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

Abstract

Solar energy at the top of the atmosphere (solar constant), rotation rate, 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 concentration, and reduced solar constant is presented. The control simulation of this study has a solar constant reduced by 10% the present amount, zero land fraction using a swamp ocean surface, CO2 concentrations of 330 ppmv, present‐day rotation rate, and is integrated under mean diurnal and seasonal solar forcing. Four sensitivity test are performed under zero land fraction and reduced solar constant conditions: (1) increased rotation rate, (2) 8 times present CO2, (3) a combination of 1 and 2, and (4) 3 with a 15% reduction in the solar constant. Results indicate the following changes in the global mean sea surface temperatures (SSTs) compared to the control simulation: (1) + 6.6 K, (2) + 12 K, (3) + 18.5 K, and (4) + 6.6 K relative to the control's global mean temperature 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 studies that have used energy balance and radiative convective models. Previous studies required a much larger atmospheric CO2 increase to prevent an ice‐covered Earth. The results of the study, suggest that because of its possible feedback with clouds, the general circulation of the atmosphere should be taken into account in understanding the climate 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 early in Earth's history.