New Study Concludes Net Effect of Human-Made Reactive Nitrogen (Nr) is a Cooling Effect

     I have written on this blog about the global warming potential (GWP) of nitrogen oxides, including nitrous oxides (N2O) from fertilizer, and about the photochemical smog (ozone) precursor nitrogen oxide (NOx) gases. It has been recently shown via a paper in Nature by scientists from the Max Planck Institute for Biogeochemistry in Jena, Germany, that the fine suspended particles of reactive nitrogen compounds mostly from combustion, which are short-lived in the atmosphere, are the biggest contributor to a net cooling effect of anthropogenic reactive nitrogen by shielding sunlight as does gaseous ammonia (NH3), also emitted by the agriculture sector. These gases are forms of reactive nitrogen (Nr) that appear in the biosphere on land and in the atmosphere. Nitrogen gas (N2), is non-reactive, or inert but all other nitrogen compounds are reactive (Nr).

     The global nitrogen cycle includes the growing amounts of anthropogenic nitrogen oxide gases (NOx, N2O) and gaseous ammonia (NH3) being emitted into the atmosphere. The authors of the paper summarized the results in the abstract:

 

“This net cooling effect is the result of increased aerosol loading, reduced methane lifetime and increased terrestrial carbon sequestration associated with increases in anthropogenic Nr, which are not offset by the warming effects of enhanced atmospheric nitrous oxide and ozone. Future predictions using three representative scenarios show that this cooling effect may be weakened primarily as a result of reduced aerosol loading and increased lifetime of methane, whereas in particular N2O-induced warming will probably continue to increase under all scenarios. Our results indicate that future reductions in anthropogenic Nr to achieve environmental protection goals need to be accompanied by enhanced efforts to reduce anthropogenic greenhouse gas emissions to achieve climate change mitigation in line with the Paris Agreement.”

 

 

     Combustion of fossil fuels and application of fertilizers are the two biggest sources of Nr. The study involved modeling the chemical reactions in the land biosphere and atmosphere to arrive at a flow chart or Nr cycle. They utilized terrestrial biogeochemistry and atmospheric chemistry. The authors admit, however, that there is still considerable uncertainty in the calculations and potentially a high margin of error since net warming and cooling effects can vary considerably in different regions.    

     It is well known that NOx, N2O, and NH3 form aerosol particles in the atmosphere and that these aerosol particles block sunlight thereby decreasing solar radiative forcing. They also contribute to smog in the lower atmosphere that has a similar effect. N2O remains long in the atmosphere like CO2, while NOx and NH3 are short-lived in the atmosphere, more like methane. Thus, N2O has a net warming effect but the shorter-lived gases have a net cooling effect due to the aerosol particles. All of these factors are accounted for in the models. Nr gases also have different individual aerosol effects and affect one another in different ways. The paper gives the equations and reactions used as model assumptions in the study. Lest we forget, air pollution does indeed slow global warming, but only for a short time. This includes the considerable aerosols from burning coal and biomass. Thus, it should perhaps be pointed out that this study only considers Nr aerosols and not all aerosol particles which also include particulate matter and sulfur oxide and sulfate compounds. When the world finally moves away from coal after China’s and India’s consumption peaks and begins to drop, the amount of combustion aerosols in the atmosphere will drop as well. Better pollution abatement equipment on coal and wood power plants, especially, but also natural gas plants can decrease aerosol production as well.

 

 

References:

Net effects of man-made nitrogen attenuate global warming, researchers find. Eberhard Fritz. Phys.org. July 24, 2024. Net effects of man-made nitrogen attenuate global warming, researchers find (msn.com)

Global net climate effects of anthropogenic reactive nitrogen. Cheng Gong, Hanqin Tian, Hong Liao, Naiqing Pan, Shufen Pan, Akihiko Ito, Atul K. Jain, Sian Kou-Giesbrecht, Fortunat Joos, Qing Sun, Hao Shi, Nicolas Vuichard, Qing Zhu, Changhui Peng, Federico Maggi, Fiona H. M. Tang & Sönke Zaehle. Nature (2024). Global net climate effects of anthropogenic reactive nitrogen | Nature