PerspectiveAtmospheric Science

Countering the Early Faint Sun

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Science  04 Jun 2010:
Vol. 328, Issue 5983, pp. 1238-1239
DOI: 10.1126/science.1189196

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In 1972, Carl Sagan and George Mullen noted the earlier discovery that our Sun should have been around 30% less luminous 4.5 billion years ago, and examined the consequences for primordial Earth (1). Main-sequence stars like the Sun burn hydrogen to helium, and as they do the star becomes more luminous. Sagan and Mullen showed that this solar evolution meant that Earth's current 33° of greenhouse warming, primarily from water vapor and carbon dioxide (CO2), could not have kept Earth's average surface temperatures above freezing prior to around 2 billion years ago (see the figure). The abundant evidence for liquid water at Earth's surface before that date, they argued, constitutes what is now called the “early faint Sun paradox.”