Climate
Data Hint At Irreversible
Rise In Seas
By Andrew C. Revkin
25 March, 2006
The New York Times
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 four degrees higher than today and that over the coming centuries,
the oceans could rise 13 to 20 feet — conditions last seen 129,000
years ago, between the last two ice ages.
The findings, being reported today 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 T. 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.," Dr. 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 last 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 Dr. Overpeck and his
fellow lead author, Bette L. Otto-Bliesner of the National Center for
Atmospheric Research in Boulder, Colo.
In a second article in Science,
researchers say they have detected a rising frequency of earthquakelike
rumblings in the bedrock beneath Greenland's two-mile-thick ice cap
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 Goran
Ekstrom of Harvard and Meredith Nettles of the Lamont-Doherty Earth
Observatory in Palisades, N.Y., part of Columbia University.
H. Jay Zwally, a NASA scientist
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," Dr. 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 over the last century and the
pace has picked up markedly since the 1990's.
Dr. Overpeck, the co-author
of the paper on rising sea levels, acknowledged the uncertainties about
the causes. But he said that in a world in which humans, rich and poor,
increasingly clustered on coasts, the risks were great enough to justify
prompt action.
"People driving big
old S.U.V.'s to their favorite beach or coastal golf course," he
said, should "start to think twice about what they might be doing."