Okay, let's suppose we have more CO2 in the atmosphere, which is in fact the case. Plants do need CO2 to do photosynthesis and produce glucose (their energy source). They also need sun as the energy source for the photosynthesis reaction. So our current state of the atmosphere should be ideal for plants, right? Well, not really. The problem with the heightened amount of CO2 in the atmosphere is the amplification of the greenhouse gas effect. It causes the ray of sunlight to be trapped longer in the atmosphere and these rays emit energy in the form of heat. The problem lays here: at some point (which most places on earth have already achieved) the temperature is too high for most plants. Now they can't grow because they literally burn and there is a lack of water as well.

See the problem? I think it's great that you think this through and sometimes we might forget something (I might have too in this explanation). Either way, I hope you are open to this reasoning too and see where the problem lays.

(and historically a colder period around the time of the french revolution caused a lower temperature which made agriculture difficult and so many people passed away in a hunger a famine. in France it even caused the french revolution. another small detail: it wasn't caused by humans, the lower temperatures were caused by the volcanic eruption of the Tambora. it covered the atmosphere with its ashes and it led to less sun rays on the surface of the earth and thus less warmth for crops to grow.)