This so-called faint young sun paradox, first identified by astronomer Carl Sagan in the early 1970s, has flummoxed many a scientist—how could Earth be so temperate without a strong sun, when it should have been frozen over?

Now a new computer simulation published in the July issue of the journal Astrobiology has lent strong support to an old solution to the paradox. According to the authors of the study, Eric Wolf and Brian Toon of the University of Colorado at Boulder, the ancient Earth could have been kept warm by high atmospheric concentrations of carbon dioxide and methane. Those greenhouse gases compensated for the dimmer sun by trapping more of its heat in the atmosphere.

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