Ammonia at the net-zero crossroads.

Ammonia is indispensable for food production and an emerging carbon-free energy vector, yet its synthesis via the fossil-fuelled Haber-Bosch process makes it a major source of carbon dioxide. We introduce the Ammonia Rainbow, a classification framework combining physics-based modelling, learning-curve analysis, and life-cycle sustainability assessment to evaluate net-zero transitions across eight ammonia production technologies. Here we show that, assuming global ammonia demand doubles to meet projected food and energy needs, net-zero ammonia is not achievable by 2050 under any current policy or technological scenario. Even optimistic deployment of green routes demands extensive infrastructure, consuming over 40 percent of available green hydrogen alongside large fractions of carbon capture and renewable energy capacity. Electrochemical routes offer long-term promise but are unlikely to reach commercial viability before 2070 without disruptive innovation. By exposing these hidden systems-level burdens, we challenge the prevailing assumption of unchecked demand growth and argue for demand-side measures, efficiency improvements, and alternative decarbonisation strategies.