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November 2006
NCAR
scientists predict a warmer, wetter Earth
New study based on computer models offers
strong evidence for more extreme weather events in the
future.
It’s rare to turn on the news these days without hearing
about a heat wave in one place, a catastrophic flood in another,
or perhaps an unusually strong snowstorm.
Claudia Tebaldi.
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Indeed, a new study by scientists in SERE and ESSL suggests
that wild weather is in fact on the rise, and that some regions
of the world will get hit harder than others. The study,
slated for publication in the December
issue of the journal Climatic Change, has already
drawn ample media coverage in the United States and abroad.
It foretells a warmer, wetter world in which the build-up
of greenhouse gases in the atmosphere causes a greater
number of extreme weather events, particularly heat waves
and intense bursts of precipitation.
The study is one of the first of its kind to draw on the
extensive computer modeling recently carried out for the
Intergovernmental Panel on Climate Change’s upcoming
report, due in
early 2007.
“We now have the first model-based consensus on how
the risk of dangerous heat waves, intense rains, and other
kinds of extreme weather will change in the next century,” says
Claudia Tebaldi (ISSE/CGD/IMAGe), who led the study. The
team includes Julie Arblaster and Jerry Meehl, both in CGD.
A fourth collaborator, Katherine Hayhoe, is based at Texas
Tech University.
The power of multiple models
Previous studies have analyzed how average temperature and
rainfall might change over the next century as greenhouse
gases increase; the new study focuses specifically on how
weather extremes would change. It is unique in that it incorporates
simulations from nine computer models, giving the scientists
greater confidence in its results.
“Past studies have looked at maybe one or two models,
but by looking at these nine models and getting similar answers,
it makes the results more credible,” Jerry says.
To generate data for the study, modeling centers around
the world used supercomputers to run simulations for both
the 20th and 21st centuries. Within this 200-year stretch,
the researchers singled out the periods 1980–1999 and
2080–2099 to compute future changes in extremes. (Simulating
past years and comparing the results to real weather observations
lets scientists verify the accuracy of their models.) Each
model simulated the future years three times, varying the
amount of greenhouse gases that could accumulate in the atmosphere
over the next hundred years to account for how society may
or may not act to reduce emissions.
Vegetation struggles to grow in
dry soil in eastern Colorado. Dry spells and droughts
could lengthen significantly across the western
United States, a hot spot for future weather extremes.
(Photo by Carlye Calvin)
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For all three scenarios, the models agree that by 2080–2099:
- The number of extremely warm nights and the length
of heat waves will increase significantly over nearly
all Earth’s land areas.
- Dry spells, which can produce and intensify droughts,
could lengthen significantly across the western United
States, the Mediterranean, eastern Brazil, and other
mid- to low-latitude areas.
- Most areas above about 40°N will experience a
significant increase in the number of days with heavy
precipitation (more than 0.4 inches). This includes the
northern tier of the United States, Canada, and most
of Europe.
- The average growing season could increase significantly
across most of North America and Eurasia.
The study pinpoints the western United States as a hot spot
for some of the worst extremes. Some places, such as the
Pacific Northwest, are expected to have longer dry spells
punctuated by heavier rainfall.
Claudia emphasizes that weather extremes, not averages, are
what damage society and ecosystems. The most ominous results
are the predictions for heat waves and warm nights, she says.
During heat waves, warm nights are often associated with
fatalities because people and buildings have less chance
to cool down.
She also notes that the scenario
with the lowest increase in greenhouse gas emissions produced
the least increase in extreme events. “We see this
finding as the only hopeful note from this study,” she
says. “Even if not positive in itself, we hope it will
have a motivating effect for initiating long overdue measures
to curb greenhouse gases, since it shows a very straightforward
relation between the severity of the most extreme events
to come and future concentrations of human-produced gases
in our
atmosphere.”
• by Nicole
Gordon
On the Web
“Going
to the Extremes: An Intercomparison of Model-Simulated
Historical and Future Changes in Extreme Events”
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