Rice grown at higher carbon dioxide (CO2) concentrations, like those possible later this century, has lower nutritional value, according to a study that evaluated rice grown in Japan and China under simulated carbon dioxide increases. Notably, some varieties of rice seemed to react differently to increased levels of CO2, say the study's authors, showing relatively smaller decreases in nutritional value, a finding that farmers and others could take advantage of. The finding that rice's nutritional quality can suffer as atmospheric CO2 concentrations increase has notable implications for populations in regions that rely on rice for primary nutrition. Rice is the primary food source for more than two billion people around the world, a key source of protein and vitamins. Experts interested in the effects of human-caused changes on agriculture crops like rice have typically focused their studies on effects on agricultural production. However, how climate change affects crops' dietary quality has received little attention. With this in mind, Chunwu Zhu et al. conducted multiyear free-air CO2 enrichment (or "FACE") experiments, in which pipes were placed around a specific plot of cropland to control the CO2 level. They did this work in Japan and China to observe the effects of anticipated end-of-century CO2 concentrations on 18 varieties of rice. They confirmed declines in protein, iron and zinc, as well as consistent declines in vitamin B, in all varieties, under increases of this greenhouse gas. The greatest impact and risk of such a result, say the authors, will be to countries consuming the most rice with the lowest GDP. Future studies investigating influences on crop nutritional quality should also look at how other human-caused atmospheric changes - like increasing temperature - could affect rice, Zhu and colleagues say.