Uncertain climate effects of anthropogenic reactive nitrogen

Green ammonia (NH₃) is produced based on green hydrogen (H₂) and has recently gained wide interest due to its potential to decarbonize ammonia production, and as a carbon-free solution for energy storage and transportation. However, the production and use of ammonia come with other climate and environmental challenges due to its alteration of the Earth’s nitrogen cycle. Before introducing new ammonia technologies on a large scale, it is important to thoroughly understand current atmospheric impacts of anthropogenic reactive nitrogen (Nr), mainly the impacts of ammonia, nitrogen oxides (NO_X) and nitrous oxide (N₂O) emissions. The present work addresses pre-industrial (1850) to present-day (2019) climate effects of Nr, and has been published in Hodnebrog et al. (2025, Nature, https://doi.org/10.1038/s41586-025-09337-9).

We use five independent latest-generation atmospheric chemistry models (OsloCTM3, CESM2, GISS ModelE, GFDL-AM4.1 and LMDZ-INCA), and find that the change over the industrial era of nitrate and sulfate aerosol abundances owing to Nr emissions varies greatly across the models. Consequently, the direct aerosol radiative forcing (RF) differs widely by model, even in sign. The positive ozone and negative methane RF due to Nr emissions also vary widely between models. While all five models show a net negative RF (i.e., cooling) due to historical anthropogenic Nr emissions, the net climate effect is the sum of several terms that vary in sign and are associated with substantial uncertainties. Future research is clearly needed, both to better define and narrow the uncertainties on the climate effects given here and to quantify climate effects for processes for which estimates do not yet exist (for example, aerosol-cloud interactions due to Nr emissions).