Radiative transfer in CO2-rich paleoatmospheres

Calculations of radiative transfer in CO2-rich atmospheres are central to modeling the early climate of Earth and Mars. Line-by-line radiative transfer models are the most accurate means of carrying out such calculations and provide the basis for all other radiative transfer approaches. We examine the sensitivity of line-by-line results to three parameterizations of line and continuum absorption by CO2, all of which yield essentially identical radiation fluxes at low CO2 abundance. However, when applied to atmospheres containing 0.1-5 bars of CO2, appropriate for early Earth and Mars, the outgoing longwave radiation calculated with the three parameterizations differs by as much as 40 W m(-2). In addition, the choice of parameters for CO2 absorption affects the sensitivity of the calculations to infrared absorption by trace gases other than CO2. Our findings imply that previous estimates of the amount of CO2 required to maintain relatively warm temperatures throughout Earth's early history and during episodes in the early history of Mars are highly uncertain. Despite these uncertainties, we conclude that early Mars probably required other infrared absorbers to reach super-freezing surface temperatures, while for the early Earth, this is not necessarily the case.