“Greening” can be a good thing short-term and only under certain environmental circumstances, but it is generally considered to be a bad thing long-term and on a global scale, due to increases in plant respiration that correspond proportionally to increases in photosynthesis. Greening effect also has an upper limit, since plants stop absorbing significant amounts of carbon dioxide as temperatures hit certain thresholds. Forests in particular could lose ~50% of their capacity to absorb carbon dioxide if temperatures get to high, and we’ve already seen evidence of this in both observational and simulated data. Not to mention the agricultural effects of greening on a global scale are pretty bad, as those conditions favor non-food-bearing plants.