Measuring water from space(Antarctica. This has been revealed by a ...)

The GRACE mission comprises two satellites, launched in 2002, which measure how the Earth's gravity field varies with time. The gravity field depends on how mass is distributed on Earth, and affects the speed of satellites in orbit. By closely monitoring the speed of both satellites, as they orbit the planet, it is possible to measure the change in mass of water or ice in different regions.

The method has shown that the Greenland ice sheet is losing about 150 gigatonnes of ice each year, two thirds of which is large chunks of ice flowing rapidly into the sea. The combined effect of ice loss from Greenland and West Antarctica has contributed about 1 mm per year to the rising seas over the past five years.

Rivers run low

Using GRACE, Cazenave and others have also looked at changes in water storage in river basins. In the period from 2002-2006, they found that some basins, including the Congo and the Mississippi, have been losing water, but river systems in the boreal regions are gaining water.

Meanwhile, scientists at the European Space Agency, collaborating with DeMontfort University in the UK, have begun to use data from the satellites that measure sea level, to assess lake and river levels on land.

Fresh inland water is much in demand, but those managing it suffer from a grave lack of information about how much of it there is. "The number of river gauges is diminishing every day, and many catchments are now entirely unmeasured," says Jrme Benveniste of the European Space Agency's data processing centre ESRIN, in Frascati, Italy. "But we have 16 years' worth of data on river and lake levels. It's just a question of processing it all."

The work Benveniste is leading can recreate water levels in reservoirs, or lakes, and reconstruct the annual ebb and flow in large river basins like the Amazon.