Surface melting of Greenland ice sheets is speeding up their flow into the sea
By Gerard Wynn
Seeping of summer melt water to the bottom of Greenland’s massive ice sheets and glaciers is speeding up their flow into the sea, a new study has found.
Each summer, the melt water seeps through crevasses and moulins to the bottom of the ice sheets.
At the bottom, the water can lubricate the base of the ice, while it also releases heat when it re-freezes, softening the surrounding ice and allowing it to flow faster.
The latest study was the first to find that such re-freezing was a major feature throughout northern Greenland, including at the bottom of Petermann Glacier, one of the largest to flow directly into the sea.
The authors found that the resulting “basal units” of re-frozen and deformed ice could be several hundred metres thick and tens of kilometres long.
They coincided with the place where the Petermann Glacier started to speed up, in its slide towards the sea 200 kilometres downstream.
“Warmer, softer ice within the basal unit together with lubrication at the ice sheet base contributes to the onset of fast flow,” they said, in the study published in the journal Nature Geoscience.
“In the onset region, the ice moves 10-100% faster than the adjacent ice.”
The flow rate of glaciers is influenced by other factors, such as the steepness of the underlying landscape, and is something that scientists are watching closely given concerns about the impact of climate change on sea level rise.
Complete loss of the Greenland ice sheet would occur over 1,000 years or more and contribute up to 7 metres of global mean sea-level rise.
In 2012, all but the entire surface of the Greenland ice sheet melted, which a recent study concluded had not happened since 1889.
The amount of melting depends on how warm the summer is, and is also influenced by soot from distant forest fires which can darken the ice surface and cause it to absorb more heat.
The melt two years ago coincided with an exceptionally warm summer.
Glaciers, ice sheets and sea ice are losing mass globally, partly as a result of manmade climate change, the Intergovernmental Panel on Climate Change (IPCC) said last year, in a review of the science to date.
“Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent,” the IPCC said in its summary for policymakers.
“The average rate of ice loss from the Greenland ice sheet has very likely substantially increased from 34 billion tonnes per year over the period 1992 to 2001 to 215 billion tonnes per year over the period 2002 to 2011.”
The Greenland ice sheet would disappear altogether after sustained, long-term warming above a certain threshold, which the IPCC estimated was 1-4C above pre-industrial levels. Temperatures have so far risen 0.8C.