfall, Columbia University's Earth Institute reported. Over that same period, rain-caused melt doubled during summer and tripled during winter. Total precipitation over Greenland did not change, but the balance of snow to rain shifted.
"We were surprised that there was rain in the winter," lead study author Dr. Marilena Oltmanns of the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany told BBC News. "It does make sense because we're seeing flows of warm air coming up from the South, but it's still surprising to see that associated with rainfall."
In order to reach their conclusions, the researchers used satellite data to determine when melting was taking place as well as automated readings from 20 weather stations to determine when rainfall occurred. They pinpointed 313 incidents over the study period when rainfall triggered melting, according to Science.
Rain can lead to more melting even if it falls in winter and refreezes right away, study author Marco Tedesco explained to BBC News. That's because it leaves the ice both darker and smoother. Darker ice absorbs more heat from the sun, leading to more melt, and smooth ice enables that melt to flow faster over its surface.
"The potential impact of changes taking place in the winter and spring on what happens in summer needs to be understood," Tedesco said.
Professor Jason Box, a Geological Survey of Denmark and Greenland glaciologist not involved with this study, told BBC News he had observed first-hand during a research trip how rain could transform the ice sheet:
"After weeks of sunshine, it started raining on us and it completely transformed the surface—it got darker.
"And I became convinced—only by being there and seeing it with my own eyes—that rain is just as important as strong sunny days in melting the Greenland ice sheet."
The winter rain usually falls in lower elevations in Greenland's south and southwest, where it is carried on warm, wet winds from the south that may be getting more common as climate change shifts the jet stream, The Earth Institute explained.
"This is what climate change looks like, it's the 'Atlantification' of the Arctic," climate scientist Ruth Mottram of the Danish Meteorological Institute in Copenhagen, who did not participate in the study, told Science Magazine. "This paper identifies a really important mechanism and we need to figure out how it plays into our predictions of sea level rise."
If all the ice in Greenland melted, it would raise global sea levels by 7 meters (approximately 23 feet). Most projections say that sea levels will rise two to four feet by 2100, The Earth Institute said, but more research is needed to determine how much Greenland and Antarctica will contribute.
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