Despite its negative impacts on plant functioning, climate change benefits plants at the cellular level. For example, the stimulation of C3 photosynthesis by elevated CO₂ can increase N₂ fixation by 73% and grain yield by 10%–11%. The global elevated atmospheric CO₂ concentration has already decreased the nitrogen content in C3 crop species and C3 woody vegetation by 14% and 21%, respectively, regardless of added nitrogen fertilizer. ¹⁵N-feeding experiments have shown that, after 19 h under elevated CO₂, the ¹⁵N concentration in the stems, roots plus rhizomes, and whole plants of Scirpus olneyi (S. olneyi) decreased by 51%, 63%, and 74%, respectively. Moreover, S. olneyi showed reduced NH₄⁺ assimilation under elevated CO₂, which decreased the amino acid contents in the stems by 25.6% for glycine and 65.0% for serine, and that in the roots plus rhizomes by 2% for gamma-aminobutyric acid (GABA) and 80% for glutamate. Wheat grain protein has also been found to decrease by 7.4% under elevated CO₂ due to reductions in threonine, valine, iso-leucine, leucine, and phenylalanine. The mineral nutrient contents in grains of rice and maize were similarly found to decrease under high CO₂ by 1.0% and 7.1% for phosphorus, 7.8% and 2.1% for sulfur, 5.2% and 5.8% for iron, 3.3% and 5.2% for zinc, 10.6% and 9.9% for copper, and 7.5% and 4.2% for manganese, respectively. In general, mineral concentrations in C3 plants are predicted to decrease by 8% under elevated CO₂, while total non-structural carbohydrates (mainly starch and sugars) are expected to increase. These decreases in grain protein, amino acids, and mineral nutrients could double the incidence of global protein-calorie malnutrition and micronutrient deficiency—especially in Africa, where agricultural soils are inherently low in nutrient elements. Additionally, the increase in total non-structural carbohydrates (mainly starch and sugars) in cereal crops could elevate diabetes incidence due to heavy reliance on starchy diets. The negative effects of elevated CO₂ on rice, maize, and wheat—the world’s three major staple crops—suggest an increase in global food insecurity with rising atmospheric CO₂ concentration.