Question: how long would it take for the earth to run out of fuel?

  1. If you mean geothermal energy, the answer would be “a very very very long time”. There is WAY more heat in the Earth than we could ever use. If you mean other types of fuels like oil, gas and coal, the answer is probably more like a few decades or perhaps a few hundred years. Its very hard to know because we haven’t actually discovered all of these fuels yet, so we don’t know how much is there. But its clear that these things are going to run out within a few human lifetimes, so we need to find alternatives.