Few will argue that we can put CO2 into the atmosphere at present rates
forever, either we'll run
out of fossil fuels, or there'll be a point where adding further CO2 clearly will be the more expensive option, and in the extreme (there's plenty
of carbon in the Earth's crust, and failing that the
solar system) it'll turn the Earth into Venus eventually, and probably before that the CO2 itself would start getting toxic (at a few ten thousand ppm it ought to get to levels that'll kill us).