Add one more consequence to the ever-growing list of climate-change repercussions: a threat to human nutrition. In a study released today in Nature, researchers reveal that rising CO2 levels are significantly reducing the amount of two critical dietary nutrients—iron and zinc—in crops. Some two billion people already have iron and zinc deficiencies, and now climate change could make the problem worse.
The lead author of the study, Samuel Myers of the Harvard School of Public Health, has long studied the intersection of human health and global environmental change. The catalyst for this study was an article Myers wrote that led him to some fairly esoteric literature on the impacts of CO2 on crops grown in open-top chambers. But these studies that Myers stumbled across were criticized for growing crops in artificial conditions, and, in any case, they had small sample sizes that made their data difficult to extrapolate to a global scale.
So Myers and his colleagues set up a huge study, building a coalition of researchers around the world to collect data on the problem. In seven locations over six growing seasons, collaborators conducted free-air CO2 enrichment (FACE) experiments (a method that involves increasing CO2 concentrations in the air surrounding the crops). CO2 levels in the growing sites were raised to 546 to 586 parts per million—the dangerous level of atmospheric CO2 concentrations that humans could reach by 2050.
Agronomists participating in the FACE experiments then sent grains and seeds from their crops to the researchers for analysis. In total, they looked at data from 41 different cultivars of legumes and grains in the U.S., Australia, and Japan.
"We found that rising concentrations of CO2 are threatening global nutrition by reducing levels of nutrients in food crops like rice, wheat, and soybeans—significant reductions of zinc, iron, and protein," says Myers. In wheat grains grown at the FACE sites, for example, the team discovered that levels of zinc, iron, and protein dropped by 9.3 percent, 5.1 percent, and 6.3 percent compared to crops grown normally.
Myers and his team studied zinc and iron specifically because they knew the grains and legumes contained significant amounts of the nutrients. But it's certainly possible that all sorts of vitamins could be affected by CO2 levels, he says: "What you want to do is identify the foods that serve as a very important source of nutrients and then see how those foods change in response to CO2." Myers' team also looked at elements like sulfur, boron, and manganese, and found their levels in crops also changed with varying levels of CO2. But the significance of those changes on human health is less clear.
The researchers still don't know why CO2 changes crop nutrients like this. One of the dominant hypotheses is the idea of carbohydrate dilution, which posits that plants make more starchy carbohydrates in the face of rising CO2, and that all the additional carbohydrates are washing out nutritional elements. But it's not clear that this is the right explanation. "If there was a washout effect, you'd expect [elements] to be washed out the same way. But some things go up, some things go down," says Myers.
Myers's team is already modeling the implications of nutrient changes for nearly 200 populations around the world. But more research also needs to be done. Could other changing conditions caused by climate change—like drought conditions, for example—also affect CO2's impact on nutrient levels? And what can kind of interventions can we perform in the face of less nutrient-packed foods?
Since some rice cultivars tested in the study had large reductions in iron and zinc, while others had much lower reductions, it's possible that agronomists could breed rice cultivars that are more resistant to changes in CO2 concentrations, either through conventional breeding or genetically modified methods.
Myers points out that his study is important even if you don't believe in the greenhouse effect. "It doesn't require climate change, per se. It's about a direct impact of CO2 on plant nutrients," he says. "There's really no debate that CO2 has been rising since pre-industrial times. You just go out and measure it."