The planet would be in serious danger of burning up. One possible answer is the biological production of methane by bacteria. We know that climate has changed a great deal in the past, producing episodes of glaciation, and could warm significantly in the future, due to atmospheric pollution. However, climate change could be much more extreme. At least for the past billion years it is unlikely that the earth's temperature was more than 15 degrees warmer or 5 degrees colder than it is today.
Earlier temperatures are very uncertain.
Calculations also suggest that under this faint early sun, the earth should have been a frozen ball. However, life arose under these conditions, and there is geological evidence of flowing water from this time. It has been suggested that a kind of greenhouse warming was in effect at that time, involving such gases as methane, ammonia, and carbon dioxide, and that this is evidence of a kind of Gaian planetary temperature control mechanism. Lovelock and Margulis argue that life solved this one, also. A warming earth stimulated greater plankton production, removing CO 2 from the atmosphere.