PNAS - "Phenomenally High Rates Of Melting" At Base Of Greenland Ice Sheet From Falling Meltwater
Researchers have observed extremely high rates of melting at the bottom of the Greenland Ice Sheet, caused by huge quantities of meltwater falling from the surface to the base. As the meltwater falls, energy is converted into heat in a process like the hydroelectric power generated by large dams. An international team of scientists, led by the University of Cambridge, found that the effect of meltwater descending from the surface of the ice sheet to the bed—a kilometer or more below—is by far the largest heat source beneath the world's second-largest ice sheet, leading to phenomenally high rates of melting at its base.
The lubricating effect of meltwater has a strong effect on the movement of glaciers and the quantity of ice discharged into the ocean, but directly measuring conditions beneath a kilometer of ice is a challenge, especially in Greenland where glaciers are among the world's fastest moving. This lack of direct measurements makes it difficult to understand the dynamic behavior of the Greenland Ice Sheet and predict future changes. With ice losses tied to both melting and discharge, the Greenland Ice Sheet is now the largest single contributor to global sea level rise.
Now, in a study published in the Proceedings of the National Academy of Sciences, the Cambridge-led team has found that the gravitational energy of meltwater forming at the surface is converted to heat when it is transferred to the base through large cracks in the ice.
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To verify the high basal melt rates recorded by the radar system, the team integrated independent temperature measurements from sensors installed in a nearby borehole. At the base, they found the temperature of water to be as high as +0.88 degrees Celsius, which is unexpectedly warm for an ice sheet base with a melting point of -0.40 degrees. "The borehole observations confirmed that the meltwater heats up when it hits the bed," said Christoffersen. "The reason is that the basal drainage system is a lot less efficient than the fractures and conduits that bring the water through the ice. The reduced drainage efficiency causes frictional heating within the water itself. When we took this heat source out of our calculations, the theoretical melt rate estimates were a full two orders of magnitude out. The heat generated by the falling water is melting the ice from the bottom up, and the melt rate we are reporting is completely unprecedented."
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https://phys.org/news/2022-02-greenland-ice-sheet-world-largest.html
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