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Saturday, November 14, 2009

Greenland's mass loss increases to 0.75 mm per year of sea level rise, a 50% increase over period before 2006

Greenland ice loss speeds ahead

by environmentalresearchweb, November 13, 2009

Between 2000 and 2008, the Greenland Ice Sheet shrank by roughly 1500 gigatons, research by a team from the Netherlands, UK and US has revealed. The mass loss was split equally between surface processes, such as runoff and precipitation, and ice dynamics. Since 2006, mass loss has accelerated and is equivalent to 0.75 mm per year of sea level rise.

The researchers say they were able to work out the contribution of individual components to the mass loss with confidence. They did this by using mass budget calculations, which sum the net change from the surface and from ice discharge, and validating the results with data from the GRACE satellite, which uses gravimetry to assess the mass of the ice sheet.
"We can state, for instance, that both glacier acceleration and enhanced surface melt contributed equally to post-1996 Greenland mass loss, but that surface-mass losses would have been much greater if it weren't for increased snowfall and refreezing," Michiel van den Broeke of Utrecht University in the Netherlands told environmentalresearchweb. "The latter comes at a price, however, because the refreezing heats up the snow pack and therefore facilitates future melt. All this we did not know with any certainty before."
The team, from Utrecht University; the University of Bristol, UK; the University of California, Irvine, US; Jet Propulsion Laboratory, US; Delft University of Technology, the Netherlands; and Royal Netherlands Meteorological Institute, estimate that without the increased snowfall and refreezing, Greenland mass loss since 1996 would have been double the amount of mass loss actually observed.

The research also revealed that surface-mass loss plays a greater role than previously believed. "Until now, glacier acceleration caught most of the attention, it being very visible," said van den Broeke. "Surface melting happens every summer in Greenland, so it is much harder to detect changes in it. But our results show that it has taken over as the most important component of Greenland mass loss in the last five years."

The team found that since 2006, high summer-melt rates have increased the mass loss of the Greenland ice sheet to 273 Gigatons per year, equivalent to 0.75 mm per year of sea-level rise. If the entire ice sheet were to melt, global sea levels would rise by around 7 metres.

Ice station
Ice station

"We noted that previous models underestimated the seasonal cycle of ice-sheet waxing and waning, while ours proved much better," said van den Broeke. "That stimulated us to start a comparison with the GRACE data, which turned out to be very good, and there you go."

According to Jonathan Bamber of the University of Bristol, UK, it's clear from these results that mass loss from Greenland has been accelerating since the late 1990s, and the underlying causes suggest this trend is likely to continue in the near future. "We have produced agreement between two totally independent estimates, giving us a lot of confidence in the numbers and our inferences about the processes," he said.

The scientists' next step is to couple a climate scenario run with an atmospheric model to an ice-dynamic model to see how the ice sheet is likely to change shape and lose mass over the next few centuries.

The researchers reported their work in Science.


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