Big ice sheets melting rapidly, say scientists

The Grace satellite measures tiny fluctuations of the Earth's gravity field. Photo / David Harrowfield

A satellite that measures gravity fluctuations on Earth due to changes in the massive ice sheets of Greenland and Antarctica has detected a rapid acceleration in the melting of glacier ice over the past decade, which could have a dramatic impact on the world's sea levels.

The sheets are losing around 300 billion tonnes of ice a year, the research indicates.

However, 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 Dr 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 published in the Earth sciences journal Nature Geoscience.

The Grace satellite measures tiny fluctuations of the Earth's gravity field resulting from the loss of ice into the sea, but it cannot yet point to a long-term trend. Ice sheets also melt because of variations in the weather due to shifting ocean currents or decade-long oscillations in the weather systems of the North Atlantic Ocean.

A few more years of observations would be needed for the Grace experiment to point to whether global warming rather than natural variability is behind the loss of ice in the Antarctic, while it could take another 10 years of data to demonstrate a link with the loss of ice in Greenland, Wouters said.

The melting of the world's two great ice sheets is one of the greatest unknowns in climate-change science. Together, the ice sheets of Greenland and Antarctica contain about 99.5 per cent of the Earth's glacier ice, which could increase average sea levels by 63m if they were to melt completely - an event that would in any case take many centuries.

- Independent