Burning all the fossil fuel reserves we have (know about), would add over 200 ppm CO2 to the atmosphere (mostly from 1 trillion tons of coal). The additions from oil and gas are smaller. We'd then have over 500 ppm CO2.
"Recoverable" coal is about 20 times proven reserves, so burning all that would add 4000 ppm CO2. Now the total would be 4500 ppm CO2, or almost 0.5 percent! Likely to be quite unhealthy.
All the while you’re doing that, plant life would be booming, and after a sufficient period of time would capture enough of that CO2 again to make it possible for non-plant life to once again flourish.
"A sufficient period of time" is underselling it a little. Coal is only formed if wood is buried without rotting first. If we burned all the coal we can find, it would take tens of millions of years to be replaced.
I had a vague impression that the processes that produced fossil fuels would probably not renew in due time, because the existing layers have something to do with the inability of microbes to break down lignin long ago. Once the capability evolved, the same process can't repeat even if one waits a long time?
"Recoverable" coal is about 20 times proven reserves, so burning all that would add 4000 ppm CO2. Now the total would be 4500 ppm CO2, or almost 0.5 percent! Likely to be quite unhealthy.
https://knoema.com/infographics/smsfgud/bp-world-reserves-of...
https://inspectapedia.com/hazmat/Carbon_Dioxide_Hazards.php