Producing food, transportation, and energy for seven billion people has resulted in massive increases in use of synthetic nitrogen (N) fertilizers and in emissions of N as forms of air pollution. In its numerous chemical forms, reactive nitrogen plays a critical role in all aspects of climate change considerations, including mitigation, adaptation, and impacts.
Mitigation: Management of the N cycle not only affects emissions of nitrous oxide (N2O), but also impacts CO2 and CH4, through effects on carbon sequestration in forests and soils and the effects of nitrogen oxides (NOX) and ozone (O3) on atmospheric CH4 concentrations. While some of these N cycling processes have a short-term cooling effect, the warming effects of N2O dominate at long time scales.
Adaptation: Although considerable progress has been made in lowering NOX emissions from energy, industry, and transportation sectors in the US, adaptations to climate change could slow or reverse this progress if energy use is increased for additional air conditioning or for pumping and treating water. Diversions of surface waters and extraction of groundwater needed for climate change adaptation will alter hydrologic flows of N and affect water quality, including drinking water nitrate and eutrophication of aquatic ecosystems. Improved agricultural nutrient management can confer some adaptive capacity of crops to climatic variability, but increased climatic variability will render the task of nutrient management more difficult.
Impacts: Higher air temperatures will result in a “climate penalty” for air quality mitigation efforts, because larger NOX emissions reductions will be needed to achieve the same reductions of O3 pollution under higher temperatures, thus imposing further challenges to avoid harmful impacts on human health and crop productivity. Changes in river discharge, due to summer drought and to extreme precipitation events, will affect the loading and processing of N within rivers and estuaries, potentially resulting in more eutrophication. Both climate change and N inputs from N deposition can provoke loss of biodiversity in aquatic and terrestrial ecosystems, because nutrient enrichment of native ecosystems often favors fast-growing, often non-native species.
As with climate change, political and economic impediments often stand in the way of mitigating losses of excess N to the environment. Policies aimed at improving N-use efficiencies in agriculture and reducing emissions from transportation and energy sectors would have multiple interacting benefits for climate mitigation and for minimizing climate change impacts on crop productivity, air and water quality, biodiversity, and human health risks.