The accelerating rate of melting makes the Greenland Ice Sheet the largest dam in the world

Water flows in Moulins and sinks to the bottom of the Store Glacier in Greenland. Credit: Paul Kristofferson

The researchers observed extremely high rates of melting at the bottom of the Greenland ice sheet, due to massive amounts of meltwater falling from the surface to the base. As meltwater falls, the energy is converted into heat in a process such as hydroelectric power from large dams.

An international team of scientists led by the University of Cambridge found that the effect of water soluble The descent from the surface of the ice sheet to the lower layer – a kilometer or more – is by far the largest source of heat under the second largest ice sheet in the world, resulting in A high degree solubility at its base.

The lubricating effect of melt water has a strong influence on glacier movement and the amount of ice discharged into the ocean, but measuring conditions directly under 1 km of ice is challenging, particularly in Greenland where the glaciers are among the fastest moving in the world.

This lack of direct measurements makes it difficult to understand the dynamic behavior of the Greenland ice sheet and to predict future changes. Because ice losses have been linked to melting and degassing, the Greenland ice sheet is now the single largest contributor to global sea level rise.

Now, in a study published in Proceedings of the National Academy of SciencesThe Cambridge-led team found that gravitational energy from meltwater formation at the surface turns into heat when it is transferred to the base through large cracks in the ice.

Every summer, thousands of lakes and melt streams form on the surface of the Greenland Ice Sheet as temperatures rise and sunlight increases daily. Many of these lakes drain quickly to the bottom of the ice sheet, falling through large cracks and fractures that form in the ice. With the continued supply of water from streams and rivers, the connections between the deck and bed often remain open.

The accelerating rate of melting makes the Greenland Ice Sheet the largest dam in the world

Tents with lake over Glacier in Store Glacier, Greenland. Credit: Tom Chodley

As part of EU funding transponder Professor Paul Kristofferson of the Scott Polar Research Institute in Cambridge has studied meltwater lakes, how and why they are draining so quickly, and their impact on the overall behavior of the ice sheet as global temperatures continue to rise.

The current work, which includes researchers from Aberystwyth University, is the culmination of a seven-year study focused on Store Glacier, one of Greenland’s largest ice cap vents.

“When studying the primary melting of ice sheets and glaciers, we look at sources of heat such as friction, geothermal energy, and latent heat released as water freezes and heat loss in the ice above,” Kristofferson said. “But what we haven’t really looked at is the heat from the meltwater discharge itself. There’s a lot of gravitational energy stored in the water that forms on the surface and when it falls, the energy has to go somewhere.”

To measure basal melt rates, the researchers used phase-sensitive radio-echo sounding, a technique developed at the British Antarctic Survey and previously used on floating ice sheets in Antarctica.

“We were not sure that this technology would also work on a fast-flowing Greenland glacier,” said first author Dr Tun Gan Young, who installed the radar system at Store Glacier as part of his PhD thesis. in Cambridge. “Compared to Antarctica, the ice deforms very quickly and there is a lot of meltwater in the summer, which complicates the work.”

The basal melt rates observed with radar were often as high as the melt rates measured at the surface with a weather station: however, the surface receives energy from the sun while the base does not. To interpret the results, the Cambridge researchers collaborated with scientists at the University of California Santa Cruz and the Geological Survey of Denmark and Greenland.

The accelerating rate of melting makes the Greenland Ice Sheet the largest dam in the world

Science camp at Store Glacier near supraglacial lake in Store Glacier, Greenland. Credit: Tom Chodley

The researchers estimated that up to 82 million cubic meters of meltwater was carried to the bottom of the Store Glacier per day during the summer of 2014. They estimated the energy generated by falling water during periods of maximum melt power produced by China’s Three Gorges Dam, the largest hydropower plant in the world. the scientist. With a melting area spanning nearly one million square kilometers at the height of summer, the Greenland ice sheet produces more hydropower than the ten largest in the world. hydroelectronic energy combined stations.

“Given what we’re seeing at higher latitudes in terms of climate change, this form of hydropower could easily double or triple, and we still don’t include those numbers even when estimating the ice sheet’s contribution to sea level rise,” he said. Christophersen.

To verify the high basal melt rates recorded by the radar system, the team combined independent temperature measurements from sensors installed in a nearby well. At the base, they found that the water has a temperature of +0.88 degrees Celsius, which is unexpectedly warm for the base of an ice sheet with a melting point of -0.40 degrees.

“The well observations confirmed that meltwater rises when it hits the bed,” Kristofferson said. “The reason is that the primary drainage system is much less efficient than the fractures and channels that move water through the ice. Low drainage efficiency leads to frictional heating within the water itself. When we excluded this heat source from our calculations, theoretical estimates of the melt rate were two whole orders of magnitude The heat generated by the falling water is melting the ice from the bottom up, and the rate of melting we are reporting is absolutely unprecedented.”

The study provides the first concrete evidence of the mechanism of ice mass loss, which has not yet been included in projections of global sea level rise. While high rates of melt are specific to heat generated in subglacial drainage pathways that carry surface water, the volume of surface water produced in Greenland is huge and growing, almost all of which drains to the bottom.

Study: Greenland ice sheet loses enough water in 20 years to cover the United States

more information:
Rapid basal melting of the Greenland ice sheet from surface meltwater discharge, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2116036119.

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