Climate change impacts of US reactive nitrogen

Technological advancements in agriculture, farming, and combustion technology in the United States have altered the global nitrogen cycle.   Pinder et al. (2012) developed a framework in which the oxides of nitrogen, and ammonia can be assessed for their climate change impact relative to the impact of carbon dioxide.  The gases were partitioned based on their source (combustion and agriculture) and were tested over 20 and 100 year spans.  Global warming and cooling effects were both taken into account.  The results of the study showed that under current conditions, the warming and cooling effects enacted by the reactive forms of nitrogen partially offset each other.  However, the authors note that recent trends show decreasing emissions of cooling reactive nitrogen.  — Anthony Li
Pinder R. W., Davidson E. A., Goodale C. L., Greaver T. L., Herrick J. D., Liu L. 2012. Climate change impacts of US reactive nitrogen.  Proceedings of the National Academy of Sciences of the United States of America 109 7671–7675

The metric used to measure the effect of reactive nitrogen on climate change relative to that of carbon dioxide was the Global Temperature Potential (GTPt) set to 20 and 100 years of time.  Spatial variability in nitrogen deposition was captured using the Community Multiscale Air Quality (CMAQ) model at 12 km horizontal resolution.  The model was set to 12 km in order to compensate for atmospheric nitrogen scattering by the wind.  Using the CMAQ model, the N deposition was calculated for four ecosystem types, including forest, cropland, grassland, and wetland.  N2O emissions were calculated using the Greenhouse Gas Emissions Inventory.  The researchers classified the nitrogen emissions based on their primary source; NOx was from combustion, while NH3 was from agriculture.  The analysis only considered man-made sources of nitrogen. 
The researchers found that cooling effects generally come from combustion sources, offsetting the warming effects generally coming from agricultural sources.  The largest contributors of these effects to the atmosphere were NOx and methane with their subsequent effects on ozone, radiative forcing, N2O emissions, and enhanced CO2uptake.  Combustion sources contributed to an overall cooling because their emissions of NOx can remove methane from the atmosphere by increasing hydroxyl radical concentrations.  The GTP20 found that the impact of NOx on ozone and methane is  –270 Tg CO2e and the aerosol effects are much less at a –29 Tg CO2eand –7.3 Tg CO2e for NOx and NH3, respectively.  On a 100 year basis, aerosols, ozone, and methane are negligible meaning that as the time of the analysis goes on, these compounds have less effect.  Although NOx also contributes to a warming effect by producing ozone, a greenhouse gas, its cooling effects outweigh this warming.   Agriculture sources contributed to an overall warming due to its general emissions of N2O, a potent greenhouse gas.  On a 20 year basis, the GTP20 found N2O to contribute 180–380 Tg CO2e, while on the 100 year basis, the GTP100 is found to be 160–350 Tg CO2e.  Both N2O and NOxcontributed to long-term cooling impacts due to the nitrogen enhancement (via run off) of carbon storage in forests.
The results of this study showed that the cooling effects brought by combustion emissions have negated the warming effects brought on by agricultural emissions.  While this may cause a sigh of relief for some, the authors of this paper warn of the future, noting that NOx emissions have been declining relative to CO2 so warming is likely to be the dominant effect in the future.  Another point to take with the results is that global N2O emissions are increasing, causing further imbalance to the NOx to CO2 ratio.  Our society should begin focusing on reducing N2O emissions by looking at nitrogen use-efficiency and denitrification management in the agriculture sector.  Such improvements could bring a 20–25% increase in nitrogen efficiency and a 30–50% decrease in nitrogen loses.  

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