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Ice
sheets covering both the Arctic and Antarctic could melt
more quickly than expected this century, according to
two studies that blend computer modeling with paleoclimate
records. Led by scientists at the National Center for
Atmospheric Research (NCAR) and the University of Arizona,
the studies show that by 2100, Arctic summers may be as
warm as they were nearly 130,000 years ago when sea levels
rose to 20 feet (6 meters) higher than they are today.
Bette Otto-Bliesner (NCAR) and Jonathan Overpeck (University
of Arizona) report their new work in two papers appearing
in the March 24 issue of Science.
Otto-Bliesner and Overpeck base their findings on data
from ancient coral reefs, ice cores, and other natural
climate records, as well as output from the NCAR-based
Community Climate System Model (CCSM), a powerful tool
for simulating past, present, and future climates.
Scientists such as Otto-Bliesner and Overpeck have learned
to "read" the Earth's environmental history
by sampling sediments and other well-preserved remains
deposited over time, said Jay S. Fein, who heads the climate
and large scale dynamics program in the National Science
Foundation's (NSF) Division of Atmospheric Sciences.
NSF, which provides primary support to NCAR, funded the
research . The study also involved researchers from the
universities of Calgary and Colorado, the U.S. Geological
Survey, and The Pennsylvania State University.
In this case, Fein added, the data describe a period
in Earth's history characterized by a high level of Arctic
warming. Based on those data, the modeling experiments
Otto-Bliesner and Overpeck conducted provide important
insights about possible future environmental changes in
a warmer world that have the potential to significantly
alter our natural and man-made environments.
"Although the focus of our work is polar, the implications
are global," says Otto-Bliesner. "These ice
sheets have melted before and sea levels rose. The warmth
needed isn't that much more than present conditions."
The two studies show greenhouse-gas increases over the
next century could warm the Arctic by 5-8 degrees Fahrenheit
(3-5 degrees Celsius) in summertime--about as warm as
it was 130,000 years ago, between the most recent ice
age and the one before it. Changes in Earth's tilt and
orbit caused the warm Arctic summers during the last interglacial
period. The CCSM accurately captured that warming, which
is mirrored in data from paleoclimate records.
Although simulation results depend on the assumptions
and conditions of different models, estimates of warming
from the CCSM are within the range projected by other
climate models, according to the authors.
"Getting the past climate change correct in these
models gives us more confidence in their ability to predict
future climate change," says Otto-Bliesner.
The CCSM suggests that during the interglacial period,
melt water from Greenland and other Arctic sources raised
sea level by as much as 11 feet (3.5 meters), says Otto-Bliesner.
However, coral records indicate the sea level actually
rose 13-20 feet (4-6 meters) or more. Overpeck concludes
that Antarctic melting must have produced the remainder
of the sea-level rise.
These studies are the first to link Arctic and Antarctic
melting in the last interglacial period. Marine diatoms
and beryllium isotopes found beneath the West Antarctic
Ice Sheet indicate parts of the ice disappeared at some
point over the past several hundred thousand years.
Overpeck theorizes that the rise in sea levels produced
by Arctic warming and melting could have helped destabilize
ice shelves at the edge of the Antarctic ice sheet and
led to their collapse. If such a process occurred today,
it would be accelerated by global-scale greenhouse-induced
warming year round, Overpeck says. In the Arctic, melting
would likely be hastened by pollution that darkens snow
and enables it to absorb more sunlight.
In the past few years sea level has begun rising more
rapidly, now at a rate of about an inch per decade, says
Overpeck. Recent studies have also found accelerated rates
of glacial retreat along the margins of both the Greenland
and West Antarctic ice sheets.
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NSF
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