One of the dumber but more-persistent climate change memes is based around the claim that CO₂ is plant food. Plants rely on it for photosynthesis, so increasing its level in the atmosphere should benefit plants by boosting their growth. There are variants on this that claim plants are currently starved for carbon dioxide, while others suggest plants will grow so fast that they’ll keep the carbon from warming the atmosphere much at all.

Fortunately, if you search the Web for “carbon dioxide is plant food,” all of the early hits you get are from people and organizations debunking the idea. While CO₂ can help plants and may have contributed to a general greening of the Earth, it’s just one of a number of factors that influence plant growth. And a new study shows that, even when trees are clearly benefitting from the rising levels of atmospheric carbon, they don’t seem to be growing any faster.

Ask the trees

Carbon dioxide is incorporated into sugars through the action of a rather inefficient enzyme, so simply having more of it around makes photosynthesis run smoothly. When there’s more around, the pores that allow it into leaves can also close up a bit more, which conserves water. For these reasons, carbon dioxide has a fertilizing effect on many plants.