Fossil fuel combustion and fertilizer application in the United States have substantially altered the nitrogen cycle, with serious effects on climate change. The climate effects can be short-lived, by impacting the chemistry of the atmosphere, or long-lived, by altering ecosystem greenhouse gas fluxes. Here we develop a coherent framework for assessing the climate change impacts of US reactive nitrogen emissions, including oxides of nitrogen, ammonia, and nitrous oxide (N2O). We use the global temperature potential (GTP), calculated at 20 and 100 y, in units of CO2 equivalents (CO2e), as a common metric. The largest cooling effects are due to combustion sources of oxides of nitrogen altering tropospheric ozone and methane concentrations and enhancing carbon sequestration in forests. The combined cooling effects are estimated at −290 to −510 Tg CO2e on a GTP20 basis. However, these effects are largely short-lived. On a GTP100 basis, combustion contributes just −16 to −95 Tg CO2e. Agriculture contributes to warming on both the 20-y and 100-y timescales, primarily through N2O emissions from soils. Under current conditions, these warming and cooling effects partially offset each other. However, recent trends show decreasing emissions from combustion sources. To prevent warming from US reactive nitrogen, reductions in agricultural N2O emissions are needed. Substantial progress toward this goal is possible using current technology. Without such actions, even greater CO2 emission reductions will be required to avoid dangerous climate change.
Natural Sciences in General