Massive ice sheets melting at rate of 300bn tonnes a year

A rapid acceleration in the melting of Antarctic and Greenland glacier ice, amounting to around 300 billion tonnes a year, has been detected by a satellite over the past decade.
The satellite that measures gravity fluctuations on Earth due to changes in the massive ice sheets of Greenland and Antarctica detected the melting which could have a dramatic impact on sea levels around the world, researchers say.
Scientists have warned that the measurements gathered since 2002 by the Gravity Recovery and Climate Experiment (Grace) flying in space are still too short-term for accurate predictions of how much ice will be lost in the coming decades, and therefore how rapidly sea levels will rise.

"In the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice - about 300 billion tonnes a year - and that the rate at which these losses occurs is increasing," said Bert Wouters of Bristol University's Glaciology Centre.
"Compared to the first few years of the Grace mission, the ice sheets' contribution to sea-level rise has almost doubled in recent years," added Wouters, the lead author of the study.
Yet, there is no consensus among scientists about the cause of this recent increase in ice sheet mass loss observed by satellites, researchers say.

The findings, published in the journal Nature Geoscience, underscore the need for continuous satellite monitoring of the ice sheets to better identify and predict melting and the corresponding sea-level rise, researchers say.

The ice sheets covering Antarctica and Greenland 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, they said.
According to researchers, the ice sheets are the largest potential source of future sea level rise - and they also possess the largest uncertainty over their future behaviour.
Satellites 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.

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, researchers said.
"So, if observations span only a few years, such 'ice sheet weather' may show up as an apparent speed-up of ice loss which would cancel out once more observations become available," Wouters said.
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.