One of the world’s largest ice sheets, the second-largest in terms of area, is melting at an unprecedented rate due in large part to the heat released by meltwater descending to the bed, a kilometer or more below the surface, according to research led by the University of Cambridge.

In Greenland, where glaciers are among the world’s fastest moving, measuring the conditions beneath a kilometer’s worth of ice is a challenge, especially because the glaciers are among the fastest moving on the planet.

Greenland’s Ice Sheet’s dynamic behavior is difficult to predict due to a lack of direct measurements. The Greenland Ice Sheet is now the world’s most significant contributor to rising sea levels, due to both melting and discharge of ice.

It has now been discovered that the gravitational energy of meltwater forming on top of the ice is converted to heat when it is transferred to the base through large cracks in the ice.

Thousands of lakes and streams on the Greenland Ice Sheet’s surface form each summer as temperatures rise and sunlight levels rise. Many of these lakes quickly drain to the ice sheet’s bottom, falling through the ice’s cracks and large fractures. Streams and rivers provide a steady supply of water, which allows surface and bed connections to remain open.

Researchers from Cambridge’s Scott Polar Research Institute have been studying these meltwater lakes as part of the EU-funded RESPONDER project, and they’ve found that they drain so rapidly because of the effect that global temperatures are having on the ice sheet.

After a seven-year investigation of Store Glacier, one of Greenland’s largest glacial outlets, researchers from Aberystwyth University are involved in the current study.

The latent heat released when water freezes and the heat losses into the ice above are all factors to consider when studying basal melting of ice sheets and glaciers, according to Christoffersen. “However, we hadn’t given much thought to the heat generated by the melting water itself,” he explains. Water on the surface contains a lot of gravitational energy, and when it falls it must release that energy somewhere.

Phase-sensitive radio-echo sounding, developed at the British Antarctic Survey and previously used on floating ice sheets in Antarctica, was used to measure basal melt rates.

When Dr. Tun Jan Young installed the radar system on Store Glacier in Greenland as part of his PhD at Cambridge, he was unsure if the technique would work on a fast-flowing glacier. A lot of melting water complicates work in summer, “compared to Antarctica, where the ice deforms quickly.”

Radar measurements of basal melt rates were often as high as the surface melt rates measured by a weather station, but the surface receives energy from the sun while the base does not. Researchers from the University of Cambridge and the Geological Survey of Denmark and Greenland collaborated to explain the findings.

During the summer of 2014, researchers estimated that Store Glacier lost an average of 82 million cubic meters of meltwater to the glacier’s bed each day. Falls of water produced power comparable to China’s Three Gorges Dam, the largest hydroelectric power station in the world, during peak melt periods. The Greenland Ice Sheet generates more hydropower than the ten largest hydroelectric power plants in the world combined during the height of summer, when the melt area reaches nearly a million square kilometers.

If climate change continues at the high latitudes, “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,” Christoffersen said.

The radar system recorded high basal melt rates, so the team incorporated temperature measurements from sensors installed in a nearby borehole to verify the findings. For an ice sheet’s melting point of -0.40 degrees Celsius, the water temperature at the base was found to be as high as +0.88 degrees Celsius.

Christoffersen said, “The borehole observations confirmed that the meltwater heats up when it hits the bed,” There’s a simple reason for this: Basal drainage is less efficient than fractures and conduits in the ice. The reduced drainage efficiency results in frictional heating of the water itself, resulting in higher temperatures. When we omitted this heat source from our calculations, our theoretical melt rate estimates were two orders of magnitude off. ‘ The ice is being melted from the bottom up as a result of the falling water’s heat, and the rate of melting we’re seeing is unprecedented.”

An ice sheet mass loss mechanism that hasn’t been included in global sea level rise projections has been documented in a study. Temperatures generated by subglacial drainage paths carrying surface water are causing Greenland’s glaciers to melt at a much higher rate than previously thought.