Radiative forcings for 28 potential Archean greenhouse gases release_6s4stkvekndthk3ung2ukxubhq

Abstract

<strong>Abstract.</strong> Despite reduced insolation in the late Archean, evidence suggests a~warm climate which was likely sustained by a stronger greenhouse effect, the so-called faint young sun problem (FYSP). CO<sub>2</sub> and CH<sub>4</sub> are generally thought to be the mainstays of this enhanced greenhouse, though many other gases have been proposed. We present high accuracy radiative forcings for CO<sub>2</sub>, CH<sub>4</sub>, and 26 other gases, performing the radiative transfer calculations at line-by-line resolution and using HITRAN 2012 line data for background pressures of 0.5, 1, and 2 bar of atmospheric N<sub>2</sub>. For CO<sub>2</sub> to resolve the FYSP alone at 2.8 Gyr BP (80% of present solar luminosity), 0.32 bar is needed with 0.5 bar of atmospheric N<sub>2</sub>, 0.20 bar with 1 bar of atmospheric N<sub>2</sub>, or 0.11 bar with 2 bar of atmospheric N<sub>2</sub>. For CH<sub>4</sub>, we find that near-infrared absorption is much stronger than previously thought, arising from updates to the HITRAN database. CH<sub>4</sub> radiative forcing peaks at 10.3, 9, or 8.3 W m<sup>−2</sup> for background pressures of 0.5, 1, or 2 bar, likely limiting the utility of CH<sub>4</sub> for warming the Archean. For the other 26 HITRAN gases, radiative forcings of up to a few to 10 W m<sup>−2</sup> are obtained from concentrations of 0.1–1 ppmv for many gases. For the 20 strongest gases, we calculate the reduction in radiative forcing due to overlap. We also tabulate the modern sources, sinks, concentrations, and lifetimes of these gases and summaries the literature on Archean sources and concentrations. We recommend the forcings provided here be used both as a first reference for which gases are likely good greenhouse gases, and as a standard set of calculations for validation of radiative forcing calculations for the Archean.
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