[1] Standard models of stellar evolution imply that the Sun, early in its history, radiated perhaps a quarter less energy than it does today. [2] Given this reduced output, straightforward calculation predicts that Earth's surface should have been frozen for much of its first two billion years. [3] Yet geological evidence—sedimentary rocks that could only have formed in liquid water—indicates that the early planet was not merely unfrozen but, by some measures, warmer than at present. [4] This discrepancy between predicted and inferred temperatures is known as the faint young Sun paradox.
[5] The most widely accepted resolution invokes the composition of the early atmosphere. [6] If that atmosphere contained far higher concentrations of greenhouse gases—carbon dioxide, methane, or both—than it does now, the enhanced trapping of heat could have compensated for the weaker solar flux. [7] The difficulty is that independent evidence for such concentrations has proven equivocal: some geochemical proxies suggest carbon dioxide levels too low to close the gap unaided. [8] This has prompted supplementary proposals, including a diminished planetary reflectivity, since a young Earth with less exposed continental land and fewer bright clouds would have absorbed a greater fraction of the light that reached it. [9] None of these mechanisms need operate to the exclusion of the others, and the current consensus favors a combination whose precise proportions remain unsettled. [10] What the paradox illustrates, more durably than any single resolution, is that a planet's climate is governed not by the luminosity of its star alone but by the properties of the planet that intercept and retain that light.
