Washington, Nov 14 : Monitoring changes to water levels in the sea, in rivers and lakes, in ice sheets and even under the ground, with the help of observations from satellites, has revealed that since the start of 1993, sea level has been rising by 3.3 mm a year, almost double the rate of the previous 50 years.
Sea level rise in one of the major consequences of global warming, but it is much more difficult to model and predict than temperature.
Since the 1990s, a number of altimeter satellites have been measuring the height of the ocean surface and this has dramatically improved our understanding of sea level rise.
Currently, three altimeter satellites cover the entire globe every 10 to 35 days, and can measure the height of the sea surface to a precision of 1 to 2 cm.
These measurements show that since the start of 1993, sea level has been rising by 3.3 mm a year, almost double the rate of the previous 50 years.
"For several years now, the rate of rise has not changed significantly," said Anny Cazenave, from the Laboratoire d'Etudes en Geophysique et Oceanographie Spatiale (LEGOS) in Toulouse.
Cazenave's team, and other groups, calculate that for 1993-2003, about half of the sea level rise was due to the oceans expanding as they became warmer, and the other half was due to shrinking land ice.
Since 2003, ocean warming has had a temporary break but sea level has continued to rise.
Now, about 80 percent of the annual sea level rise can be attributed to accelerated land ice loss from glaciers, Greenland and Antarctica.
This has been revealed by a brand new satellite technique, called space gravimetry.
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.
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.