Carbon dioxide is the life breath of trees and other green vegetation. At the moment we have a carbon dioxide famine, and this is leading to desertification. It is ironic that the "green" activists are attempting to deprive the world of oxygen producing plants. Water vapour is the major cause of the greenhouse effect, and this is removed from all the reports, and this distorts the results. When a plant is deprived of carbon dioxide, it "gasps for breath" and opens its pores. This leads to the evaporation of water, and a transfer from the ground into the atmosphere. It is easy to understand how this leads to desertification.
There is no lack of co2 in the air. The very fact that it's increasing year over year is because plants can't eat all of it out of the atmosphere. You'll notice that co2 goes up and down in a saw like pattern as it's going up.
If there was a co2 famine the concentration of co2 in the atmosphere would be decreased.
Also I find the oxygen producing role of plants not that useful. The atmosphere is currently 20% oxygen. Even if no more oxygen was produced we've got our supply for thousands of years.
There is no set-point for carbon dioxide.
Under current conditions a sudden increase of carbon dioxide, will cause a lot more plants and plant matter to die due to the change in precipitation patterns and the rate of evaporation.
Yes, plants eat co2, but that doesn't mean that more co2 on a global level is good for plants.
The earths current sensitivity to co2 is affected by many factors. The primary ones are albedo (reflectivity) which is affected by where the landmasses currenlty are on the planet and how much energy the sun is putting out.
Over time the sun has been burning hotter (over the long term, not over the last decade or so).
For most plants co2 is the limiting factor.
There is also another major consideration being missed and that is the rate of change. Plants and ecosystems can't just up and move as the climate changes.
Climate change at the rate we're driving it is past the point where plants can adapt. They're not calling our times the sixth mass extinction for no reason.
I'll also put the whole thing in a simple analogy.
Even though water is necessary for human survival
What a drowning man needs in his throat is not more water
but more air.
Similarly, even though co2 is what plants use as food
what a heat stressed, dried up plant doesn't need more of
is co2, which would change the conditions around the plant
to make its existense impossible.