Reducing CO2 levels to per-industrial levels would reduce tree growth in the Northern hemisphere by 30% and farming yields by 12 to 13%.
CO2 is plant food.
CO2 is Green... and Green is Good!
More CO2 in the air means more plant growth.
Earth’s current atmospheric CO2 concentration is almost 390 parts per million (ppm). Adding another 300 ppm of CO2 to the air has been shown by literally thousands of experiments to greatly increase the growth or biomass production of nearly all plants. This growth stimulation occurs because CO2 is one of the two raw materials (the other being water) that are required for photosynthesis. Hence, CO2 is actually the “food” that sustains essentially all plants on the face of the earth, as well as those in the sea. And the more CO2 they “eat” (absorb from the air or water), the bigger and better they grow (see table below).
http://www.plantsneedco2.org/default.aspx?MenuItemID=103