According to Willis Eschenbach there isn’t enough fossil fuels left in the ground to double CO2 in the atmosphere even if we burned it all by 2100.
Physics says the direct warming caused by a doubling of CO2 is only around 1.2C.
Then there are theories about feedbacks that have yet to be proven.
sunshine hours 
The estimate for total known and potentially, not economically, recoverable fossil fuel reserves are around 5000 Gtc. That’s more than double the estimates used by Bern and comes from various sources familiar with the industry. Just fyi.