I tried adding a similar caveat to my message, but my feeble attempts muddied my simple message. Even your caveat is slightly over-simplified because there's a significant time component: most ecosystems would probably be OK if the oil was burned over a few million years rather than a few decades.
Yes. Oil and gas and coal proven reserves of burned would produce about 4.5 trillion tons of CO2. About half of that would remain in the atmosphere. There’s currently about 2 trillion tons of CO2 in the atmosphere today at 400ppm. So in proven reserves alone, we have enough to more than double the current CO2 levels, or triple/quadruple them from pre-industrial levels (when they were ~280ppm).
...but the real kicker is that proven reserves are just a small fraction of total likely recoverable reserves long-term. Proven reserves basically assumes no tech improvement and no exploration, two things we’re continually doing. So proven reserves have actual GROWN over time and likely will. There are truly vast amounts of fossil fuels available that could be tapped if we had to tap them.
For instance, there’s over a trillion barrels of kerogen/oilshale oil (which is expensive to extract, like Canada’s oil sands) in JUST the Green River formation in Colorado/Wyoming, more than all the world’s proven oil reserves combined. But it sits untapped because oil prices are so low. And practically all of northern Alaska has coal underneath it if you drill to the right depth, 5 trillion tons (~18 trillion tons of CO2), more than four times all the world’s coal proven reserves combined. Source: https://pubs.usgs.gov/dds/dds-077/
America alone has enough fossil fuels in the ground to single-handedly double or triple or sextuple the CO2 level. Add in similar fossil fuel amounts in much of Africa, South America, Russia, Canada, Antarctica, Australia, and the world’s oceans, and the planet would be unrecognizable.
There’s enough fossil fuels (if we tried hard enough) to get the CO2 levels in the 10,000 ppm range where there are serious long term health effects from just breathing the stuff. And psychological effects start becoming apparent at just 500-1000ppm, which we will see by the end of the century or so.
So yeah, we’ll run out of atmosphere to dump the CO2 into before we run out of fossil fuels.
Yes. We have more than twenty years of fossil fuel reserves. We need to be carbon neutral in less than twenty years if we want to stay below 2° of warming.
