Melted drainage makes the Greenland ice sheet the world’s largest ‘dam’

O thawing at the base of the ice cap Greenland is occurring faster than previously estimated. An international team of scientists, led by Cambridge University (United Kingdom), found that the effect of water going down from the surface of the ice sheet to the bed, situated a kilometer or more belowis by far the biggest source of heat under the second largest ice cap in the world (after Antarctica). Energy conversion occurs similarly to hydroelectric power generation in large dams.

The lubricating effect of molten water greatly influences the glacier movement and not amount of ice that pours into the ocean, but directly measuring conditions under a kilometer of ice is a challenge, especially in Greenland, where glaciers are among the fastest moving on Earth. His icy cape is also currently the biggest contributor to sea level rise in the world due to melting ice.

The effect of water running from the surface of the Greenland ice sheet to the base a kilometer or more below was considered by far the biggest source of heat under the second largest ice sheet.

Now, as published this week in the magazine PNASthe authors found that the gravitational energy of water of the melt that forms on the surface is converted into heat when it is transferred to the base through big cracks on ice.

Every summer they form thousands of lakes and streams of meltwater on the surface of the Greenland ice sheet as temperatures and daily sunlight increase. Many of these lakes quickly drain to the bottom, falling through cracks and large fractures that form in the ice. With a continuous supply of water from streams and rivers, connections between the surface and the bed often remain open.

Scientific camp at the Glacier Store, near its supraglacial lake. / Tom Chudley

in the frame of RESPOND projectfunded by the EU, Professor Poul ChristoffersenThe Cambridge Scott Polar Research Institute, studied these meltwater lakes, how and why they drain so quickly, as well as the effect they have on the overall behavior of the ice sheet as global temperatures continue to rise. The work focused on Glacierone of the biggest outlets of the Greenland ice sheet.

Heat generated by falling water

“By studying the basal melting of ice sheets and glaciers, we have observed heat sources such as friction, geothermal energy, latent heat released when water freezes, and heat losses in the ice above. ”, explains Christoffersen, “but what we hadn’t really studied was the heat generated by the runoff of the meltwater itself. there is a lot gravitational energy stored in the water that forms on the surface and when it falls, that energy has to go somewhere.”

To measure basal fusion rates, the researchers used radio echoes, a technique formerly used on Antarctica’s floating ice sheets. “We weren’t sure it would also work on a fast-flowing glacier in Greenland,” explains the first author, Tun Jan Young, who installed the radar system at Store Glacier as part of his PhD at Cambridge. “Compared to Antarctica, the ice deforms very quickly and there is a lot of thawing in the summer, which complicates the work”, he points out.

Tents with supraglacial lake at Store Glacier, Greenland. / Tom Chudley

At basal melt rates observed with radar were often as high as those measured at the surface with a weather station, although the surface receives energy from the sun and the base does not. To explain the results, the Cambridge researchers teamed up with scientists from the University of California, Santa Cruz, and the Geological Survey of Denmark and Greenland.

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The authors calculated that until 82 million cubic meters of meltwater were transferred to the Glacier Store bed every day during the summer of 2014. They estimate that the energy produced by falling water during periods of maximum melt was comparable to that produced by the Three Gorges Dam in Chinathe largest hydroelectric power plant in the world.

The Greenland ice sheet produces more hydropower than the world’s ten largest hydropower plants combined.

The authors calculated that until 82 million cubic meters of meltwater were transferred to the Glacier Store bed every day during the summer of 2014. They estimate that the energy produced by falling water during periods of maximum melt was comparable to that produced by the Three Gorges Dam in Chinathe largest hydroelectric power plant in the world.

With a melt surface that expands to nearly a million square kilometers in midsummer, the Greenland ice sheet produces more hydroelectric power than the world’s 10 largest hydropower plants combined, according to the researchers.

“Given what we are witnessing at high latitudes in terms of climate change, this form of hydroelectric power could easily double or triple, and we have not yet included these numbers when estimating the contribution of the ice sheet to sea level rise,” emphasizes Christoffersen.

Sensors detect very hot water at the bottom

To verify the high rates of basal melt recorded by the radar system, the team integrated independent temperature measurements of sensors installed in a nearby well. At the basefound that the water temperature reached +0.88 degrees centigradewhat results unexpectedly hot to the bed of a layer of ice with a melting point of -0.40 degrees.

Scientists drilling wells at Store Glacier, Greenland. / Poul Christoffersen

The results indicate that the baseline fusion rates reached an average of 14 millimeters per day and a maximum of 57 millimeters per day in August, when the basal water temperature reached 0.88 °C. The measured basal melt rate is two orders of magnitude higher than previous estimates for an ice sheet and is comparable to sun-driven fusion at the surface of a glacier.

The study presents the first concrete evidence of a mass loss mechanism from the ice sheet, which is not yet included in the projections of global sea level rise.

“Observations from the wells have confirmed that meltwater is heated when it hits the bed,” says Christoffersen, adding: “The reason is that the basal drainage system is much less efficient than the fractures and conduits that carry the water to the bottom. bed. the ice. The lower drainage efficiency causes frictional heating within the water itself. The heat generated by its fall is melting the ice from the bottom up, and the melt rate we are reporting is unprecedented.”

The study presents the first concrete evidence of an ice sheet mass loss mechanism, which is not yet included in the sea ​​level rise projections in the world. The results are significant considering that the volume of surface water produced in Greenland is huge and growing, and almost all of it drains to the bottom.

Reference:

TunJanYoung et al. “Rapid basal melting of the Greenland ice sheet from surface meltwater drainage”. PNAS2022

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