The faint young Sun problem

The Sun was almost certainly much less bright during the early stages of its main-sequence lifetime. In the absence of some compensating factor, the Earth's mean surface temperature would therefore have been below the freezing point of water prior to ~2 Gyr ago. Geologic evidence for liquid water as early as 3.8 Gyr ago implies that the Earth was never this cold. This discrepancy can be resolved if the greenhouse effect of the early atmosphere was much larger than today. The most likely cause of an enhanced greenhouse effect is an increase in atmospheric carbon dioxide (CO₂) concentrations by a factor of ~1000 or more compared to today. Such an increase could have resulted from feedbacks inherent in the carbonate-silicate geochemical cycle that controls the atmospheric CO₂ level over long time scales. -from Authors