The faint young Sun paradox or faint young Sun problem describes the apparent contradiction between observations of liquid water early in Earth‘s history and the astrophysical expectation that the Sun‘s output would be only 70 percent as intense during that epoch as it is during the modern epoch.[1] The issue was raised by astronomers Carl Sagan and George Mullen in 1972.[2] Proposed resolutions of this paradox have taken into account greenhouse effects, astrophysical influences, or a combination of the two.

An unresolved question is how a climate suitable for life was maintained on Earth over the long timescale despite the variable solar output and wide range of terrestrial conditions.