How Much Warming Could Happen as a Result of Global Warming?

The IPCC estimates that there are 18,635 gigatons of carbon in the Earth's fossil fuel reserves. The question is not how much fossil fuel there is, but how much might ultimately be recoverable with future technology. Releasing 3,000 gigatons would take us to carbon dioxide concentrations in the atmosphere of around 2,000 parts per million. For comparison, pre-industrial carbon dioxide concentrations were 278 parts per million and current concentrations are 415 part per million. Given this, how much warming could happen as a result?