This newest paper builds on a major study published in Nature in 2014, finding that elevated levels of carbon dioxide reduced the amount of zinc and iron found in wheat, rice, field peas and soybeans. In both studies, researchers installed pipes that emitted carbon dioxide onto small open-air plots — rather than simply testing crops in enclosed greenhouses — to simulate future real-world conditions.

T he finding that extra carbon dioxide can make crops less nutritious may sound counterintuitive. Plants, after all, rely on carbon dioxide as an ingredient for photosynthesis, so it seems like more CO2 should be beneficial, helping them grow. But what scientists have also found is that the chemical composition of a plant depends on the balance of the carbon dioxide it takes in from the air and the nutrients it absorbs from the soil. Upset this balance, and the plant can change in unexpected ways.

In plants like rice and wheat that undergo what is known as C3 photosynthesis, higher levels of carbon dioxide may spur plants to produce more carbohydrates, which dilute some of the more nutritious components. But scientists are still trying to understand exactly why some compounds, like vitamin B, get diluted and others don’t, or why some varieties of rice see sharper declines in vitamin B than others.

With further research, scientists might try to breed or genetically engineer new crop varieties that preserve much of their nutritional value in the face of rising carbon dioxide. But this could prove challenging, Dr. Ziska said, given that all of the tested rice lines in their study showed significant declines in vitamin B.

“We still don’t understand why some plant genotypes show a bigger response to higher levels of carbon dioxide,” said Andrew Leakey, a crop biologist at the University of Illinois who was not involved in the latest study. “And that’s important if we want to move from understanding the problem to solving it.”