Scientific American published a great article on the topic in which they stated;
In less than 1,000 years, if consumption continues to increase at the
current rate, we will have exhausted the currently known reserves of
coal and oil. By that time we will have multiplied the carbon dioxide
tonnage of the air 18 times. When the ocean–atmosphere system comes back
to equilibrium, the concentration of carbon dioxide in the air will be
10 times greater than it is today, and the earth will be 22 degrees
warmer. In another few thousand years, when the carbonate content of the
oceans has reached equilibrium, the concentration will still be four
times greater than it is today. The earth's temperature will then fall
to about 12.5 degrees above its present average.'
So, things look to warm up quite a bit, but will not be terribly punishing to humanity for a number of centuries. [Note: Most of the doom and gloom predictions assume that nothing changes and that we will all continue to throw CO2 into the air with wild abandon. Yet in all likelihood, humanity will have managed to kill off itself much sooner than that]! There exists an elephant standing right in the middle of our room that everyone is ignoring. Please see my post on the coming Water Wars for what is even more likely to happen in just a few decades!