Venus is hot from greenhouse effects, but it gets twice the sunlight and it's 'daytime' last a couple of months, it has no magnetic field of note either. Earth couldn't get that hot even if our entire atmosphere was CO2, because we get way less light and our day is 24 - not 2400 - hours long. Nor could it happen, by best estimates, there is literally not enough coal and oil in the world to do such a thing, and at the increased temperature and rainfall such initial warming would bring, plant life would be able to chew up the increased CO2 into plant matter at a far faster rate. The joke on biofuels, that I always like, is that they will eventually become cost effective as the increase in CO2 will let us grow more of it on the same amount of land till eventually that increase growth curve intersects with the rarity increase from fossil fuels.