The Climate Future of Greenland's Ice Sheet Looks Worse Than Previously Thought

The Greenland Ice Sheet occupies some 1,710,000 square kilometres of territory, almost the entire surface of Greenland itself. This makes it the second-largest ice sheet in the world, behind Antarctica. At points, the sheet's surface is a full two kilometers above the actual land surface below, and that surface has remained fairly mysterious until recently.

It's no big secret that the Greenland Ice Sheet is melting, perhaps terminally. A full-melt scenario would leave Greenland indeed pretty green, at least the parts not covered by water. The massive breadth and depth of the Greenland sheet means that such an event would boost global sea levels by around 7.2 meters (24 feet). And given existing models, Greenland is melting at its faster rate in recorded history.

Researchers from the University of Cambridge have developed a new model for the sheet's future, published in this week's edition of Nature Communications. Unlike previous estimates, this one takes into account the soft, spongy nature of the ground below. This softness changes the overall dynamics of the sheet's melt, which is estimated to currently occur at a rate of 200 gigatonnes per year (equaling about 0.6 millimeters of sea level rise per year).

The new model is particularly concerned with the melting process related to ice flow, one of two general mechanisms by which ice is lost. "When these large ice sheets melt, whether that's due to seasonal change or a warming climate, they don't melt like an ice cube," said Marion Bougamont, lead author of the new report, in a statement.

"Instead, there are two sources of net ice loss: melting on the surface and increased flow of the ice itself, and there is a connection between these two mechanisms which we don't fully understand and isn't taken into account by standard ice sheet models."

Previous models have assumed out of convenience that the ice sheet is moving along a surface of solid rock rather than the water-saturated, muddy surface one would find at the bottom of a lake. Recent surveys have confirmed that the latter is more likely to be the case. The new Cambridge model takes into account meltwater from the bottom of the sheet permeating into the sediment below. And, as the sediment gets weaker and oversaturated, there should be less resistance to the flow of ice above.

This means faster melting, perhaps much faster. As the climate warms, the result is more surface melt and more accumulation of water on the ice surface. Short-lived lakes form as a result, only to empty rapidly as the ice fractures underneath, dumping the surface water into this squishy subglacial environment. So: surface melt impacts the melting that occurs as a result of ice flow.

The Cambridge paper doesn't suggest anything super-extreme, like full-melt within this century. But generally, the surface melting situation is tending toward the extreme, particularly in light of 2012's startling Greenland melt event. More and more it looks possible that full-melt could occur not within centuries, plural, but at time-scales approaching "sudden."

In other words, all of these feedback mechanisms seem to point toward more surface melt and better subglacial conditions for ice flow, all of which are happening at an accelerating rate and all of which seem to boost some other melting factor.