[RedDevil]

Originally Posted by jamesqf

What this is overlooking is the temperature dependence of photosynthesis. C3 efficiency starts dropping around 25C, and shuts down entirely around 45C. C4 photosynthesis works at a bit higher temperature, peaking around 35C, but even it shuts down around 55C: https://kuensting.org/school/bb/cell...osynthesis.htm

So you put enough CO2 into the air to raise temperatures into this range (even if only in the tropics), and then goes a chunk of the biosphere. And of course it's a positive feedback: fewer plants recycling CO2 means more goes into the atmosphere, which means more temperature increase, which means even less photosynthesis...

There also is a dependency to CO₂ levels; higher CO₂ allows photosynthesis at higher temperatures in C3 plants and reduces their water usage.

So on the one hand CO₂ raises the global temperature which will increase the size of regions where temperature is the limiting factor on photosynthesis, on the other hand it allows some plants to make use of conditions that would otherwise impede them. Provided nutrients suffice.