Climate change impacts of US reactive nitrogen

Abstract

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 (N 2 O). We use the global temperature potential (GTP), calculated at 20 and 100 y, in units of CO 2 equivalents (CO 2 e), 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 CO 2 e on a GTP 20 basis. However, these effects are largely short-lived. On a GTP 100 basis, combustion contributes just −16 to −95 Tg CO 2 e. Agriculture contributes to warming on both the 20-y and 100-y timescales, primarily through N 2 O 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 N 2 O emissions are needed. Substantial progress toward this goal is possible using current technology. Without such actions, even greater CO 2 emission reductions will be required to avoid dangerous climate change.C ombustion, fertilizer use, and biological nitrogen fixation transform inert N 2 into reactive nitrogen-forms of N that are chemically, biologically, or radiatively active (1). Reactive nitrogen includes oxides of nitrogen (NO x ), ammonia (NH 3 ), and nitrous oxide (N 2 O). NO x is largely from combustion, whereas NH 3 and N 2 O are largely from agriculture. These compounds can impact the climate in a myriad of interconnected ways. NH 3 and NO x contribute to climate change indirectly. They alter the production and loss of climate forcers, atmospheric constituents that perturb the Earth's energy balance by trapping heat (greenhouse gases) or scattering incoming solar energy (aerosols). NO x impacts greenhouse gases by (i) increasing the formation of ozone, contributing to warming, and (ii) increasing the removal of methane (CH 4 ), contributing to cooling. Both NO x and NH 3 can enhance light-scattering aerosols. When deposited out of the atmosphere into ecosystems, reactive N can stimulate plant growth and alter the uptake of greenhouse gases. N 2 O has a direct effect on climate change; it is a powerful greenhouse gas. These climate change impacts are summarized in Table 1.Previous studies have examined a subset of these impacts on atmospheric climate forcers (2-4) and greenhouse gas fluxes (5-9). A few studies have assessed the combined impacts on a global (10) and European (11) scale. However, prior efforts have not specifically assessed the impacts of US react...

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