The ice sheets of Antarctica -- the world's largest reservoir of fresh water -- are shrinking faster than new snow can fall, scientists reported Thursday in the first comprehensive satellite survey of the entire continent.
Researchers at the University of Colorado determined that between 2002 and 2005 Antarctica lost ice at a rate of 36 cubic miles a year, rather than growing from heavier snowfalls as had been predicted. That amount of ice is equivalent to about 30 times the fresh water used by Los Angeles every year.
"It is the first time we can say that if you look at the entire ice sheet, it is losing mass," said geophysicist Isabella Velicogna, whose findings were published online Thursday by the journal Science.
This month, an independent research team at the Jet Propulsion Laboratory in La Canada Flintridge reported that the Arctic glaciers of Greenland were melting twice as fast as five years ago, adding an extra 38 cubic miles of fresh water to the Atlantic Ocean every year.
Taken together, the findings suggest that a century of steady increases in global temperatures is altering the seasonal balance of the world's water cycle, in which new snow and ice neatly offset thaw and rainfall runoff every year to maintain the current level of the seas.
If so, experts say, increasing global temperatures -- the 10 warmest years on record all occurred after 1990 -- may be hastening the demise of the polar icecaps and estimates of the pace of sea-level rise could be too low.
By previous calculations, Antarctica's coastal glaciers shed enough meltwater every year to raise sea levels by 0.02 of an inch, even as new snow falling in the interior locked up the same amount in the icecap. The result was that sea level remained more or less the same from year to year.
"A little bit of change in one of these things could throw it all out of balance and, evidently, that is what is going on," said University of Colorado geophysicist John Wahr, who helped analyze the new satellite measurements.
Portions of the Antarctic coast are 4.5 degrees Fahrenheit warmer than 60 years ago, research has shown.
Those same areas have lost an estimated 5,500 square miles of ice in the last 30 years, calving icebergs the size of Belgium and Rhode Island. In 2002, an entire ice shelf collapsed into the sea.
But the newest work signals a broader loss across the entire continent -- an amount equal to more than 13% of the annual sea level rise measured in recent years, the researchers said. The shrinkage is concentrated in the West Antarctic Ice Sheet, which has enough fresh water to raise sea levels more than 20 feet.
The researchers based their findings on unique gravity measurements collected by a pair of orbiting satellites, the Gravity Recovery and Climate Experiment, launched in 2002.
Eric Rignot at JPL called the gravity measurement technique "a breakthrough" because the satellites allow researchers for the first time to measure changes across immense swaths of Earth's surface.
In a separate measure of the effects of changing temperatures on a continental scale, researchers in South Africa reported Thursday in Science that even minor changes in rainfall caused by climate change could drastically affect lakes, rivers and streams across one quarter of Africa by the end of the century.
Climate-driven changes in water supplies "potentially have devastating implications," said researchers Maarten de Wit and Jacek Stankiewicz of the University of Cape Town.
They used computer databases of drainage basins and global climate models to calculate the potential impact of changing rainfall patterns on drainage across Africa.