A Revised, Hazy Methane Greenhouse For The Archean Earth

摘要

Geological and biological evidence suggests that Earth was warm during most of its early history, despite the fainter young Sun. Upper bounds on the atmospheric CO_2 concentration in the Late Archean/Paleoproterozoic (2.8-2.2 Ga) from paleosol data suggest that additional greenhouse gases must have been present. Methanogenic bacteria, which were arguably extant at that time, may have contributed to a high concentration of atmospheric CH_4, and previous calculations had indicated that a CH_4-CO_2-H_2O greenhouse could have produced warm Late Archean surface temperatures while still satisfying the paleosol constraints on pCO_2 Here, we revisit this conclusion. Correction of an error in the CH_4 absorption coefficients, combined with the predicted early onset of climatically cooling organic haze, suggest that the amount of greenhouse warming by CH_4 was more limited and that pCO_2 must therefore have been ≥0.03 bar, at or above the upper bound of the value obtained from paleosols. Enough warming from CH_4 remained in the Archean, however, to explain why Earth's climate cooled and became glacial when atmospheric O_2 levels rose in the Paleoproterozoic. Our new model also shows that greenhouse warming by higher hydrocarbon gases, especially ethane (C_2H_6), may have helped to keep the Late Archean Earth warm.