Evaluating the climatic response to changes in CO2 and solar luminosity

A series of simulations was conducted using the latest version of the NCAR Community Climate Model (CCM3) to investigate the equilibrium response of surface temperature and other key variables to atmospheric CO_2 concentrations and varied solar luminosity. The CCM3 is a general circulation model (GCM) of the Earth's global climate. Eight GCM simulations with CO_2 concentrations of 180, 230, 355, 710, 1000, 2000 and 3000 ppmv were run for a minimum of thirty two seasonal cycles each. The 355 ppmv simulation was designated as the control run for the present-day climate. The results showed the same basic non-linear behavior of temperature to CO$_2$ concentrations obtained previously with a similarly designed experiment with CCM1, an earlier version of the NCAR GCM. The magnitude of the sensitivities, however, were much lower in the new CCM3 runs than in the older CCM1 runs. Four possible reasons for the reduced sensitivity in CCM3 are attributed to: a non-local boundary layer scheme; the parameterization of atmospheric convection; the treatment of the implied ocean heat transports; and, possibly the parameterization of long wave radiation. An additional four GCM simulations with solar luminosity values of +5%, +2%, -2% and -5% of the present day value of 1367 watts per meter square are currently being completed. Preliminary results of these latter experiments suggest a relatively larger response than to the changes in CO2, although more detailed analyses are currently being completed and will be presented in the Conference paper and presentation. The model results to changes in these very different forcings also have profound implications for the hydrologic cycle, water vapor and sea ice feedbacks, all of which are vital for the study of past and future climates.