publication of the American Chemical Society
May 30, 2007

Models underestimate global warming impacts

A spate of research finds the effects of warming
racking up faster than scientists had predicted.

It used to be that climate scientists worried
about how to make the public care about changes
that might not happen for a century. Today they
have a bigger problem: some of the changes aren’t
waiting around that long.

Following the latest projections by the UN’s
Intergovernmental Panel on Climate Change (IPCC),
new research shows that models in the report
underestimate some changes that are already under
way. Sea ice is melting and sea level is rising
faster than models had predicted, and one brake
on warming, the uptake of CO2 by oceans, appears
not to be working as well as scientists had

Results published in Geophysical Research Letters
in May show that ice-free summers could be even
more likely this century than estimated in
February’s IPCC report. Julienne Stroeve of the
National Snow and Ice Data Center led a group
that analyzed nearly 60 years of sea ice records
from satellites, ships, and airplanes, concluding
that ice has disappeared at an average rate of
7.8% per decade since 1953, compared with 2.5%
per decade in computer simulations.

And the Southern Ocean is not exactly doing its
part, taking up less CO2-5-30% less per
decade-than expected, according to a study
published online May 17 in Science. Models hadn’t
accounted for increased winds that push currents
to bring deep carbon to the surface, where it
percolates back into the atmosphere.

Stefan Rahmstorf, a climatologist at Potsdam
University (Germany), points out that models tend
to underestimate sea level rise, too. “As
climatologists, we’re often under fire because of
our pessimistic message, and we’re accused of
overestimating the problem,” he says. “But I
think the evidence points to the opposite-we may
have been underestimating it.”
Disappearing Arctic sea ice is one of several
changes happening faster than models predict.
Peter West/National Science Foundation

Disappearing Arctic sea ice is one of several
changes happening faster than models predict.

Modelers don’t purposely err on the conservative
side, says Marika Holland of the National Center
for Atmospheric Research, but some processes “are
just not well understood, and because of that
have not been incorporated into climate models.”
Holland has published model results on the fate
of sea ice and coauthored the recent paper
showing that ice is melting faster than models
predicted. There are many reasons for the
underestimates, she says. For example, models
don’t fully capture heat transport between ocean
and atmosphere, or faster warming as reflective
ice gives way to darker, heat-absorbing waters.

But Rahmstorf says that modelers might
unwittingly make models more conservative by
applying “one-sided filters”, weeding out models
that clearly overestimate the changes seen so
far, but hanging onto ones “where everything is
too well behaved and stable.”

In January, Rahmstorf published sea-level-rise
predictions in Science, noting that the actual
rise tracks the uppermost limits of 2001 IPCC
projections. Despite the previous underestimate,
this year’s IPCC report gave even smaller
sea-level-rise projections, partly because
authors omitted any estimate of accelerating ice
flow. “There’s absolutely no reason to assume sea
level rise is going to be lower than previously
thought,” Rahmstorf says.

The underestimates started to become clear last
year, when Eric Rignot of NASA’s Jet Propulsion
Laboratory used new satellite techniques to track
a decline in Greenland’s ice. Within months,
satellite results showed the Antarctic ice sheet
losing mass, too. Before those data came out,
scientists had assumed polar ice sheets were in
balance for lack of better information. In 2001,
the IPCC said that loss of ice sheets, leading to
faster sea level rise, was “very unlikely during
the 21st century.” The latest IPCC report
abandons that position, concluding that the
Antarctic ice sheet is already contributing to
sea level rise.

“Unfortunately the story gets worse the more data
we collect,” says climate scientist Brenda
Ekwurzel of the Union of Concerned Scientists, an
environmental advocacy group. She notes that the
public may become more motivated as climate
change moves from models to the backyard.

“We need more creative thinking, because we’re
going to see this happening more rapidly than we
had thought,” says Joel Smith, vice president of
Stratus Consulting and lead author of a climate
change impacts chapter in the recent IPCC report.
“I wouldn’t advise people to take the IPCC
literally and assume the upper end [of sea level
rise] reported there is really the upper limit,”
he says. “Don’t overreact,” he adds, “but ask a
lot of questions.” -ERIKA ENGELHAUPT

Copyright © 2007 American Chemical Society


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