6 M ay 2 01 1 Faint young Sun paradox remains

The Sun was fainter when the Earth was young, but the climate was generally at least as warm as today; this is known as the ‘faint young Sun paradox’. Rosing et al. [1] claim that the paradox can be resolved by making the early Earth’s clouds and surface less reflective. We show that, even with the strongest plausible assumptions, reducing cloud and surface albedos falls short by a factor of two of resolving the paradox. A temperate Archean climate cannot be reconciled with the low level of CO2 suggested by Rosing et al. [1]; a stronger greenhouse effect is needed. During the Archean eon, the Earth received 76% to 83% of the energy from the Sun that it does today. If the Earth’s greenhouse effect and albedo were the same as now, the Earth would have been in continual deep freeze until one billion years ago, with glaciers reaching the Equator. However, Archean glacial sediments are rare and geological evidence indicates that the Archean was typically warmer than today (we are in a glacial period now). With the amount of energy reaching the Earth given by F = 1 4 (1 − α) = 239Wm (using the present-day solar constant S = 1368Wm and albedo α = 0.3), the radiative deficit in the Archean would have been (1 − 0.79)F < 50Wm. Resolution of the ‘faint young Sun paradox’ requires a positive radiative forcing—from reducing the albedo or increasing the greenhouse effect— of more than 50Wm.