Climate Model Projections for 21st Century

NCAR Climate Model Projections for 21st Century: Earth Warms by 3 Degrees F; Winter Rain/Snow Increases 40% in Southwest and Great Plains

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BOULDER--Carbon dioxide emissions over the next century could increase wintertime precipitation in the U.S. Southwest and Great Plains by 40% as global average temperature rises 3 degrees Fahrenheit (2 degrees Celsius), according to latest results from a new climate system model developed at the National Center for Atmospheric Research by NCAR, university, and other laboratory scientists. 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 degrees F (1.5 degrees C). The model results were announced this week in Atlanta. The study was funded in part by the National Science Foundation (NSF), NCAR's primary sponsor.

The NCAR model simulated the 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% 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 business as usual 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. On the continental scale, carbon dioxide stabilization reduces climate warming over Eurasia more than over North America.

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 noticeably 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 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.

Data from these NCAR climate system model runs are available to the scientific community for research into possible effects of climate change on human health, water resources, agriculture, natural ecosystems, and the economy. Scientists from both NCAR and the National Oceanic and Atmospheric Administration worked on the study, which was funded by NSF and ACACIA (A Consortium for the Application of Climate Impact Assessments). ACACIA is a joint NCAR-industry program sponsored by electric utility research organizations in the United States, Japan, and The Netherlands, and by NCAR and NSF. The simulations were run on supercomputers at NCAR and in Japan.

For more information, see Tom Wigley's background paper on the World Wide Web at http://www.cgd.ucar.edu/cas/ACACIA/index.html For model data, see the U.S. National Assessment site at http://www.nacc.usgcrp.gov NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.

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