Faint Young Sun Paradox
						q Solar luminosity was much weaker (~30%) in the early part of  Earth’s history (a faint young Sun). q If Earth’s albedo and greenhouse effect remained unchanged at that time, Earth’s mean surface temperature would be well below the freezing point of water during a large portion of its 4.5 Byr history. q That would result in a “snowball” Earth, which was not evident in geologic record.