A GENERAL-CIRCULATION MODEL STUDY OF THE EFFECTS OF FASTER ROTATION RATE, ENHANCED CO2 CONCENTRATION, AND REDUCED SOLAR FORCING - IMPLICATIONS FOR THE FAINT YOUNG SUN PARADOX

Solar energy at the top of the atmosphere (solar constant), rotation r ate, and carbon dioxide (CO2) may have varied significantly over Earth 's history, especially during the earliest times. The sensitivity of a general circulation model to faster rotation, enhanced CO2 concentrat ion, and reduced solar constant is presented. The control simulation o f this study has a solar constant reduced by 10% the present amount, z ero land fraction using a swamp ocean surface, CO2 concentrations of 3 30 ppmv, present-day rotation rate, and is integrated under mean diurn al and seasonal solar forcing. Four sensitivity test are performed und er zero land fraction and reduced solar constant conditions: (1) incre ased rotation rate, (2) 8 times present CO2, (3) a combination of 1 an d 2, and (4) 3 with a 15% reduction in the solar constant. Results ind icate the following changes in the global mean sea surface temperature s (SSTs) compared to the control simulation: (1) + 6.6 K, (2) + 12 K, (3) + 18.5 J, and (4) + 6.6 K relative to the control's global mean te mperature of 264.7 K. Sea ice is confined to higher latitudes in each experiment compared to the control, with ice-free areas equatorward of the subtropics. The warm SSTs are associated with a 20% reduction in clouds for the rotation rate experiments and higher CO2 concentrations in the other experiments. These results are in contrast to previous s tudies that have used energy balance and radiative convective models. Previous studies required a much larger atmospheric CO2 increase to pr event an ice-covered Earth. The results of the study, suggest that bec ause of its possible feedback with clouds, the general circulation of the atmosphere should be taken into account in under standing the clim ate of early Earth. While higher CO2 concentrations are likely in view of the results, very large atmospheric CO2 concentrations may not be necessary to counterbalance the lower solar constant that existed earl y in Earth's history.