Faint Young Sun Paradox The luminosity of the Sun has increased as it has aged; 3.8 billion years ago the Sun was ~25 % fainter than today. This is a conundrum because there was liquid water on the Earth at least 3.7 billion years ago and a simple argument leads to a prediction for what is referred to as the Equilibrium Temperature, Te for the Earth which at that time, would be below the freezing point of water, Te = -40 C!. Note that Te is determined by simply finding the temperature for the Earth where it radiates exactly the same amount of energy per second as it receives from the Sun in the absence of clouds and an atmosphere. Further, if we were to include an atmosphere with the composition of our current atmosphere, the temperature would rise but would still be less than the freezing point of water.

The answer to the question of then, why do we have liquid oceans? requires that our atmosphere in the past had a much different chemical composition than today so that the Greenhouse Effect could maintain liquid oceans or, perhaps, the Sun was much brighter in the past than we now believe.