While photosynthesis does pull carbon dioxide out of the atmosphere, much of that gas goes right back into the air. The reason: At night, the chemical reactions in plants essentially run backward. In a process known as respiration, plants pump out carbon dioxide instead of pulling it in.

“Part of the story is that photosynthesis is going up, and part of the story is that so is respiration,” said Dr. Campbell.

While the increase in photosynthesis is greater than that of respiration, the ultimate benefit to crops has been small — and it doesn’t explain our modern agricultural revolution.

“The driving factor has to be the fertilizers, the seed varieties, the irrigation,” Dr. Campbell said.

Extra Carbon Dioxide Can Make Plants Less Nutritious

A number of studies indicate that plants that grow in extra carbon dioxide often end up containing lower concentrations of nutrients such as nitrogen, copper and potassium.

As more carbon dioxide gets into the atmosphere, the problem will grow. “There’s definitely strong evidence that quality will be affected,” said Dr. Campbell.

It’s not clear why this happens. In a paper published in the journal Current Opinion in Plant Biology in June, Johan Uddling of the University of Gothenburg in Sweden and his colleagues speculated that microbes are to blame.