Greenland Will Lose Mass Faster due to Ice “Lid”
January 6, 2016
Dark Snow Project Chief Scientist Jason Box contributed to a new study of Greenland mass loss that is getting attention.
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 up. 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 Kjær, 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.
Scientists previously thought that melting process might be slowed down by a porous layer on the surface of the ice called firn, which is partway between ice and snow.
Climate models expected the firn layer to absorb up to 30 to 40 per cent of any meltwater that travels across the ice sheet, allowing it to refreeze instead of pouring into the ocean.
But a new international study has found that during 2012, an unusual amount of melting caused the top of the firn to freeze into solid ice. That meant it could no longer absorb the meltwater, which instead formed thousands of new rivers snaking across the surface of the ice sheet to the ocean.
“That hadn’t been seen before,” said William Colgan, a researcher at York University in Toronto who co-authored the new study.
The Intergovernmental Panel on Climate Change’s predictions of sea level rise are based on models that assume the firn would fill up gradually over the course of a century, reducing the amount of meltwater that Greenland pours into the oceans.
The new study, led by Horst Machguth at the Geological Survey of Denmark and Greenland and published this week in Nature Climate Change, suggest those predictions are underestimates.
“In a year like 2012… an extra 60 gigatonnes of water or thereabouts went into the ocean,” Colgan said.
Rising sea levels are already causing flooding in some coastal areas, and could become a more serious problem more quickly than anticipated.
While firn is theoretically renewable — it reforms from fresh snowfall — the top 40 metres of firn take 80 years to develop, and warmer years like 2012 are becoming increasingly common, Colgan said.
“Now the firn is being capped off much faster than it’s being recreated.”
And the phenomenon may not be restricted to Greenland — many other ice caps like those in Canada are covered in firn too.
“Evidence is emerging to show Canadian Arctic firn is also capping off,” Colgan said. He added that relative to their size, glaciers in Canada contribute more to sea level rise than the Greenland ice sheet.
Approximately half of Greenland’s current annual mass loss is attributed to runoff from surface melt1. At higher elevations, however, melt does not necessarily equal runoff, because meltwater can refreeze in the porous near-surface snow and firn2. Two recent studies suggest that all3 or most3, 4 of Greenland’s firn pore space is available for meltwater storage, making the firn an important buffer against contribution to sea level rise for decades to come3. Here, we employ in situ observations and historical legacy data to demonstrate that surface runoff begins to dominate over meltwater storage well before firn pore space has been completely filled. Our observations frame the recent exceptional melt summers in 2010 and 2012 (refs 5,6), revealing significant changes in firn structure at different elevations caused by successive intensive melt events. In the upper regions (more than ~1,900 m above sea level), firn has undergone substantial densification, while at lower elevations, where melt is most abundant, porous firn has lost most of its capability to retain meltwater. Here, the formation of near-surface ice layers renders deep pore space difficult to access, forcing meltwater to enter an efficient7 surface discharge system and intensifying ice sheet mass loss earlier than previously suggested3.