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Greenland's extraordinary rainfall and melt event could portend faster ice melt

A recent satellite image of the southwestern portion of the Greenland Ice Sheet shows the existence of cerulean blue pockmarks — phenomena that provide scientists with a worrying message about sea-level rise and the risk of massively consequential changes in ocean behavior.

Why it matters: The melt ponds, rivers and moulins, cracks in the ice where surface water can plummet to where the ice sheet meets bedrock, are a symptom of a summer season that has brought large spikes in melt extent.

  • This is significant since ice melt from Greenland is the largest contributor to global sea level rise.
  • In addition, there's increasing evidence that the cold freshwater coming from the ice sheet may be helping to disrupt one of the most important ocean circulation systems on Earth, which includes the Gulf Stream.

Driving the news: The melt ponds and other surface water formations are partly the result of a heavy rainfall event and widespread melting that occurred during mid-August.

Details: On Aug. 14, rain was observed for the first time on record at the National Science Foundation's Summit Station, typically the coldest spot on the world's largest island.

  • The station is located at the top of the Greenland Ice Sheet, some 10,551 feet in elevation.
  • The rain lasted for several hours and the temperature stayed above freezing for nine hours, according to the National Snow and Ice Data Center (NSIDC). How much rain fell is not known, in part because the weather station there lacks a rain gauge, since rain is not something meteorologists expected when it was built.
  • There have been three Summit Station melt events, or periods with temperatures above freezing, in just the past 10 years: 2012, 2019 and this year. This frequency is itself a huge departure from the past.
  • Ted Scambos, a climate scientist at the University of Colorado, told Axios that prior to 1990, melt events likely occurred at the summit on the order of about once every 150 years.

Context: In terms of melt extent, the August 14-16 event was exceptional, but ranked behind another spike this past July, as well as a record event in 2012. It was, however, unusual for its occurrence so late in the season, when colder weather typically starts to set in.

  • The NSIDC found that melt extent peaked at 337,000 square miles on Aug. 14, with melting observed across more than 50% of the ice sheet's surface. This marked this season's second melt event that exceeded 309,000 square miles, the first occurred in July.
  • According to University of Liege scientist Xavier Fettweis, the heavy rains that fell during the warm spell led to a rate of surface mass loss that was the highest ever recorded in mid-August since 1950, and seven times larger than average.
  • During the warm episode, a total of 7 billion tons of rain fell on the ice, which is about 20% of the annual rainfall total for the typically frigid island. This makes it the largest single rainfall event on the ice sheet since reliable records began more than 70 years ago, Fettweiss told Axios via email.

What's next: The rain at higher elevations of Greenland may set the ice sheet up for a "huge" sea level rise contribution next summer, Fettweiss noted, because of the formation of "ice lenses" or "slabs" more than six feet thick at the surface. These would prevent melt runoff from percolating down into the snowpack and expand Greenland's runoff area.

  • This means 2022's meltwater may go directly into the ocean, adding to sea level rise, rather than percolating down into the snow and ice and refreezing.
Another view of melt ponds and surface water on the ice sheet, plus surface darkening, in southwestern Greenland on Aug. 22, 2021. (NASA Worldview)
  • "When you put rain on ice, in terms of melting it's the same thing as putting gasoline on a fire. It really makes it much stronger and bigger," Columbia University climate researcher Marco Tedesco told Axios in an interview.
  • "This is changing Greenland because it's really switching from a frozen state to a more liquid state," he said, noting this can lead to "an acceleration of sea level rise."

Between the lines: The cause of the warm episode that triggered the rainfall was the combination of a high pressure area southeast of Greenland, which pumped mild southwesterly winds over the icebound island, and a low pressure area over northeastern Canada, which reinforced that air flow.

  • Atmospheric circulation changes are a future ice melt wild card that is not well incorporated into projections of ice sheet melt, Tedesco said.

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