Jim Hansen himself has addressed this. Even with slightly generous estimates of remaining oil and gas reserves, the total level of global warming will be bad assuming we use them all up, but not catastrophic.

The really big problem is coal. We DO have enough of it to do tremendous damage (push natural geological/biological feedforwards into a big acceleration), and it looks like we are exactly on track to do so.

The back-to-the-future substitution of coal for depleting oil and gas will accelerate. That's worse even per joule/BTU of raw combustion energy because there's no hydrogen in coal, unlike oil and gas---all the energy comes from oxidization of carbon, resulting in greenhouse emissions.

And then there's the extra energy inefficiency of coal-to-liquids, coal-to-gas etc, requiring additional primary energy to get the useful end product to substitute for gas and oil.