It depends a little on who you ask. The original number was 550 ppm (parts per million). more recently however, scientists have realized that the number is actually probably much lower than that. (If we go above 550 we are probably in some REALLY serious trouble). Some say that 450 ppm is still too high and that the safest upper limit is really 350 which we have already surpassed. Some argue that with aggressive efforts we could get back down below 350 but being that we are still pumping out CO2 not curbing it that is going to be a real challenge.
For it to be lethal, CO2 must be in concentrations of 50,000ppm or higher, generally in an enclosed area. During the Cretaceous, CO2 levels were as high as 1,000ppm (about 3 times the present level). Despite that, life continued and thrived during the Cretaceous; the CO2 gave rise to all manner of unique plant life.
It’s debatable how much CO2 is too much in terms of climate change, but I would wager that concentrations could return to the Cretacean levels without too many repercussions. Climate systems tend to find a way to balance themselves, the Earth is very resilient.
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