Study finds continent is shrinking faster than it can grow. Experts
say changes to the global water cycle could hasten the pace of sea-level rise.
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
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 Cañada 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