It's Raining on the Greenland Ice. In the Winter.
Changing Weather Is Triggering Melting, Says a New Study
by Kevin Krajick | March 7, 2019
Rainy weather is becoming increasingly common over parts of the Greenland ice sheet, triggering sudden melting events that are eating at the ice and priming the surface for more widespread future melting,
says a new study. Some parts of the ice sheet are even receiving rain in winter—a phenomenon that will spread as climate continues to warm, say the researchers. The study appears this week in the European scientific journal The Cryosphere.
Greenland has been losing ice in recent decades due to progressive warming. Since about 1990, average temperatures over the ice sheet have increased by as much as 1.8 degrees C (3.2F) in summer, and up to 3 degrees C (5.4F) in winter. The 660,000-square-mile sheet is now believed to be losing about 270 billion tons of ice each year. For much of this time, most of this was thought to come from icebergs calving into the ocean, but recently direct meltwater runoff has come to dominate, accounting for about 70 percent of the loss. Rainy weather, say the study authors, is increasingly becoming the trigger for that runoff.
The researchers combined satellite imagery with on-the-ground weather observations from 1979 to 2012 in order to pinpoint what was triggering melting in specific places. Satellites are used to map melting in real time because their imagery can distinguish snow from liquid water. Automated weather stations spread across the ice offer concurrent data on temperature, wind and precipitation. Combining the two sets of data, the researchers zeroed in on more than 300 events in which they found the initial trigger for melting was weather that brought rain. “That was a surprise to see,” said the study’s lead author,
Marilena Oltmanns of Germany’s GEOMAR Centre for Ocean Research. She said that over the study period, melting associated with rain and its subsequent effects doubled during summer, and tripled in winter. Total precipitation over the ice sheet did not change; what did change was the form of precipitation. All told, the researchers estimate that nearly a third of total runoff they observed was initiated by rainfall.
Melting can be driven by a complex of factors, but the introduction of liquid water is one of the most powerful, said
Marco Tedesco, a glaciologist at Columbia University’s Lamont-Doherty Earth Observatory and coauthor of the study. Warm air, of course, can melt ice directly, but is not very efficient by itself, he said. However, warmer air can produce cascading effects. For one, it makes it more likely that atmospheric conditions will pass the threshold where precipitation comes down as rain, not snow. Liquid water carries a great deal of heat, and when it soaks into a snowy surface, it melts the snow around it, releasing more energy. Meanwhile, the warm air that brought the rain often forms clouds, which hem in the heat.
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