AR6, Working group 1 report, chapter 2. Relevant section starts at page 365:
https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6...
The conclusion is the same, though they've added a paragraph talking about browning in some areas "somewhat slowing" the rate of aggregate increase since the late 90s. Conclusion is unchanged, and in fact, they strengthened it versus the draft by directly attributing it to CO2:
"The increased greening is largely consistent with CO2 fertilization at the global scale, with other changes being noteworthy at the regional level (Piao et al., 2020)"
> So it sounds like a combination of the CO2 increases up to about the year 2000, along with agricultural intensification and various other factors have indeed increased the amount of plant cover, but we are already seeing changes to that picture with further rises to CO2 levels.
Not really. The observations are also made in uninhabited areas. See above.
> And my central point is that the model you're implying there is one in which there's a monotonic relationship between CO2 levels and plant growth.
I said nothing about a monotonic relationship. I said that the earth will have more plants (plant mass, really) with more CO2. This is inevitable. It could follow a monotonic relationship, or it could do something else as factors shift. For example, one big, unpredictable factor that likely swamps everything else, is the randomness of human behavior.
> However in reality things are clearly more complex than that, and there is indeed legitimate argument over what factors are dominant in different scenarios.
No. Greening is occurring, and has been for some time. We have multiple lines of evidence. The IPCC report confidence is high. The only debate is over what might happen in the future, which, again, is fortune telling -- involving not only the climate system, but the actions of people.
> In systems with significant feedback loops it seems dangerous to assume that changes will only happen slowly unless you're very confident that you fully understand all the system dynamics.
I grant you that one can imagine theoretical scenarios in which all sorts of doomy feedback loops happen. The problem with that kind of imaginative exercise is that you have to bring evidence of their existence. So far, with regard to global vegetation, no such evidence exists, and in fact, the opposite of the doom loop scenario is occurring.
Could this change? Maybe! But that's just storytelling right now.