Sagan and Mullen proposed that the Sun's luminosity (energy flux) was probably low enough to freeze the oceans some 2 billion years ago if the atmospheric composition were the same as now (the 'faint young Sun paradox'), yet for the last 3.8, possibly even the last 4.4, billion years the Earth has had liquid oceans, a fact established from the fossil and marine sedimentary record and the isotopic record of Hadean zircons. The solution to this paradox is that atmospheric compositions have not been constant over geologic time; rather, higher levels of the greenhouse gases carbon dioxide and likely methane were present in the past, especially the Early Precambrian, to generate warming compensating the fainter Sun, even overcompensating, generating higher temperatures than at present.
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