NSF PR 99-28 - April 21, 1999
Media contact: |
Cheryl Dybas, NSF |
(703) 292-8070 |
cdybas@nsf.gov |
Media contact: |
Anatta, NCAR |
(303) 497-8604 |
Program contact: |
Cliff Jacobs, NSF |
(703) 306-1521 |
cjacobs@nsf.gov |
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Climate Projection for the 21st Century: Earth Will
Warm by 3 Degrees
Carbon dioxide emissions over the next century could
increase global average temperatures 3 degrees Fahrenheit
(2 degrees Celsius) while wintertime precipitation
over the U.S. Southwest and Great Plains could rise
by 40% according to latest results from a new climate
system model developed at the National Center for
Atmospheric Research (NCAR). Meanwhile, the new model
suggests that reducing the buildup of carbon dioxide
concentrations over the next century by one half could
largely dry up the extra rain and snow, and slow the
global temperature rise to 2 degrees F (1.5 degrees
C). The study was funded in part by the National Science
Foundation (NSF), NCAR's primary sponsor.
"The results from the NCAR model offer new insight
into the behavior of Earth's climate system in response
to human activity," says Cliff Jacobs, program director
in NSF's division of atmospheric sciences, which funds
NCAR.
The NCAR model simulated the earth's climate from 1870
to 1990, -- then continued the simulation to 2100
under two different scenarios. The first was a "business-as-usual"
increase in greenhouse gases in which atmospheric
carbon dioxide doubles over the next century. In the
second, carbon dioxide increases are stabilized at
50 percent above today's concentrations. In the first
projection, changes in precipitation vary markedly
by region and by season. Within the United States,
the greatest increases occur in the Southwest and
Great Plains in winter and substantially exceed the
range of natural variability. Precipitation changes
are reduced when carbon dioxide emissions are limited,
according to the model.
Global average temperature climbs by 3 degrees F (2
degrees C) for the business as usual scenario and
2 degrees F (1.5 degrees C) when carbon dioxide emissions
are limited. These changes are three to four times
larger than the warming that has occurred since 1900.
NCAR scientist Tom Wigley says, "These results show
that we will experience not only future climate change,
but also the results of policies to reduce these changes,
in ways that are not simply related to changes in
the global mean temperature. Policy decisions about
reducing greenhouse emissions should not, therefore,
be dictated by projected changes in global mean temperature
alone."
The model shows no clear separation between the business-as
usual and the stabilization cases until around 2060,
even though the carbon dioxide concentrations begin
to diverge in 2010. The half-century lag until the
changes in greenhouse emissions begin to affect the
climate is the result of large thermal inertia in
the earth's climate system, especially in the oceans,
say the scientists.
The NCAR model's special features help push the science
of climate modeling into new territory. It is one
of the world's first global models not to require
special corrections to keep the simulated climate
from drifting to an unrealistic state. It is also
one of only a handful of models in the world capable
of realistically simulating the chemistry and transport
of individual greenhouse gases and sulfur compounds.
The model employs a more realistic scenario for future
emissions of sulfur dioxide, a form of industrial
pollution that cools the climate. Assuming that societies
take steps to reduce sulfur dioxide emissions over
the next century, the scientists incorporate this
decline into the model. The sulfur dioxide cooling
effect gradually diminishes, allowing the simultaneous
greenhouse warming to emerge more clearly.
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