Large concentrations of atmospheric CO2 in the atmosphere of the early earth have been proposed as a possible explanation of the apparent absence of frozen earth in spite of a faint early sun. However, the most thorough treatments of this question, by Owen et al. (1979) and Kasting et al. (1984), apparently disagree as to the warming effects of large amounts of CO2. We recalculate the evolution of surface temperature over the last 4.25 billion year time period, using the same scenario for CO2 partial pressures and solar constant as employed by the previous authors. We find good agreement with Kasting et al. (1984) and also explain why the results of Owen et al. are at variance with our findings and those of Kasting et al. Using the concept of direct radiative forcing, we present analytic relations between the solar luminosity and CO2 mixing ratio required to maintain the troposphere close to its present thermal structure. As a by-product, we present new broadband parameterizations for the 961 and 1064 cm−1 CO2 bands that can be used in climate models. We also consider the direct radiative forcing of large amounts of CH4, or changes in model clouds, and discuss how these might reduce the CO2 mixing ratio required to balance the faint early sun.