Dietary deficiencies of zinc and iron are a substantial global public health problem. An estimated two billion people suffer these deficiencies, causing a loss of 63 million life-years annually^,. Most of these people depend on C₃ grains and legumes as their primary dietary source of zinc and iron. Here we report that C₃ grains and legumes have lower concentrations of zinc and iron when grown under field conditions at the elevated atmospheric CO₂ concentration predicted for the middle of this century. C₃ crops other than legumes also have lower concentrations of protein, whereas C₄ crops seem to be less affected. Differences between cultivars of a single crop suggest that breeding for decreased sensitivity to atmospheric CO₂ concentration could partly address these new challenges to global health.