By Capt. Paul (Eclogite) on Sunday, April 23, 2006 - 09:31 pm:
True Walter, the sun is getting more powerful with age. One of the big questions right now in geology is: if the sun has been getting brighter, what were conditions like in the Archean on Earth? If the sun was cooler then, earth's average temperature should have been 20+ degrees cooler than today and Archean oceans should have been frozen solid. However, we see fossil evidence that liquid water has existed since at least 3.8 GA, so the earth could not have been this cold. This conflict between calculation and observation is called the "Faint young sun paradox".
So if the sun 'was' less bright then, why don't we see evidence for frozen water? The answer lies in the fact that early in earth's history before the arrival of photosynthetic lifeforms the atmosphere was enriched in carbon dioxide, producing a greenhouse effect. This grennhouse effect would have increased the surface temperature of the earth enough to counter the effects of a 'faint young sun' and thus prevented the icing of liquid water.
Or so the theory goes........