The Greenland ice sheet is losing mass at an accelerating rate, researchers reported this week in a study published in Science.

Two factors are contributing equally to the loss: an increase in the number of icebergs released into the ocean as the flow of Greenland’s outlet glaciers speeds up, and an increase in the amount of meltwater flowing off the ice sheet’s surface.

Using satellite observations and a state-of-the art regional atmospheric model, the researchers determined that recent warm summers brought Greenland’s ice mass loss to 273 gigatonnes per year in the period from 2006 to 2008 (1 gigatonne is the mass of 1 cubic kilometre of water). That rate of loss equates to a global sea level rise of 0.75 millimetres per year.

“It is 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,” said Jonathan Bamber, one of the study’s authors and a researcher at the University of Bristol. “We have produced agreement between two totally independent estimates, giving us a lot of confidence in the numbers and our inferences about the processes.”

The entire Greenland ice sheet contains enough water to cause a global sea level rise of seven metres. Since 2000, the ice sheet has lost a total of about 1,500 gigatonnes, which equates to a global sea level rise of 5 millimetres since then.

While surface melting of the ice sheet started to increase around 1996, snowfall also increased at approximately the same rate, masking surface mass losses for nearly a decade. Moreover, a significant part of the additional meltwater refroze in the cold snowpack that covers the ice sheet. Without these moderating effects, post-1996 Greenland mass loss would have been double the amount of mass loss observed now.

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