Rain is becoming more common in Greenland, accelerating the melting of its ice, according to a new study.
Scientists say they are "surprised" when they discover a rainfall during the long Arctic winter.
The huge Greenland ice sheet is being watched closely because it contains a huge supply of frozen water.
And if all this ice had melted, the sea level would rise seven meters and threaten the coastal population centers around the world.
Precipitation usually occurs in winter as snow ̵
What did the scientists find?
The effects of rain on Greenland ice sheet
The scientists studied satellite imagery of the ice sheet, which shows areas where melting takes place.
And they combined these images with data from 20 automated weather stations that were recorded when there was rainfall.
The results, published in The Cryosphere magazine, show that there were about two winter rains in the winter each year, beginning the period of the study period, which had risen to 12 spells by 2012.
On more than 300 occasions between 1979 and 2012, the analysis found that rainfall triggered ice melt.
Most of them were in the summer, when the air often rises above zero.
But in the winter months there is a growing number of people who expect temperatures in the dark winter to keep temperatures well below freezing in the polar winter.
What happens when it rains? 19659025] Image copyright
The lead author of the study, dr. Marilena Oltmanns from the GEOMAR Marine Research Center in Germany told BBC News: "We were surprised that there was rain in the winter.
" It makes sense because we are. It still looks surprisingly warm air flows from the south stream.
Another study scientist, Professor Marco Tedesco of Columbia University in New York, said the increase in rainfall was important implications.
Even if it falls in winter and then quickly freezes again, the rain changes the properties of the surface, making it smoother and darker and "preconditioned" to melt faster in the summer.
The darker the ice, the more heat it absorbs from the sun, causing it to melt faster.
"This opens a door to a world that is extremely explorable," said Prof. Tedesco.
"The potential impact of changes in winter and spring on what happens in summer must be understood."
A smoother surface, especially a "lens" of ice, allows the meltwater to flow and darken much faster, when more sun rays are absorbed, which leads to an acceleration. Warm up.
Images of a British research team caught in a rainstorm on the ice sheet last year show how a bright, highly reflective landscape of snow and ice became a much darker scene.
Why is that important?
Although Greenland is extremely remote, a vast island located at the northern end of the Atlantic Ocean, the sheer volume of ice cover means its fate can have global repercussions.
In stable times, winter snowfall will equalize ice melted in the ice or breaking off in the sea. However, research has shown that the ice sheet has lost large amounts of mass in recent decades.
While this only adds up to a relatively small amount of sea-level rise – the remainder comes from thermal expansion as the oceans heat up – it is feared that meltwater flow could accelerate with rising temperatures.
Two years ago, the Greenland BBC reported the risk of faster melting due to algae growth, which darkens the ice and warms more.
This effect of algae works in addition to the darkening. caused by soot and other forms of wind pollution in the Arctic.
This is in the context of growing concern that the region will heat up twice as fast as the rest of the world, possibly affecting the flow of the high-altitude jet stream.
This may disturb the weather in Europe and other regions, and may also explain how the warm, humid air from the Atlantic reaches Greenland in the winter.
What do other scientists say?
According to Professor Jason Box, a glaciologist not involved in the new study, the study relies on earlier work by him and his colleagues published in 2015, which found that summer rain could increase the rate
that due to the high heat content of water, only 14 mm of rain is required to melt 15 cm of snow, even if the snow has a minus temperature of minus 15 ° C.
"There is a simple threshold, the melting point, and when the temperature rises above it will rain instead of snow," he said.
"So in a warming climate, rocket science is not about having more rain than snow, and that's another reason why the ice can go into deficit instead of being in excess."
Prof Box has Even sudden rainstorms experienced as he camped on the ice cover.
"After weeks of sunshine, it started to rain on us and it completely changed the surface – it got darker.
" And I was convinced that I'm only there and see it with my own eyes – that the Rain is just as important as strong sunny days when melting the Greenland ice sheet.
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