(CN) – Rain – even in the dead of winter – is gradually wearing down a 660,000-square-mile sheet of ice in Greenland and about 270 billion tons of ice melts away each year, researchers reported Thursday.
A once common characteristic of the Greenland landscape has been snow, but the new study says warming conditions have made it difficult for snow to stick and that has given way to unseasonal rainfall.
Runoff from ice is a relatively common occurrence, but as temperatures over Greenland increase less snow will be able to form, more rain will wear down the ice sheet and more sunlight will make its way to the surface of the ice. The researchers published their findings in the scientific journal The Cryosphere.
On average, temperatures over Greenland’s ice sheet have increased by as much as 3.2 degrees Fahrenheit in the summer and up to 5.4 degrees Fahrenheit in the winter. While scientists believe most of the loss of the ice sheet early on was due to icebergs calving into the ocean, meltwater runoff now makes up about 70 percent of all loss and feeds into a cycle.
Glaciologist Marco Tedesco from Columbia University's Lamont-Doherty Earth Observatory said liquid water carries a great deal of heat and when it soaks into a snowy surface, it melts the snow around it and more energy is released. Meanwhile, the warm air that brought the rain often forms clouds which hems the heat in like a blanket.
As a result, this pulse of melting feeds on itself and outlasts the rain that kicked off the event, often by several days.
Lead author Marilena Oltmanns of Germany's GEOMAR Centre for Ocean Research said over the study period melting associated with rain and its subsequent effects doubled during summer and tripled in winter. The researchers estimate nearly a third of total runoff they observed was initiated by rainfall.
Over the decades, this warming and melting stretched out into longer periods of time, according to weather observations from 1979 to 2012.
Researchers reviewed satellite imagery that can differentiate between snow and liquid water in real time, while weather stations on the ground can measure temperature, wind and rainfall. With all this data, they were able to focus on more than 300 events where they found that the catalyst for the melting events was due to weather that was followed by rain.
What’s more, melted ice water that runs off tends to refreeze. While that might sound like a good thing, it transforms snow – which reflects light and therefore heat – into dark, dense ice. The dark ice absorbs sunlight and melts again.
Now researchers are seeing earlier melting in the summer.
"If it rains in the winter, that preconditions the ice to be more vulnerable in the summer," said Tedesco, a co-author of the study. "We are starting to realize you have to look at all the seasons."
Right now, winter rainfall is limited to lower elevations in south and southwest Greenland and brought in by moist, warm ocean winds from the south. But shifts in the jet stream induced by climate change are making the warm winds more common and could push them farther inland – turning what should fall as snow into rain.
"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," said Oltmanns.
Global sea-levels have risen at an accelerated rate from about 2.2 millimeters a year to 3.3 millimeters between 1993 and 2014, with melting on Greenland largely to blame.
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