Rising CO2 boosts photosynthesis in C3 plants (wheat, rice, soy, most trees) because their carbon-fixing enzyme, Rubisco, is not CO2-saturated at today's concentration. C4 plants (maize, sorghum, sugarcane) already run an internal CO2-concentrating pump, so extra CO2 barely helps them — the simulator's crop selector shows both regimes:
Yield index Y(C) = 100 · [1 + k_Y · ln(C / 400)]
Protein index P(C) = 100 · [1 − k_P · (1 − f_N) · ln(C / 400)]
Zn/Fe index M(C) = 100 · [1 − k_M · (1 − f_N) · ln(C / 400)]
k_Y ≈ 0.41 (C3) / 0.05 (C4) k_P ≈ 0.30 k_M ≈ 0.25
f_N = nitrogen-fertilization offset (0 / 0.15 / 0.30)
Free-Air CO2 Enrichment (FACE) field trials — the only method that tests real crops in open fields rather than sealed chambers — measured exactly this pattern. Zhu et al. (2018, Science Advances) found rice grown at +200 ppm CO2 gained yield but lost 10.3% protein, 9.6% zinc and 5.1% iron. Myers et al. (2014, Nature) pooled FACE trials across wheat, rice, maize, soy and field peas and found C3 grains lose roughly 3–17% of their zinc, iron and protein content near 550 ppm CO2 — the level projected for mid-century.
- Crop — switches between a CO2-responsive C3 pathway and the near-saturated C4 pathway, and reloads the field with that crop's plant geometry.
- Atmospheric CO2 — the driver: more CO2 means more greenhouse-gas radiative forcing globally, and, at the leaf level, more substrate for photosynthesis — but nitrogen and mineral uptake from the soil don't scale up to match, so each extra unit of starch dilutes the nutrients already in the grain.
- Nitrogen fertilization — extra soil nitrogen can offset part (not all) of the protein dilution by feeding more N into the grain alongside the extra carbon.
- Growing season day — scrubs the field through a logistic growth curve from seedling to harvest; the readouts are for the current CO2/fertilization scenario at full maturity, while the visible plant height tracks the chosen day.
Ecological consequence: because rice, wheat and other C3 staples supply most of the dietary protein, zinc and iron for roughly two billion people, this "hidden hunger" effect from rising atmospheric CO2 is a real, measured nutritional cost of the greenhouse effect that sits alongside its better-known warming impacts.