At the icy peak of the Greenland Ice Sheet, three kilometers above sea level and more than 800 kilometers above the Arctic Circle, something extraordinary happened last Saturday: It rained for the first time.
The rain at a research station – not just a few drops or a drizzle, but a stream for several hours while temperatures rose slightly above freezing – is another worrying sign of a changing Arctic, warming faster than any other region the earth .
“It’s incredible because it’s writing a new chapter in the book of Greenland,” said Marco Tedesco, researcher at Columbia University’s Lamont-Doherty Earth Observatory. “This is really new.”
At the station, which is called Summit and is manned year-round under the auspices of the National Science Foundation, there has been no rain since observations began in the 1980s. And computer simulations don’t show evidence going back further, said Thomas Mote, a climate scientist at the University of Georgia.
Temperatures above freezing are almost as rare at the Summit. Prior to this century, ice cores showed they had occurred only six times in the past 2,000 years, wrote Martin Stendel, a senior researcher at the Danish Meteorological Institute, in an email.
But in 2012, 2019 and this year temperatures above freezing have occurred at the summit – three times in less than 10 years.
The Greenland ice sheet, which is up to two miles thick and covers approximately 650,000 square miles, has lost more ice over the past few decades and has contributed more to sea level rise as the earth has warmed from man-made emissions of carbon dioxide and other heat-storing gases .
The surface of the ice sheet is gaining mass every year because the accumulation of snowfall is greater than the surface melt. But overall, the sheet loses more ice from melting where it meets the ocean and from breaking off icebergs. On average over the past two decades, Greenland has lost more than 300 billion tons of ice each year.
This year will likely be an average year for surface accumulation, said Dr. Stendel, who is also the coordinator of Polar Portal, a website that disseminates the results of Danish Arctic research. Heavy snowfall at the beginning of the year suggested it might be an above average year for accumulation, but two warming spells in July and another in early August changed that by causing widespread surface melt.
The warming that accompanied the rain last Saturday also caused more than 50 percent of the ice sheet surface to melt.
Dr. Mote said that these melt episodes were each “one-off” occurrences. “But these events seem to be happening more and more frequently,” he said. “And that tells the story that we are seeing real evidence of climate change in Greenland.”
Last Saturday, it was the first time since satellite surveillance began in 1979 that more than half of the surface melted in mid-August, said Dr. Mote. There is usually a peak meltdown in mid-July, as happened in 2012 when there was a huge meltdown event.
“By the time you arrive in mid-August, you usually see a rapid drop in melt activity and a drop in temperature,” he said.
Dr. Tedesco said the rain on the summit wouldn’t contribute directly to sea level rise as the water drains into the ice rather than the ocean. “But if that happens at the summit, the effect will be more severe at lower elevations,” he said. “And this ice actually goes into the sea.”
Dr. Tedesco described the rain on the summit as “worrying” because it shows that even a small warming in the region can have an effect.
“Half a degree of warming can really change the state of the Arctic because you can go from frozen to liquid,” he said. “That’s exactly what we see.”
Last Saturday’s rain and melt occurred as the jet stream plunged south over northeastern Canada instead of flowing west to east in its normal pattern. This brought low pressure air over warmer water, where it took in heat and moisture.
The jet stream then retreated north, bringing that air to southwest Greenland, from where it swept over the ice sheet. The warm air, and even the moisture-laden clouds themselves, caused temperatures to rise at the summit and precipitation fell as rain rather than snow, said Dr. Mote.
Some scientists have linked such jet stream disturbances, often referred to as “ripple”, to climate change in the Arctic, although this is still controversial. But they do occur and also create what are known as blocking patterns that can block high pressure air over an area.
That’s what happened in the earlier melt episodes this summer. High pressure air stalled over the ice sheet resulted in a clear sky that allowed more sunlight to come to the surface and more snow melted.