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Summer 1999

Update: CSM sees a wet, hot 21st century

by Anatta

The NCAR climate system model (CSM) has completed two simulations of climate change through the 21st century; results were released on 12 April. The Climate of the 21st Century Project was first reported in the UCAR Quarterly Summer 1998 issue.

If future carbon dioxide emissions continue to grow at their current rate, wintertime precipitation over the U.S. Southwest and Great Plains could increase by 40% as global average temperature rises 2°C (3°F). Reducing the buildup of CO2 concentrations by one-half over the next century largely dries up the extra rain and snow and slows the global temperature rise to 1.5°C. Globally, CO2 stabilization has more effect on temperatures in Europe and Asia than in North America.

The CSM 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 CO2 doubles over the next century. In the second scenario, CO2 increases are stabilized at 50% above today's concentrations.

"These values are three to four times as large as the warming that has occurred over the 20th century," says Tom Wigley, director of ACACIA (A Consortium for Application of Climate Impact Assessments), the joint NCAR-industry program that sponsored the computer runs. The results are especially noteworthy because the CSM is less sensitive to anthropogenic greenhouse-gas forcing than are most other models, Wigley notes.

In the business-as-usual 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. Limiting CO2 emissions reduced the precipitation changes.

The model shows no clear separation between the business-as-usual and the stabilization cases until around 2060, even though the CO2 concentrations begin to diverge in 2010. The half-century lag is the result of thermal inertia in the earth's climate system, especially in the oceans.

The CSM is 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 scenario for future emissions of sulfur dioxide, which cools the climate, that the researchers believe is more realistic than those used by the Intergovernmental Panel on Climate Change, which do not take possible policy changes into consideration. The ACACIA scenario assumes that societies will take steps to reduce sulfur dioxide emissions over the next century as the public-health consequences of the emissions increase. In the ACACIA scenario, the sulfur dioxide cooling effect gradually diminishes, allowing the simultaneous greenhouse warming to emerge more clearly.

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."

Data from these NCAR climate system model runs are available to scientists studying the effects of climate change on human health, water resources, agriculture, natural ecosystems, and the economy. Besides the NCAR group, scientists from NOAA also worked on the study, which was funded by NSF and ACACIA. The simulations were run on supercomputers at NCAR and in Japan.

For more information, see Wigley's background pape. For model data, see the U.S. National Assessment site.


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