About 3 billion years ago, the Sun was only 80 percent as luminous as it is currently  Such conditions today would result in the freezing of Earth' s oceans, but geological evidence shows that water rather than ice filled the oceans at that time  Heat is trapped within Earth' s atmosphere through the presence of carbon dioxide, which, like methane, is a greenhouse gas   Only if the level of greenhouse gases were higher 3 billion years ago than it is today would Earth have retained enough heat to keep the oceans from freezing
It is likely, ttherefore, that the level of carbon dioxide in the atmosphere was significantly higher then than it is today
Sufficient heat to keep the oceans liquid 3 billion years ago could not have been generated through geological processes such as volcanic activity.