Roughly half the nitrogen in the protein of a person alive today passed through an industrial process invented in 1909, which fixes atmospheric nitrogen into ammonia under heat and pressure. No other single technology supports a comparable share of the human population. The same process is responsible for a large share of agriculture's emissions, both through the natural gas it consumes and through the nitrous oxide released when fertiliser is applied to soil. Proposals to reduce its use therefore run into an arithmetic that admits no comfortable answer: the yield gains are real, the population depending on them is real, and the available alternatives close only part of the gap.