Climate Projection For The 21st Century: Earth Will Warm By 3 Degrees

April 21, 1999

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.
Additional contact:
Anatta, NCAR

National Science Foundation

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