Measuring water from space( Observations from satellites now all...)

Observations from satellites now allow scientists to monitor changes to water levels in the sea, in rivers and lakes, in ice sheets and even under the ground. As the climate changes, this information will be crucial for monitoring its effects and predicting future impacts in different regions.

Sea level rise in one of the major consequences of global warming, but it is much more difficult to model and predict than temperature. It involves the oceans and their interaction with the atmosphere, the ice sheets, the land waters and even the solid Earth, which modifies the shapes of ocean basins. Measurements from tidal gauges show that for most of the twentieth century, sea levels rose by 1.8 mm per year on average.

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. "It could be that we are seeing a decadal fluctuation, and in the near future the rate will fall again," says Anny Cazenave, from the Laboratoire d'Etudes en Gophysique et Ocanographie Spatiale (LEGOS) in Toulouse, "but I do not think so. For several years now, the rate of rise has not changed significantly."

Melting ice fills the sea

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% of the annual sea level rise can be attributed to accelerated land ice loss from glaciers, Greenland and
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