Studies hint at irreversible rising seas
By Andrew C. Revkin The New York Times
SATURDAY, MARCH 25, 2006
NEW YORK Within the next 100 years, the growing human influence on
Earth's climate could lead to a long and irreversible rise in sea levels by
eroding the planet's vast polar ice sheets, according to new observations and
analysis by several teams of scientists.
One team, using computer models of climate and ice, found that by about
2100, average temperatures could be 4 degrees higher than today and that over
the coming centuries, the oceans could rise 13 to 20 feet, or 4 to 6 meters -
conditions last seen 129,000 years ago, between the past two ice ages.
The findings, reported Friday in the journal Science, are consistent with
other recent studies of melting and erosion at the poles. Many experts say
there are still uncertainties about timing, extent and causes.
But Jonathan Overpeck of the University of Arizona, a lead author of one
of the studies, said the new findings made a strong case for the danger of
failing to curb emissions of carbon dioxide and other gases that trap heat in a
greenhouselike effect.
"If we don't like the idea of flooding out New Orleans, major portions of
South Florida, and many other valued parts of the coastal U.S.," Overpeck said,
"we will have to commit soon to a major effort to stop most emissions of carbon
to the atmosphere."
According to the computer simulations, the global nature of the warming
from greenhouse gases, which diffuse around the atmosphere, could amplify the
melting around Antarctica beyond that of the previous warm period, which was
driven mainly by extra sunlight reaching the Northern Hemisphere.
The researchers also said that stains from dark soot drifting from power
plants and vehicles could hasten melting in the Arctic by increasing the amount
of solar energy absorbed by ice.
The rise in sea levels, driven by loss of ice from Greenland and West
Antarctica, would occur over many centuries and be largely irreversible, but
could be delayed by curbing emissions of the greenhouse gases, said Overpeck
and his fellow lead author, Bette Otto-Bliesner of the National Center for
Atmospheric Research in Boulder, Colorado.
In a second article in Science, researchers say they have detected a
rising frequency of earthquakelike rumblings in the bedrock beneath Greenland's
ice cap, which is two miles, or 3.2 kilometers, thick, in late summer since
1993.
They say there is no obvious explanation other than abrupt movements of
the overlying ice caused by surface melting.
The jostling of that giant ice-cloaked island is five times more frequent
in summer than in winter, and has greatly intensified since 2002, the
researchers found. The data mesh with recent satellite readings showing that
the ice can lurch toward the sea during the melting season.
The analysis was led by Göran Ekström of Harvard and Meredith Nettles of
the Lamont-Doherty Earth Observatory in Palisades, New York, part of Columbia
University.
H. Jay Zwally, a scientist with the National Aeronautics and Space
Administration studying the polar ice sheets with satellites, said the seismic
signals from ice movement were consistent with his discovery in 2002 that
summer melting on the surface of Greenland's ice sheets could almost
immediately spur them to shift measurably. The meltwater apparently trickles
through fissures and lubricates the interface between ice and underlying rock.
"Models are important, but measurements tell the real story," Zwally
said. "During the last 10 years, we have seen only about 10 percent of the
greenhouse warming expected during the next 100 years, but already the polar
ice sheets are responding in ways we didn't even know about only a few years
ago."
In both Antarctica and Greenland, it appears that warming waters are also
at work, melting the protruding tongues of ice where glaciers flow into the sea
or intruding beneath ice sheets, like those in western Antarctica, that lie
mostly below sea level. Both processes can cause the ice to flow more readily,
scientists say.
Many experts on climate and the poles, citing evidence from past natural
warm periods, agreed with the general notion that a world much warmer than
today's, regardless of the cause of warming, will have higher sea levels.
But significant disagreements remain over whether recent changes in sea
level and ice conditions cited in the new studies could be attributed to rising
concentrations of the greenhouse gases and temperatures linked by most experts
to human activities.
Sea levels have been rising for thousands of years as an aftereffect of
the warming and polar melting that followed the last ice age, which ended about
10,000 years ago. Discriminating between that residual effect and any new
influence from human actions remains impossible for the moment, many experts
say.
Satellites and tide gauges show that seas rose about eight inches, or 20
centimeters, over the past century and that the pace has picked up markedly
since the 1990s.
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