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Study disrupts model of how lake drainage within glaciers affects sea level


Study disrupts model of how lake drainage within glaciers affects sea level

Columbia University researcher Meredith Nettles (left) and Oxford University’s Laura Stevens approach the Greenland Ice Lake by helicopter. (Marianne O’Carr, UNAVCO)

Some models of sea level rise may be incorrect based on our understanding of how glaciers respond to warming, a new study found it. This could have major implications for future projections of global sea level rise from the Greenland and Antarctic ice sheets.

Sea levels are rising as ice on land melts and ocean water expands. The magnitude and rate of water level rise in the near future will depend in part on the frequency of glacial calving events—that is, when chunks of ice from glaciers that terminate in the ocean (called tidewater glaciers) separate and break off as icebergs . The faster tidewater glaciers flow into the ocean, the more ice they produce, and the faster the sea level rises.

During warm summer months, the surfaces of Greenland’s glaciers melt and form large lakes that sometimes flow into the glacier base. Studies of inland glaciers far from the coast have shown that this reduces friction between the ice and the ground, causing the ice to slide faster over a period of days. However, until now, it was unclear whether such events would affect the speed of tidewater glaciers and therefore the speed of disintegration events.

To investigate this, a team of researchers at the Lamont-Doherty Earth Observatory at the University of Oxford and the Columbia School of Climate used Global Positioning System (GPS) observations of the Helheim Glacier in southeastern Greenland, a part of Greenland’s largest single contributor to island sea level rise. They captured a near-perfect natural experiment: high-resolution observations of the glacier’s response to a lake’s sudden drainage over several days. Usually satellites are used to track glacier speed, but their temporal resolution is about a week, so events like GPS capture usually go unnoticed.

“No one has observed the response of Greenland’s main tidewater glaciers to the sudden drainage of an icy lake,” said study co-author Meredith Nettles, a Lamont-Doherty seismologist.

The researchers found that Helheim behaved differently from inland ice, which typically exhibits rapid downslope movements during lake drainage events. In contrast, Helheim exhibited relatively small, short pulses of increased movement, followed by slower than usual movements; finally, there was no net increase in velocity.

Using numerical models of the subglacial drainage system, the researchers speculate that the Helheim bed may have an efficient system of channels and cavities. This allows the drain to be drained quickly without causing an increase in overall movement.

While this appears to be good news in terms of the impact of sea level rise, the researchers suspect that glaciers without effective drainage may have a different impact, with surface melt currently low but likely to increase in the future due to climate Change. This seems to be the case in Antarctica.

To study this prospect, the team also ran a mathematical model based on the conditions of cooler Antarctic tidewater glaciers. The results suggest that lake drainage under these conditions may lead to a net increase in glacier movement. This is mainly due to the inefficiency of subglacial drainage systems in winter.However, so far, no on site Antarctic tidewater-observation of glacier response to lake drainage.

“Tidewater glaciers are tricky,” says lead author Laura Stevens Oxford University. “We still have a lot to learn about how meltwater drainage works and regulates tidewater glacier velocity before we can confidently model their future responses to atmospheric and ocean warming.”

The study was also co-authored by James Davis, Timothy Creyts and Jonathan Kingslake of Lamont-Doherty; Ian Hewitt of Oxford University; and Aaron Stubblefield of Dartmouth College.

Adapted from Oxford University press release.




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