“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.” Poul Christoffersen, Scott Polar Research Institute (PNAS)
The rapid melt of the ice streams on Greenland has created a significant source of heat miles under the ice at the bedrock. The gravitational energy from the descent of surface meltwater through cracks in the Store glacier to the base is so intense that it is larger than the power generated by the world’s largest hydroelectric dams, such as China’s Three Gorges Dam.
Researchers at the University of Cambridge found excessive melting rates at the base of the Store glacier. The meltwater ultimately pours into the ocean. The phenomenal melting rate at the bottom is not included in any climate model, raising concerns about rising seas across the world’s coastlines.
The study was published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).
“When studying basal melting of ice sheets and glaciers, we look at sources of heat like friction, geothermal energy, latent heat released where water freezes and heat losses into the ice above,” said Christoffersen. “But what we hadn’t really looked at was the heat generated by the draining meltwater 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 used previously on floating ice sheets in Antarctica.
“We weren’t sure that the technique would also work on a fast-flowing glacier in Greenland,” said first author Dr Tun Jan Young, who installed the radar system on Store Glacier as part of his PhD at Cambridge. “Compared to Antarctica, the ice deforms really fast and there is a lot of meltwater in summer, which complicates the work.”
The basal melt rates observed with radar were often as high as the melt rates measured on the surface with a weather station: however, the surface receives energy from the sun while the base does not. To explain the results, the Cambridge researchers teamed up with scientists at the University of California Santa Cruz and the Geological Survey of Denmark and Greenland.
The researchers calculated that as much as 82 million cubic metres of meltwater was transferred to the bed of Store Glacier every day during the summer of 2014. They estimate the power produced by the falling water during peak melt periods was comparable to the power produced by the Three Gorges Dam in China, the world’s largest hydroelectric power station. With a melt area that expands to nearly a million square kilometres at the height of summer, the Greenland Ice Sheet produces more hydropower than the world’s ten largest hydroelectric power stations combined.
“Given what we are witnessing at the high latitudes in terms of climate change, this form of hydropower could easily double or triple, and we’re still not even including these numbers when we estimate the ice sheet’s contribution to sea-level rise,” said Christoffersen.