Global spatial variation in crop-livestock ammonia emission intensity and key drivers

Ammonia (NH₃) volatilization from livestock production threatens public health, ecosystems, and planetary boundaries, yet global disparities in Ammonia Emission Intensity (AEI, NH₃ emissions per unit nutritional output) and their drivers remain poorly quantified. This study systematically calculated livestock AEI (kg NH₃ emission per Gcal and kg crude protein) for 186 countries, analyzed inter-country/continent differences, and quantified the contributions of species composition, farming scale, and trade. Global crop-livestock systems emitted 32 Tg NH₃-N in 2010, with a global average AEI of 26 kg NH₃-N per Gcal and 0.5 kg NH₃-N per kg protein, showing striking spatial heterogeneity (Africa > Asia > Americas > Oceania > Europe). Bovine meat and mutton had the highest AEI, while chicken meat and industrial monogastric systems exhibited the lowest. Ruminant dominance, low nitrogen use efficiency, and small-scale backyard farming were key drivers of high AEI, whereas industrialization and optimized manure management reduced it. Global livestock trade redistributed substantial embodied NH₃, with modest environmental optimization (low-AEI countries supplying high-AEI importers) except for dysfunctional bovine meat trade. This study establishes a global AEI comparison framework, validates AEI as a useful metric for balancing food security and emission reduction, and provides targeted insights for international cooperation and policy design to mitigate NH₃ pollution while meeting growing animal product demand.