Haqq-Misra Jacob D, Domagal-Goldman Shawn D, Kasting Patrick J, Kasting James F
Department of Meteorology, The Pennsylvania State University, University Park, PA 16802, USA.
Astrobiology. 2008 Dec;8(6):1127-37. doi: 10.1089/ast.2007.0197.
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 CO2 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 CH4, and previous calculations had indicated that a CH4-CO2-H2O greenhouse could have produced warm Late Archean surface temperatures while still satisfying the paleosol constraints on pCO2. Here, we revisit this conclusion. Correction of an error in the CH4 absorption coefficients, combined with the predicted early onset of climatically cooling organic haze, suggest that the amount of greenhouse warming by CH4 was more limited and that pCO2 must therefore have been 0.03 bar, at or above the upper bound of the value obtained from paleosols. Enough warming from CH4 remained in the Archean, however, to explain why Earth's climate cooled and became glacial when atmospheric O2 levels rose in the Paleoproterozoic. Our new model also shows that greenhouse warming by higher hydrocarbon gases, especially ethane (C2H6), may have helped to keep the Late Archean Earth warm.
地质和生物学证据表明，尽管早期太阳较弱，但地球在其大部分早期历史中都是温暖的。来自古土壤数据的太古宙晚期/古元古代（28亿至22亿年前）大气二氧化碳浓度上限表明，当时必定存在其他温室气体。产甲烷细菌在当时可能已经存在，这可能导致大气中甲烷浓度很高，而且此前的计算表明，甲烷 - 二氧化碳 - 水温室效应本可以产生温暖的太古宙晚期地表温度，同时仍满足古土壤对二氧化碳分压的限制。在此，我们重新审视这一结论。对甲烷吸收系数误差的修正，再加上预测的气候变冷有机霾的早期出现，表明甲烷造成的温室变暖程度较为有限，因此二氧化碳分压必定为0.03巴，等于或高于从古土壤获得的值的上限。然而，太古宙时期仍有足够的甲烷变暖现象，能够解释为什么在古元古代大气氧气水平上升时，地球气候变冷并进入冰川期。我们的新模型还表明，更高碳氢化合物气体，特别是乙烷（C₂H₆）造成的温室变暖，可能有助于保持太古宙晚期地球的温暖。
