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The Greenland ice sheet lost a record amount of ice in 2019, equivalent to one million tons per minute for the entire year, satellite data shows.
The climate-related loss is likely to be the worst in centuries and is raising sea levels.
The climate crisis is heating up the Arctic at twice the rate at lower latitudes, and the ice sheet is the biggest contributor to rising sea levels, which already threatens coasts around the world. The ice sheet shrank by 532 billion tons last year when its surface melted and glaciers fell into the ocean and would have filled seven Olympic-size pools per second.
Satellite data has been collected since 2003. The 2019 loss was double the annual average since then of 255 billion tons. Almost that amount was lost in July 2019 alone.
Scientists knew that Greenland's ice loss had accelerated rapidly in recent decades and that there had been high melt rates in 2019. But satellite data show new snowfall and allow the net loss to be calculated. The researchers said the scale of the 2019 loss was shocking and likely the largest in centuries or even millennia.
If the entire Greenland ice sheet melts, the sea level would rise six meters. But the researchers said it was not certain that the foil had passed the point of no return and that reducing carbon emissions would slow down the melting, which would take centuries to complete.
Scientists attributed the extreme ice loss in 2019 to climate “lock patterns” that kept the air warm over Greenland for longer periods. These are becoming more and more prevalent as the world warms up. Almost 96% of the ice sheet melted at some point in 2019, compared to an average of 64% between 1981 and 2010.
"2019 was really shocking and depressing in terms of numbers," said Ingo Sasgen of the Alfred Wegener Institute in Bremerhaven, Germany, who led the analysis. "But it's not very surprising either, because we had other years of strong melting in 2010 and 2012, and I hope we see more and more."
Snowfall in Greenland was low in 2019, also due to the blocking pattern, which means that relatively little new ice was added. "The real message is that the ice sheet is very unbalanced," Sasgen said.
He also said that an additional concern were feedback mechanisms that increase ice loss, including meltwater that weakens the ice sheet and accelerates its fall into the ocean. Warmer weather also melts the white snow on top of the layer, revealing darker ice underneath, which absorbs more heat from the sun.
"These results come at a crucial moment," said Yara Mohajerani of the University of California at Irvine in the United States, who was not part of the study team. "2019 broke the previous 2012 record by 15%, a record unmatched in recent centuries or millennia."
He said Arctic warming is likely to increase further in the coming years. "Therefore, it is crucial to closely monitor changes in the ice mass of the sheet, and Sasgen and his colleagues have taken an important step in that direction."
The research, published in the journal Communications Earth & Environment, used data from NASA's Grace satellites, which take gravity measurements and actually weigh the ice mass in Greenland.
The first Grace satellite finished its data collection in June 2017 and its replacement began in May 2018. Data from the second satellite was used to determine how much had been lost in the intervening period.
The researchers found that 2017 and 2018 had unusually low ice loss, due to a reversal of the lock pattern that resulted in cold, snowy conditions in Greenland. But even under these conditions, the cap still lost ice, which means that cold years do not compensate for warm years as in the past.
"It really shows that we've entered a completely different state," with a trend of increasing ice loss and more variability each year, Sasgen said. "Greenland has gone bipolar in a way."
Professor Stefan Rahmstorf, from the University of Potsdam in Germany, said the new analysis was compelling and showed that the transition from the old to the new satellite had gone smoothly.
"Since meltwater is fresh water, it dilutes the salt content of the surrounding ocean, which contributes to slowing down the Gulf Stream system," Rahmstorf said. "If we wanted the 500 billion tonnes of freshwater added in 2019 to be as salty as ocean water, some 200,000 Panamax-class cargo ships filled with salt would have to dump their cargo into the Atlantic."
Weather data and computer models allow us to calculate losses since 1948. “If we look at the record melting years, the top five occurred in the last 10 years, and that's a concern. But we know what to do about it: reduce CO2 emissions ”.
Despite rapid melting, the Greenland ice sheet is not necessarily doomed to melt completely. First, as glaciers retreat, they lose contact with warmer ocean waters and thus less melt. Second, the melting of the sheet with hot air takes centuries, during which time the rise in global temperatures could be reversed.
"If we reduce CO2, we will reduce Arctic warming and therefore also reduce the contribution to sea level rise from the Greenland ice sheet," said Sasgen. "So while it could eventually disappear to a large extent, it happens much slower, which would be better as it would allow more time for the 600 million people who live near the coast to move."