Unbeknownst to him, this same weather system was causing a historic melt event 660 miles atop Greenland. On August 14, 2021, the system drew unusually warm and humid air northward from southern latitudes, raising temperatures to about 32 degrees (18 degrees Celsius) above normal. Rain, not snow, fell on the Greenland summit for the first time on record. Melting persisted over the next two weeks, covering 46% of the ice sheet. It was the largest melting event to occur so late in the year.
“The weather was terrible,” said Box, a professor at the Geological Survey of Denmark and Greenland. “I didn’t know it was that big.”
The “excruciating” weather was caused by a narrow, hot band of water vapor in the sky, known as an atmospheric river. The term “atmospheric river” has recently become popular in the media due to its role in extreme weather conditions. When the water vapor plume hits the ground, it precipitates as rain or snow. In fall and winter, they bring much of California’s annual rainfall, but can also trigger intense flooding. In July 2021, an atmospheric river caused flooding in Germany, which killed more than 200 people.
In Greenland, these warm rivers in the sky also play a role in the melting of the ice sheet. In a study published Thursday, Box and his colleagues explain how an atmospheric river caused the August 2021 melt 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 heat. And the heat melted tremendously,” Box said. “What I would say against nature is that there is more heat in the system and you get bigger extremes.”
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Atmospheric rivers tend to originate from southern latitudes far away from Greenland, where warm air causes ocean water to evaporate into the atmosphere. Atmospheric guiding winds help carry vapor over great distances. Box described the rivers as a natural process of transferring energy from the tropics to the pole, stating “there is an enormous amount of heat in these circulation systems”.
When an atmospheric river landed in Greenland in August, the study found most of the melting resulted from high air temperatures, which darkened the surface of the ice sheet and increased sunlight absorption. . Satellite data showed that melting snow was rising at higher elevations and exposing relatively dark bare ice. Where snow remained, surface melt distorted the snow crystals and darkened them, leading to further melting under sunny skies in the following days.
“The surface is in a dark state and stays that way for over a week. And so it effectively doubles the melt,” Box said.
Box and his colleagues determined that blackened snow increased melting by 28% at one location at an elevation of 6,036 feet (1,840 meters). At one location 1,270 meters above sea level, they determined that the melt would have been halved had the bare ice not been exposed. The flow of the Watson River, in the Kangerlussuaq region of west-central Greenland, was also the highest for this period in August in 16 years of records.
“One of the main things they point out … was how do you get this sequence of these events where warm air comes in and rains down on the snow,” said Cooperative Research Institute researcher Bill Neff. in environmental sciences from the University. of Colorado Boulder and not involved in the study. “Warm air and radiation from warm clouds can melt snow and change the properties of snow so that when the sky is clear and the sun is shining, it continues to melt.”
Neff said the August melt was similar to other recent major melt events. Over the past decade, Greenland has experienced three major melt years, 2012, 2019 and 2021, all of which were related to atmospheric rivers. Prior to 2012, he said the last major melting event linked to an atmospheric river was over 100 years ago.
The number of atmospheric rivers over Greenland “hasn’t really changed much. They’ve gone up and down a lot over the last hundred years, but what they’re doing is tapping into the hottest parts of the Earth,” he said. “You can have the same number of atmospheric rivers bringing in warmer air from everywhere. If there are more places getting hotter, then there’s a greater chance of more melting. “
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Neff’s research examines how heat waves played a role in the Greenland melting events. He previously found that Greenland’s 2012 melt season, which is still the biggest melt season on record, was driven in part by warm air from a record-breaking North American heat wave. A heat wave suddenly developed in the Midwest, which created a pulse of warm air that was carried to Greenland via an atmospheric river.
In July 2019, warm air from a heat wave in Europe headed towards Greenland and helped trigger one of the largest surface melt days on record.
Neff said the August 2021 melt event was associated with warm air masses moving across North America that likely combined with moisture from the South Atlantic, before moving on. to the southwest coast of Greenland.
Climate change intensifies 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 more water to enter the vapor phase. His research revealed that the amount of moisture transported during the summer melt season in Greenland has increased over the past decades.
“If you add up the amount of moisture atmospheric rivers carry over each melt season, it shows an upward trend,” said Mattingly, who was not involved in the study released today. “I’m pretty sure there’s a connection between increased atmospheric river moisture transport and increased Greenland melting.”
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As this year’s melt season begins in Greenland, researchers are unsure what to expect as it is difficult to predict events more than a week in advance. Melting activity has been close to normal so far, with the exception of some mild melting in late May at the periphery of the ice sheet.
“There’s nothing so far to indicate that we’re going to have a particularly big melt season,” Mattingly said.
But then again, Mattingly said the record melting season of 2012 also got off to a slow start before picking up speed in midsummer.
“You never really know until it happens,” Mattingly said.