Annual Scientific Report
Climate and Global Dynamics Division



Climate System Model

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Simulation of the Climates of the 20th and 21st Centuries

Under the direction of the Climate System Model Chemistry and Climate Change Working Group, simulations were carried out for the climates of the 20th and 21st centuries. These simulations used the climate system model with realistic time varying chemical composition of the atmosphere. For the 20th century simulations, gas concentrations were prescribed based on observations of greenhouse gases. Stratospheric and tropospheric ozone changes were obtained from a combination of satellite data, ozonesonde information and a chemical transport model. Sulfate aerosol distributions were based on results from a sulfur chemistry model run offline. Solar variability was based on the Hoyt and Schatten reconstruction. Formation of stratospheric water vapor through the oxidation of methane was accounted for in these simulations. The gas distributions, excluding CO2 and O3, were determined by fixing the near surface mixing ratios and allowing the atmospheric model to transport the gas species. Four simulations of the 20th century were carried out. The control simulation held the atmospheric gas concentrations fixed at 1870 levels throughout a 130 year integration. The transient simulation employed the time varying concentrations for CO2, CH4, N2O, CFC11, CFC12, other halocarbons, ozone and sulfates. The solar variability simulation included the reconstructed solar variation in addition to the transient atmospheric concentration. The last simulation included only transient concentrations in greenhouse gases, and hence ignored the effects of sulfate aerosols and solar variability. (This Figure, 30KB, shows the anomaly in global mean surface temperature for the four simulations.)

For simulations of the 21st century, a fully interactive sulfur chemistry model was added to the CSM. This allowed for the response of the sulfur cycle to changes in the hydrologic cycle. These simulations were started from the 1980 state of the transient 20th century simulation. For ozone, tropospheric concentrations were held fixed at present values, while the stratospheric ozone concentrations were scaled by the projected amount of reactive stratospheric chlorine. Three simulations of the 21st century were carried out. The first simulation assumed a business-as-usual increase in carbon dioxide concentration. Increases in other greenhouse gases were based on estimates of industrial growth in the future. Sulfur emissions accounted for geographic shifts in industrial activity, such as increased emissions in southeast Asia, up to 2010. Emissions in sulfur dioxide were projected to decrease after this time due to assumed air quality enforcement. The second simulation assumed that emission of carbon dioxide would stabilize, such that carbon dioxide concentration would reach its peak just after 2100. The third simulation was carried out as a part of the Intergovernmental Panel on Climate Change's (IPCC) Third Assessment Report. This scenario was quite similar to the business-as-usual scenario. (This Figure, 22 KB, shows the anomaly in global mean surface temperature from the three scenarios for the 21st century.)








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