Combining measurements from space with ground-based observations and scientific models, scientists have found that the Greenland icesheet is melting at an alarming rate. Between 2006 and 2008, two hundred and severty three gigatons - or 273 cubic kilometres of water - were lost, sufficient to increase global sea-levels by 0.75mm per year. This was also an increase on a previous average annual loss of 166 gigatons per year since 2000. The scale of Greenland icesheet loss was hinted at previously when researchers published the first results from a satellite system called GRACE (Gravity Recovery and Climate Experiment), which uses the relative gravitational acceleration exerted by the Earth on a pair of satellites to "weigh" the planet's surface.
The loss of ice from a land surface causes local mass loss and hence a drop in gravity, which GRACE can detect and quantify. However, this system could only measure the overall drop in ice-mass, rather than resolve the difference into its relative components including fresh precipitation that can off-set the effect. For this reason, Utrecht University scientist Michiel van den Broeke and his colleagues, writing in Science, have built a computer simulation to model and predict the behaviour of the Greenland icesheet. It takes into account ice-discharge in glaciers and fresh snowfall, using measurements made on the ground. The team have then validated their model by comparing it's predictions with the real values recorded by GRACE. The resulting model shows that the rate of loss of ice appears to be accelerating and, moreover, has been under-appreciated by GRACE because this failed to take into account increased snowfall in recent years. Without this moderating effect, the team say, the loss would have been double the 1500 billion tonnes of water that has been lost since 2000. And since Greenland holds enough ice to raise global sea level by 7 metres, understanding the dynamics of this apparently fragile system will be critical to predicting the impact of rising global temperatures in future.