CURRENT models for the evolution of the Sun require an increase in solar luminosity by 25% since the formation of the Solar System1. Such an increase in the solar constant should have profound effects on the terrestrial climate, but there is no evidence from the fossil record of a corresponding change in the Earth's global mean temperature2. This apparent conflict cannot be explained by the apparent inability of solar models to account for the low observed neutrino flux3. Even models that are forced to fit the neutrino data require a similar increase in the solar luminosity. As Newman and Rood1 state: ``a faint young Sun is one of the most unavoidable consequences of stellar structure considerations''. We discuss here whether CO2-H2O in a weakly reducing atmosphere could have caused this change in the early Earth's temperature by the so-called greenhouse effect.