Greenland’s ice sheet is vanishing from the bottom up – increasing flood risks across the planet, according to new research.
Huge quantities of water falling from the surface to the base are accelerating melt rate.
The energy is converted into heat in a process likened to hydroelectric power – turning the vast glacial land mass into the world’s biggest ‘dam’.
Global sea levels would rise by another twenty feet if it were to completely disappear.
The shock discovery is the culmination of a seven-year project focused on Store Glacier – one of the largest outlets.
Co-author Professor Poul Christoffersen, of Cambridge University, said: ‘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.
‘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 UK-led team calculated up to 82 million cubic metres of meltwater a day was transferred to the bed during the summer of 2014.
During peak periods it produced as much power as the Three Gorges Dam in China – the world’s biggest hydroelectric station.
The world’s second largest ice sheet is three times the size of Texas – extending 656,000 square miles and covering most of the island.
With a melt area of over 600,000 miles, it’s the single major contributor to global sea level rise.
The ice sheet produces more hydropower than the ten largest stations combined.
The scientists used radio-echo sounding – a technique developed by the British Antarctic Survey.
First author Dr Tun Jan Young, also from Cambridge, said: ‘We weren’t sure it would also work on a fast-flowing glacier in Greenland.
‘Compared to Antarctica, the ice deforms really fast and there’s a lot of meltwater in summer – which complicates the work.’
Melt rates at the bottom were as high as those measured by a weather station on the surface – which receives energy from the sun.
Prof Christoffersen said: ‘Given what we are witnessing at the high latitudes in terms of climate change, this form of hydropower could easily double or triple.
‘We’re still not even including these numbers when we estimate the ice sheet’s contribution to sea level rise.’
The effect of meltwater descending more than half a mile to the bed is by far the largest heat source beneath the world’s second-largest ice sheet.
Gravitational energy is converted to heat when it falls through large cracks.
The lubricating effect fuels the movement of glaciers and ice discharged into the ocean.
Each summer, thousands of meltwater lakes and streams form as temperatures rise and daily sunlight increases.
They quickly drain through large cracks and fractures in the ice. Gravitational energy is converted to heat.
With a continued supply of water, connections between surface and bed often remain open.
Independent temperature measurements from sensors installed in a nearby borehole ratified the results.
The water was nearly one degree centigrade (0.88C) – unexpectedly warm for an ice sheet base with a melting point of -0.40C.
Prof Christofferson said: ‘The borehole observations confirmed the meltwater heats up when it hits the bed.
‘The reason is 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 first concrete evidence of an ice-sheet mass-loss mechanism is not yet included in sea level projections.
High melt rates are specific to heat produced in sub-glacial drainage paths carrying surface water.
But the volume of surface water produced in Greenland is huge and growing – and nearly all of it drains to the bed.
The study is in Proceedings of the National Academy of Sciences.
