How is something critical if it has been transgressed and we are still alive? Or on the very first moment every single one is past we all suddenly die?
> Had Earth system remained forced by 1988 conditions (350 ppm and 85%/50%/85% of tropical/temperate/boreal forest cover remaining), the simulations show that temperature over the global land surface would not have increased by more than an additional 0.6°C in the subsequent 800 years
> If climate and land system change can be halted at 450 ppm and forest cover retained at 60%/30%/60% of boreal/temperate/tropical natural cover, then the simulation indicates a mean temperature rise over land of 1.4°C by 2100 (in addition to 0.7°C between preindustrial time and 1988) and 1.9°C after 800 years as vegetation evolves in a warmer climate
The latter is an optimistic projection assuming we will do more to stop climate change, the paper goes further into the odds of a >3C increase.
To put that in context, a 1.5°C increase in average temperature is considered a doomsday scenario where wildfires and storms ravage the earth, killing over half of the global population.