‘Unprecedented’ melt rate at Greenland Ice Sheet found by University of Cambridge scientists could change sea level rise projections
Phenomenally high rates of melting have been discovered at the base of the Greenland Ice Sheet, which is being heated by enormous quantities of meltwater falling from the surface.
An international team, led by University of Cambridge scientists, say the effect is like the hydroelectric power generated by large dams.
But such is the vast size of the world’s second largest ice sheet that it produces more hydropower than the world’s 10 largest hydroelectric power stations combined.
The Greenland Ice Sheet is the largest single contributor to global sea-level rise, but measuring conditions beneath its kilometre of ice has been a challenge - not least because the glaciers there are among the fastest-moving.
But the Cambridge-led team has shown the gravitational energy of meltwater forming at the surface is converted to heat as it is transferred to the base via large cracks in the ice.
“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 Prof Poul Christoffersen, from Cambridge’s Scott Polar Research Institute.
“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.”
The study represents the first proof of ice-sheet mass-loss mechanism - something not yet included in projections of global sea-level rise.
Thousands of meltwater lakes and streams form on the surface of the Greenland Ice Sheet each summer as temperatures rise and daily sunlight increases.
Many of the lakes quickly drain to the bottom of the ice sheet through cracks and large fractures.
The team, including researchers from Aberystwyth University, focused their seven-year study on Store Glacier, which is one of the largest outlets from the Greenland Ice Sheet.
They used a technique developed at the British Antarctic Survey that had previously been used on floating ice sheets in Antarctica, known as phase-sensitive radio-echo sounding.
“We weren’t sure that the technique would also work on a fast-flowing glacier in Greenland,” said the study’s 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.”
They were surprised to find the melt rate at the base was often as high as that measured on the surface, which is heated by the sun.
Teaming up with scientists at the University of California Santa Cruz and the Geological Survey of Denmark and Greenland, they calculated that as much as 82 million cubic metres of meltwater were being transferred to the bed of Store Glacier every day in the summer of 2014.
At its peak, the power here was comparable to that produced by the world’s largest hydroelectric power station - the Three Gorges Dam in China.
But the ice sheet’s melt area expands to nearly a million square kilometres at the height of summer. The volume of surface water is growing - and nearly all of it drains to the bed.
“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 Prof Christoffersen.
The team verified the high basal melt rates they recorded with the radar system by integrating independent temperature measurements from sensors installed in a nearby borehole.
They found the temperature of water at the base to be as high as +0.88 degrees Celsius - which was unexpectedly warm considering the base’s melting point is -0.40 degrees.
“The borehole observations confirmed that the meltwater heats up when it hits the bed,” said Prof 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.”
The study, part of the EU-funded RESPONDER project, was published in the Proceedings of the National Academy of Sciences.