Unbeknown to him, this same weather system was causing a historic melt event 660 miles away at the summit of Greenland. On Aug. 14, 2021, the system drew exceptionally warm and moist air from southern latitudes northward, increasing temperatures around 32 degrees (18 Celsius) higher than normal. Rain, not snow, fell on Greenland’s summit for the first time on record. Melting persisted over the next two weeks, covering 46 percent of the ice sheet. This was the largest melt event to occur so late in the year.
“The weather was atrocious,” said Box, a professor at the Geological Survey of Denmark and Greenland. “I didn’t know that it was as big as it was.”
The “atrocious” weather was caused by a warm, narrow band of water vapor in the sky, known as an atmospheric river. The term “atmospheric river” has recently become popularized in media due to its role in extreme weather. As the plume of water vapor makes landfall, it precipitates as rain or snow. In the fall and winter, atmospheric rivers bring much of California’s annual precipitation but can also unleash intense flooding. In July 2021, an atmospheric river brought flooding to Germany, which killed more than 200 people.
In Greenland, these warm rivers in the sky also play a role in melting the ice sheet. In a study released Thursday, Box and his colleagues illuminate how an atmospheric river caused the August 2021 melt event and brought rain to the summit. The explanation foretells a future that could be increasingly common as global temperatures rise due to human-caused climate change.
“What we realize is that the atmospheric river is much more about the heat. And the heat did a tremendous amount of melting,” Box said. “What I would argue unnatural is there’s more heat in the system and you’re getting greater extremes.”
Atmospheric rivers tend to originate from southern latitudes far removed from Greenland, where warm air causes ocean water to evaporate into the atmosphere. Atmospheric steering winds help transport the vapor great distances. Box described the rivers as a natural process to transfer energy from the tropics to the pole. “There’s a tremendous amount of heat in these circulation systems,” he said.
As an atmospheric river landed over Greenland in August, the study found the majority of melt resulted from elevated air temperatures, which darkened the ice sheet surface and increased the absorption of sunlight. Satellite data showed melting snow crept up to higher elevations and exposed relatively dark bare ice. Where snow remained, surface melting deformed the snow crystals and made them darker, which led to additional melting under sunny skies in the following days.
“The surface is in a darkened state and stays like that for more than a week. And so that effectively doubles the melting,” Box said.
Box and his colleagues determined that the darkened snow increased melt by 28 percent in one location at an altitude of 6,036 feet. At a location 4,167 feet high, they determined that melting would have been cut in half if the bare ice were not exposed. Discharge from the Watson River, in the Kangerlussuaq region in central-western Greenland, was also the highest for this period in August in 16 years of records.
“One of the major things they point out … was how you get these sequence of these events where warm air comes in and gets rain on snow,” said Bill Neff, a research scientist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder who was not involved in the study. “The warm air and the radiation from warm clouds can melt the snow and it changes the properties of the snow so that when the sky is clear and the sun is shining, it keeps melting.”
Neff said the August melting was similar to other recent major melt events. In the past decade, Greenland experienced three major melt years, 2012, 2019 and 2021, which were all tied to atmospheric rivers. Before 2012, he said the last major melt event connected to an atmospheric river was more than 100 years ago.
The number of atmospheric rivers over Greenland “hasn’t really changed that much. They go up and down a lot over the last hundred years, but what they’re doing is tapping into hotter regions of the Earth,” he said. “You can have the same number of atmospheric rivers bringing warmer air from all over the place. If there are more places getting hotter, then there’s more chance to be more melting.”
Neff’s research looks at how heat waves have played a part in Greenland melt events. He previously found that Greenland’s 2012 melt season, which is still the largest melt season ever observed, was partly spawned from warm air originating from a record heat wave in North America. A heat wave developed suddenly in the Midwest, which created a pulse of hot air that was transported to Greenland via an atmospheric river.
In July 2019, warm air from a heat wave in Europe headed to Greenland and helped trigger one of the biggest surface melt days on record.
Neff said the August 2021 melt event was associated with warm air masses moving across North America that probably combined with moisture from the south Atlantic, before moving to the southwest coast of Greenland.
Climate change is intensifying these events by increasing the amount of water vapor an atmospheric river can hold, said Kyle Mattingly, an atmospheric scientist at the University of Wisconsin-Madison. A warmer atmosphere increases the rate of evaporation and allows for more water to enter the vapor phase. His research found the amount of moisture transported over the summer melt season in Greenland has been increasing in recent decades.
“If you sum up the amount of moisture that atmospheric rivers transport during each melt season, that is showing an increasing trend,” said Mattingly, who was not involved in the study released Thursday. “I feel pretty certain that there is a link there between increasing atmospheric river moisture transport and increases in Greenland melt.”
As this year’s melt season begins in Greenland, researchers are unsure what to expect as predicting events more than a week or so in advance is challenging. Melt activity has been close to normal so far, aside from slight melting in late May on the ice sheet’s periphery.
“There’s nothing so far to indicate that we’re going to have a particularly really large melt season,” Mattingly said.
But then again, Mattingly said the record 2012 melt season also started out slow before accelerating in the middle of the summer.
“You never quite know until it happens,” Mattingly said.