Early Solar Output
Early in the Earth's history, the Sun's output would have been only 70% as intense 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 solar luminosity 4 billion (4 × 109) years ago and with greenhouse gas 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 temperature record of the Earth, with the exception of a cold phase, the Huronian glaciation, about 2.4 to 2.1 billion years ago. Water-related sediments have been found that date to as early as 3.8 billion years ago. Hints of early life forms have been dated from as early as 3.5 billion years, and the basic carbon isotopy is very much in line with what is found today. A regular alternation between ice ages and warm periods is only to be found occurring in the period since one billion years ago.
Read more about this topic: Faint Young Sun Paradox
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