When the International Panel on Climate Change published its latest projections for the future of the planet a couple of years ago, it pointed out an important caveat to its conclusions.

One question that nobody could answer was the role that various feedback mechanisms would play in the melting of the Greenland Ice Sheets. Since then, it has looked to many as if the rate of melting is increasing rapidly. That’s important because the ice sheets contain enough water to increase global sea levels by more than 7 metres.

Now Ralf Greve and Shin Sugiyama at the Institute of Low Temperature Science, Hokkaido University in Japan, have carried out an impressive set of simulations to see how the melting will proceed. The simulations start with the current ice sheet and run until 2350.

The main feedback mechanism they consider is that meltwater streams lubricate the movement of glaciers and so speed up their rush towards the sea. So how serious is this mechanism?

Greve and Sugiyama conclude that the effect significantly accelerates the decay of the Greenland Ice Sheet as a whole in the 21st century and beyond. But that they do not expect a catastrophic decay of the ice sheet.

They predict that the Greenland ice sheets will lead to an 18 cm rise in global sea level by 2100. (That may not be catastrophic for them but it will be for many low lying regions of the world).

One interesting point is the basis on which they dismiss the possibility of a catastrophic decay of the ice sheet. The simulation depends on certain technical parameters that determine things like the surface melt rate. Greve and Sugiyama say that for certain choices, the ice edge moves at speeds of up to 100 kilometres per year.

They dismiss this result on the basis that such speeds are unrealistic. But it’s not clear, at least not to me, why such speeds should be disreguarded. Why couldn’t the the catastrophic break up of a glacier lead to the ice edge moving at such speeds?

Leaving that aside, another question arises which is whether this simulation captures all the feedback mechanisms that can occur in ice sheets as they melt and fracture.

That’s something that other studies need to urgently address.

Ref: arxiv.org/abs/0905.2027: Decay of the Greenland Ice Sheet Due to Surface-Meltwater-Induced Acceleration of Basal Sliding