BOULDER--Carbon dioxide emissions over the next century could increasewintertime precipitation in the U.S. Southwest and Great Plains by 40%as global average temperature rises 3 degrees Fahrenheit (2 degreesCelsius), according to latest results from a new climate system modeldeveloped at the National Center for Atmospheric Research by NCAR,university, and other laboratory scientists. Reducing the buildup ofcarbon dioxide concentrations over the next century by one half largelydries up the extra rain and snow and slows the global temperature riseto 2 degrees F (1.5 degrees C). The model results were announced this weekin Atlanta. The study was funded in part by the National ScienceFoundation (NSF), NCAR's primary sponsor.
The NCAR model simulated the earth's climate from 1870 to 1990 and thencontinued the simulation to 2100 under two different scenarios. Thefirst was a "business-as-usual" increase in greenhouse gases in whichatmospheric carbon dioxide doubles over the next century. In the second,carbon dioxide increases are stabilized at 50% above today'sconcentrations. In the first projection, changes in precipitation varymarkedly by region and by season. Within the United States, the greatestincreases occur in the Southwest and Great Plains in winter andsubstantially exceed the range of natural variability. Precipitationchanges are reduced when carbon dioxide emissions are limited, accordingto the model.
Global average temperature climbs by 3 degrees F (2 degrees C) forbusiness as usual and 2 degrees F (1.5 degrees C) when carbon dioxideemissions are limited. These changes are three to four times larger thanthe warming that has occurred since 1900. On the continental scale,carbon dioxide stabilization reduces climate warming over Eurasia morethan over North America.
NCAR scientist Tom Wigley says, "These results show that we willexperience not only future climate change, but also the results ofpolicies to reduce these changes, in ways that are not simply related tochanges in the global mean temperature. Policy decisions about reducinggreenhouse emissions should not, therefore, be dictated by projectedchanges in global mean temperature alone."
The model shows no clear separation between the business-as-usual andthe stabilization cases until around 2060, even though the carbon dioxideconcentrations begin to diverge in 2010. The half-century lag until thechanges in greenhouse emissions begin to affect the climate noticeablyis 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 climatemodeling into new territory. It is one of the world's first globalmodels not to require special corrections to keep the simulated climatefrom drifting to an unrealistic state. It is also one of only a handfulof models in the world capable of realistically simulating the chemistryand transport of individual greenhouse gases and sulfur compounds. Themodel employs a more realistic scenario for future emissions of sulfurdioxide, a form of industrial pollution that cools the climate. Assumingthat societies will take steps to reduce sulfur dioxide emissions overthe next century, the scientists incorporated this decline into themodel. The sulfur dioxide cooling effect gradually diminishes, allowingthe simultaneous greenhouse warming to emerge more clearly.
Data from these NCAR climate system model runs are available to thescientific community for research into possible effects of climatechange on human health, water resources, agriculture, naturalecosystems, and the economy. Scientists from both NCAR and the NationalOceanic and Atmospheric Administration worked on the study, which wasfunded by NSF and ACACIA (A Consortium for the Application of ClimateImpact Assessments). ACACIA is a joint NCAR-industry program sponsoredby electric utility research organizations in the United States, Japan,and The Netherlands, and by NCAR and NSF. The simulations were run onsupercomputers at NCAR and in Japan.
For more information, see Tom Wigley's background paper on the WorldWide 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 atmosphericand related sciences.
Materials provided by National Center For Atmospheric Research (NCAR). Note: Content may be edited for style and length.
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