It is no secret that the Greenland ice sheet has the potential to be a major contributor to global sea-level rise: if it melted entirely, the seas would rise by seven metres. This is because, unlike sea ice, which is already in the water, melted land ice eventually ends up in the ocean, adding to what water is already there. As scientists get a better grip on the mechanics behind what happens during the ice sheet’s annual melting season, they are finding that its melting may, in fact, beget even more melting.
One of the ways that water reaches the ocean is through what are known as moulins (one of which is pictured above). These holes in the ice sheet that extend for more than a kilometre down to the bedrock at the very base of the ice sheet are so effective at leading water off the surface that it takes just an hour for one to drain a million cubic metres of water, or the equivalent of 400 Olympic-sized swimming pools. Once a moulin forms, it stays open for the entire melting season, thanks to the continued flow of water from the streams and rivers that criss-cross the surface of the ice sheet (see videos at end of article).
While that is concern enough, given the amount of water that they could potentially be leading into the ocean, recent research identifies two ways that falling is contributing to the destabilisation of the ice sheet. In the process known as basal melting sketched out in the graphic below, Greenland’s ice sheet is worn away from the bottom, in part by the friction caused by its own weight, and in part by the warmth of the Earth below. The water it creates acts a lubricant, speeding up the rate at which the glaciers at the edge of the ice sheet that serve as their outlets to the ocean move forward; the more water there is, the easier will be for the glacier to slide over the bedrock below it.
Basal melting is a natural process, but the additional water that ends up on the surface of the ice sheet that flows down to the bedrock drives the glacier forward faster. Fieldwork carried out on Store Glacier (in the area marked “central west” on the map at the end of the article) suggests that as much as 82 million cubic metres of meltwater was drained away from its surface to the bedrock every day during the summer of 2014.
What’s more, all this falling water itself wears away ice, firstly as it falls down the moulins, but then, when it hits the bedrock below, it is converted into heat, suggests a paper published in the Proceedings of the National Academy of Sciences on 22 February. During peak melt periods on Store Glacier, the falling water is estimated to have released as much energy as was put out during the same period by the Three Gorges Dam in China, the world’s largest hydroelectric power station. If this calculation is accurate, the ice sheet, whose melt area covers more than a million square kilometres at its greatest annual extent, may be releasing more energy the world’s ten largest hydroelectric power stations combined.
Currently the energy released by water as it drains through moulins is not taken into account when scientists try to calculate how fast the ice sheet is melting, and, thus, how much water is being emptied into the oceans. But, as significant as this could be, the amount of energy could double or even triple as temperatures rise and even more water becomes available to generate heat, according to Poul Christoffersen, who led the expedition to the Store Glacier.
Dr Christoffersen and his team began to investigate whether the falling water may itself be releasing energy after measurements using radar concluded that melt rates at the base of glaciers were often as high as melt rates measured on the surface, despite not being exposed to the sun. In order to confirm that the falling water was indeed generating heat, the team recorded the temperature of the water as it flowed out from underneath the Store Glacier and found that it was nearly a full degree Celsius, where they would have otherwise expected it to be more than a full degree colder.
“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,” he says. “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.”
That it is being stuck where the sun doesn’t shine may mean that we will be up the creek sooner than we expected.
Greenland’s glaciers and the speed at which they are moving into the ocean
Kevin McGwin, PolarJournal
Featured image: A science camp at the edge of a glacial lake on Greenland’s ice sheet (Photo: Tom Chudley / Cambridge University)
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