My understanding is that all the Carbon which is fixed and stored under the ground in petroleum, coal, natural gas, and other "fossil" fuels was in the air of the earth as CO2 before it was fixed by plants and buried. So it would seem that even with ALL the fossil fuel carbon in the atmosphere, the earth supports life. Considering the adaptability of human life, especially with modern technology, I would be surprised if it was concluded that humanity would be wiped out by this.