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Huge ice sheets melting 'at rate of 300bn tonnes a year'

WASHINGTON: Massive ice sheets of Antarctica and Greenland are losing around 300 billion tonnes of ice every year, new research has indicated. 

These ice sheets contain about 99.5 per cent of the Earth's glacier ice which would raise global sea level by some 63m if it were to melt completely. 

The ice sheets are the largest potential source of future sea level rise - and they also possess the largest uncertainty over their future behaviour. 

Since 2002, the satellites of the 
Gravity Recovery and Climate Experiment (GRACE) detect tiny variations in Earth's gravity field resulting from changes in mass distribution, including movement of ice into the oceans. 

Using these changes in gravity, the state of the ice sheets can be monitored at monthly intervals. 

Bert Wouters, currently a visiting researcher at the University of Colorado, said that in the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice about 300 billion tonnes each year and that the rate at which these losses occurs is increasing. 

He said that compared to the first few years of the GRACE mission, the ice sheets' contribution to sea level rise has almost doubled in recent years. 

Yet, there is no consensus among scientists about the cause of this recent increase in ice sheet mass loss observed by satellites. Beside anthropogenic warming, ice sheets are affected by many natural processes, such as multi-year fluctuations in the 
atmosphere (for example, shifting pressure systems in the North Atlantic, or El Nino and La Nina events) and slow changes in ocean currents. 

The team of researchers compared nine years of 
satellite data from the GRACE mission with reconstructions of about 50 years of mass changes to the ice sheets. 

They found that the ability to accurately detect an accelerating trend in mass loss depends on the length of the record. 

At the moment, the ice loss detected by the GRACE satellites is larger than what we would expect to see just from natural fluctuations, but the speed-up of ice loss over the last years is not. 

The findings have been published in Nature Geoscience.

Times of India (15-07-2013)