"CO2 is plant food"

"Plants, as far as I know, are still, still bending toward the light, and if we dance until the heart explodes, it'll make this place ignite."

~MGMT, "Flash Delirium"

"CO₂ is not a pollutant; it is green plant food."

~Pierre R. Latour

"The current focus on carbon dioxide has nearly run its course, as it has been clearly shown that it is not harmful. In fact, the slight increase in its tiny concentration will assist agriculture and help to feed the starving people of the world."

~Edward B. Boyle, Jr.

Alas, the world is far more complicated. While it's true that a higher concentration of CO₂ would, in a vacuum, improve plant growth, the overall effect on agricultural yields requires a bit of work to tease out.

The changes in climate wrought by more carbon dioxide will improve yields in some areas while hurting yields in others. A recent study published in Science examined the effects of changes in CO₂ concentration, temperature, and precipitation on the yields of the four stapliest of staple crops--corn, wheat, rice, and soy--from 1980 to 2008. They estimated that globally, climate change improved rice and soy yields by 2.9% and 1.3%, while corn and wheat yields declined by 3.8% and 2.5%, respectively. These changes caused global food prices to increase by 6.4%.

Another study concluded that by 2030, 370 million people, mostly those who are already poor and vulnerable, will experience reduced crop yields due to climate change.

Furthermore, higher levels of CO₂ will affect different plants differently. The yellow starthistle, "the worst weed of the West", would grow 600% larger in elevated concentrations of CO₂ compared to only 15% for native plants.

So yes, there is a seed of truth in the claim that carbon dioxide is yummy plant food. But the effect of more CO₂ on agricultural yields is overall a negative one.