The benefits of increased atmospheric CO2 on crops are so extensive that a long article or book needs to be written to do justice to the subject and to the results of thousands of research trials. The improvement in photosynthesis efficiency at higher CO2 levels does not tell the whole story, but it is a good place to start, since all plant growth relies on this process.
Photosynthesis is the process by which plants utilize visible light energy (e.g. sunlight) to convert aerial CO2 and water (from roots) into plant matter. This process also requires phosphorus and nitrogen.
There are three photosynthesis ‘pathways’, known as C3, C4 and CAM. CAM is unimportant for food crops, being the method used by cacti, succulents and agaves. Pineapple is the only food crop of any importance to use CAM, so CAM can be neglected for the present purposes. World food security depends on C3 and C4 photosynthesis.
Less than 1% of all plant species in the world use the C4 photosynthesis pathway. Of the 86 plant species that supply most of the world’s food, only five use the C4 photosynthetic pathway, of which only four are of much importance (corn [=maize], sorghum, millet, and sugarcane) yet these four constitute some 20% of all the food crops grown. Because of their high photosynthetic efficiency, the C4 crops corn and sugarcane are favoured for ethanol production by those who want to produce liquid biofuels rather than food, thus increasing food prices and poverty.