Faint young Sun paradox

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

Early solar output

Early in the Earth's history, the Sun's output would have been only 70% as intense during that epoch as it is during the modern epoch. In the then current environmental conditions, this solar output would have been insufficient to maintain a liquid ocean. Astronomers Carl Sagan and George Mullen pointed out in 1972 that this is contrary to the geological and paleontological evidence.

According to the Standard Solar Model, stars similar to the Sun should gradually brighten over their main sequence lifetime. However, with the predicted ago and with concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would freeze. However, the geological record shows a continually relatively warm surface in the full early of the Earth, with the exception of a cold phase, the , about 2.4 to 2.1 billion years ago. Water-related...
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