And just how do you know that we've used HALF of the earth's supply of fossil fuels? Is there any scientific way to determine that, given we haven't even explored in many vast areas of the globe?
Technology confuses how much fossil fuel remains. We've just started exploiting oil shales, for example. If we exploit undersea methane deposits, there will be more fossil fuels to burn, increasing "what remains".
It is my guess that half the OIL accumulated has been burned to the atmosphere. (Oil that took billions of years to accumulate—all gone in a geological "poof" in time).