“Basically, the entire day of Saturday, it was raining every hour that [staff] was making weather observations,” said Zoe Courville, a research engineer at the Cold Regions Research and Engineering Laboratory. “And that’s the first time that’s been observed happening at the station.”
The rain coincided with warmer temperatures that caused extensive melting across the ice sheet. Some areas were more than 18 degrees Celsius warmer than the average temperature. At the summit, temperatures peaked at 33 degrees Fahrenheit — within a degree above freezing.
The melt extent peaked at 337,000 square miles on Saturday, according to the National Snow and Ice Data Center (NSIDC). This was slightly smaller than the melting event that occurred this summer on July 28, which covered 340,000 square miles of the ice sheet, but it is still significant. Only 2012 and 2021 had multiple melt events covering more than 309,000 square miles in a year.
“The anomalous thing for this year was this huge rain event that encompassed a lot of southern Greenland.” said Von Walden, a professor at Washington State University and member of the university’s Laboratory for Atmospheric Research. “Imagine the difference of having rain fall on an ice sheet rather than snow. The rain can melt snow basically.”
Rain fell on and off for 13 hours at the station, but staff are not certain exactly how much rain fell from ground measurements. After all, there are no rain gauges at the summit because no one expected it to rain at this altitude.
“[Rain is] not something you would expect to measure [there],” said Jennifer Mercer, Program Officer for the National Science Foundation Office of Polar Program’s Arctic Sciences and Arctic Research Support and Logistics section.
The summit reaches 10,551 feet in elevation, where temperatures are almost always below freezing. In fact, temperatures at the site only rose above freezing three times before this in the past 32 years, according to observations at Summit Station from 1989. Before that, ice core data shows that the last time melting occurred at the ice cap top dates to the 1880s.
Rain at the summit can affect the ice sheet differently than if it occurred on the periphery, Walden said. Rain around the periphery during an anomalously warm event can generate a lot of melt water that runs off the ice sheet. The melt that occurred at the summit, however, will percolate into the snowpack to colder temperatures and probably refreeze. It will not drain all the way to the coast.
Climate “modelers will have a good idea of what happened elsewhere and along the periphery of the southern part of the ice sheet,” Walden said. “That’s really important to know what happened there in terms of how much it’s going to contribute to runoff and the mass loss or gain.”
Despite the anomalous rain at Summit Station, the event was initiated through a rather common air circulation pattern responsible for many past melt events — a blocking event in the north-central Atlantic Ocean, characterized by strong high pressure.
According to the NSIDC, a moderately strong lower-pressure center, flanking the high-pressure system, moved across the Hudson Bay toward Baffin Island. This created a strong pressure gradient and forced a strong wind event onto the southwestern Greenland coast. The resulting punch of warm air traveled along the southwest side of the ice sheet and up Summit Station located in the center of the island, and then spread around the coast.
“These blocking events are not that uncommon, but their severity appears to have increased in recent years, causing these large melt events,” said Richard Cullather, a climate modeler at the Global Modeling and Assimilation Office at NASA’s Goddard Space Flight Center.
2021 marks the third year with a major melting event in the past decade. The previous large melting events, in 2012 and 2019, were driven by air temperatures and cloud cover and occurred in June and July.
“To see this many melt events at this intensity in such a short period is absolutely remarkable in the historic records that are available to us,” said Christopher Shuman, a glaciologist with University of Maryland, Baltimore County, based at NASA Goddard. Shuman and his colleagues at NASA helped alert field staff at Summit Station of the impending rain as predicted by their models days ahead of time.
The Saturday melt event occurred nine days later than the previous melt event exceeding about 309,000 square miles — a surprise considering that mid- to late August marks a change into autumn in the far north. The sun moves lower in the sky and temperatures generally cool, decreasing the chances for large melt events.
“This event by itself does not have a huge impact, but it’s indicative of the increasing extent, duration, and intensity of melting on Greenland," wrote Ted Scambos, a senior research scientist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder. "Like the heat wave in the [U.S. Pacific] northwest, it’s something that’s hard to imagine without the influence of global climate change.”
This August, the United Nations released its latest report on our changing climate, showing an increase in extreme heat events. Additional global warming will continue to exacerbate melting at our polar caps. If the planet warms more than 2 degrees Celsius, the change “could trigger the inexorable collapse of the Greenland ice sheet,” The Washington Post reported. A sea-level rise of more than six feet could swamp coastal communities.
“Greenland, like the rest of the world, is changing,” Scambos said. “We now see three melting events in a decade in Greenland — and before 1990, that happened about once every 150 years. And now rainfall: in an area where rain never fell.”