Sorry, I misunderstood the question then. This article goes a bit into it, so might be an interesting lead: http://www.bbc.com/earth/story/20151130-how-hot-cou

Sorry, I misunderstood the question then. This article goes a bit into it, so might be an interesting lead: http://www.bbc.com/earth/story/20151130-how-hot-could-the-earth-get http://www.bbc.com/earth/story/20151130-how-hot-could-the-ea... Since Earth is not like Venus now, this wouldn't have happened as Earth would've lost most of its atmosphere. But Earth have been a fireball very early on, and cooled down from that since solar output was much lower then. As the solar output continue to increase, Earth will become like Venus or a fireball, before it's engulfed. As solar output "only" increased 4% over some hundred of million years it would seem unlikely to occur from greenhouse gases alone.