Radiative Transfer in the Early Atmospheres of Mars and Earth

Introduction: The early climates of Earth and Mars appear to have been relatively warm, despite the existence of a less luminous young Sun [1]. Most attempts at reconciling these apparently contradicting observations rely on atmospheres that are richer in greenhouse gases (GHGs) than the present atmospheres of either planet [2]. The major contribution to most plausible greenhouses is from CO2 and H2O vapor. However, paleo-pCO2 reconstructions from Proterozoic paleosols [3, 4] fall several to several hundred times short of the amount of CO2 required for sustained habitability over Earth’s history [5], whereas on Mars CO2 condensation complicates matters by decreasing the tropospheric lapse rate and forming CO2 clouds that scatter solar radiation [6]. While these clouds also scatter IR radiation [7], the sign and magnitude of the forcing they provide remain under debate [8, 9]. Other infrared absorbers have been suggested, such as SO2 [10, 11] and CH4 [12], each with their associated difficulties. The fundamental question, whether a CO2-H2O greenhouse can account for the observations or whether other absorbers are necessary, is compounded by uncertainty in radiative transfer through CO2-rich atmospheres. Using a line-by-line (LBL) radiative transfer model, we show that small differences in the formulation of absorption by CO2 result in very large differences in radiative forcing when applied to atmospheres containing 0.1 to 5 bars of CO2. Because band models and the more sophisticated three dimensional models in which they are embedded depend on the results of LBL calculations, this uncertainty pervades any attempt at modeling the early climate of Mars and Earth.