Researchers predict wet future for Southwest, Great Plains

If carbon dioxide emissions are stabilized at 50 percent above today's concentrations, the Southwest will not experience a wetter climate

April 20, 1999
Web posted at: 3:30 PM EDT

If nothing is done to curb carbon dioxide emissions over the next century, wintertime precipitation over the U.S. Southwest and Great Plains could increase by 40 percent, the National Center for Atmospheric Research announced April 12.

Using the latest version of the Climate System Model, researchers found that if atmospheric carbon dioxide concentrations double over the next 100 years, as they are expected to do, global average temperature will rise by 3 degrees Fahrenheit.

However, reducing the buildup of carbon dioxide concentrations over the next century by one half largely dries up the extra rain and snow and slows the global temperature rise to 2 F.

The model simulated Earth's climate from 1870 to 1990 and 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 F for business as usual and 2 F when carbon dioxide emissions are limited. These changes are three to four times larger than the warming that has occurred since 1900. On the continental scale, carbon dioxide stabilization reduces climate warming over Eurasia more than over North America.

"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," said National Center for Atmospheric Research scientist Tom Wigley.

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 noticeably is the result of large thermal inertia in Earth's climate system, especially in the oceans, said the scientists.

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 will take steps to reduce sulfur dioxide emissions over the next century, the scientists incorporated 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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