Elevated CO2 and the hidden hunger crisis: mechanisms, magnitudes, and mitigation of micronutrient dilution in staple crops.

Hidden hunger results from inadequate intake of iron (Fe), zinc (Zn), and other key micronutrients. Over 2 billion people worldwide are affected, especially those who rely on staple cereals for daily food. Elevated CO2 often boosts photosynthesis and yield in C3 crops. However, it also reduces grain mineral levels via multiple physiological pathways. These include less transpiration-driven mineral delivery, an imbalance between carbon accumulation and mineral uptake, and changes in mineral uptake, transport, and grain loading. This review collects current evidence for Fe, Zn, copper (Cu), manganese (Mn), boron (B), molybdenum (Mo), selenium (Se), and iodine (I). It draws on free-air CO2 enrichment (FACE) experiments, controlled studies, reviews, and meta-analyses from major cereals and legumes. Grain Fe and Zn show the most consistent and significant declines, and often drop more under combined CO2 enrichment with drought, warming, or low nitrogen. Cu and Mn show greater variability, and data for Se, I, B, and Mo remain limited. This highlights big gaps in climate-nutrition research. The review also considers other factors that affect micronutrient dilution. It identifies research priorities and assesses strategies such as breeding, biofortification, agronomic management, soil-microbiome support, fortification, and dietary diversity. Overall, the evidence indicates that rising CO2 levels threaten the nutritional quality of grains. There is a clear need to include micronutrient density in climate-resilient crop improvement and food system planning.