Rising global temperatures may be affecting the Greenland ice sheet — and its contribution to sea-level rise — in more serious ways that scientists imagined, a new study finds. Recent changes to the island’s snow and ice cover appear to have affected its ability to store excess water, meaning more melting ice may be running off into the ocean than previously thought.
That’s worrying news for the precarious Greenland ice sheet, which scientists say has already lost more than 9 trillions tons of ice in the past century — and whose melting rate only continues to increase as temperatures keep warming. NASA estimates that the Greenland ice sheet is losing about 287 billion tons of ice every year, partly due to surface melting and partly due to the calving of large chunks of ice. Because of the ice sheet’s potential to significantly raise sea levels as it runs into the ocean, scientists have been keeping a close eye on it — and anything that might affect how fast it’s melting.
The new study, published Monday in the journal Nature Climate Change, focuses on a part of the ice sheet known as “firn” — a porous layer of built-up snow that slowly freezes into ice over time. It’s considered an important part of the ice sheet because of its ability to trap and store excess water before it’s able to run off the surface of the glacier, an essential service that helps mitigate the sea-level rise that would otherwise be caused by the runoff water.
“As this layer is porous and the pores are connected, theoretically all the pore space in this firn layer can be used to store meltwater percolating into the firn whenever melt occurs at the surface,” said the new paper’s lead author, Horst Machguth of the Geological Survey of Denmark and Greenland, in an email to The Washington Post. Over time, the percolating meltwater trickles down through the firn and refreezes.
Until recently, many scientists have assumed that most of Greenland’s firn space is still available for trapping meltwater. But the new research shows that this is likely no longer the case. Through on-the-ground observations, the scientists have shown that the recent formation of dense ice layers near the ice sheet’s surface are making it more difficult for liquid water to percolate into the firn — meaning it’s forced to run off instead.
“If you look at some of the other studies which have been arguing that you have unlimited capacity for retention of water in the firn, this study shows that that is not the case,” said Kurt Kjaer, a curator and researcher at the Natural History Museum of Denmark, who has studied glacier dynamics on the Greenland ice sheet but was not involved in the study.
The researchers conducted their study by examining ice cores drilled into West Greenland’s firn between 2009 and 2015. They wanted to find out how a series of particularly warm summers, which caused especially significant melting events in 2010 and 2012, might have affected the ice sheet.
“I think the most notable result of our study is showing that the firn reacts faster to an atmospheric warming than expected,” Machguth said in his email. By examining the cores, the researchers found that the deluge of meltwater in recent years had trickled into the firn and frozen into chunks called “ice lenses.” These lenses then began to hinder any additional liquid water from trickling down through the firn, meaning the meltwater began to accumulate and freeze near the surface, increasing the number and thickness of the existing lenses in a kind of vicious cycle.
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SOURCE: The Washington Post, Chelsea Harvey