Greater fuel efficiency is potentially preferable to reducing NOx emissions for aviation's climate impacts.

Aviation emissions of nitrogen oxides (NOx) alter the composition of the atmosphere, perturbing the greenhouse gases ozone and methane, resulting in positive and negative radiative forcing effects, respectively. In 1981, the International Civil Aviation Organization adopted a first certification standard for the regulation of aircraft engine NOx emissions with subsequent increases in stringency in 1992, 1998, 2004 and 2010 to offset the growth of the environmental impact of air transport, the main motivation being to improve local air quality with the assumed co-benefit of reducing NOx emissions at altitude and therefore their climate impacts. Increased stringency is an ongoing topic of discussion and more stringent standards are usually associated with their beneficial environmental impact. Here we show that this is not necessarily the right direction with respect to reducing the climate impacts of aviation (as opposed to local air quality impacts) because of the tradeoff effects between reducing NOx emissions and increased fuel usage, along with a revised understanding of the radiative forcing effects of methane. Moreover, the predicted lower surface air pollution levels in the future will be beneficial for reducing the climate impact of aviation NOx emissions. Thus, further efforts leading to greater fuel efficiency, and therefore lower CO2 emissions, may be preferable to reducing NOx emissions in terms of aviation's climate impacts.

Trading off Aircraft Fuel Burn and NO x Emissions for Optimal Climate Policy.

Aviation emits pollutants that affect the climate, including CO2 and NO x, NO x indirectly so, through the formation of tropospheric ozone and reduction of ambient methane. To improve the fuel performance of engines, combustor temperatures and pressures often increase, increasing NO x emissions. Conversely, combustor modifications to reduce NO x may increase CO2. Hence, a technology trade-off exists, which also translates to a trade-off between short-lived climate forcers and a long-lived greenhouse gas, CO2. Moreover, the NO x-O3-CH4 system responds in a nonlinear manner, according to both aviation emissions and background NO x. A simple climate model was modified to incorporate nonlinearities parametrized from a complex chemistry model. Case studies showed that for a scenario of a 20% reduction in NO x emissions the consequential CO2 penalty of 2% actually increased the total radiative forcing (RF). For a 2% fuel penalty, NO x emissions needed to be reduced by >43% to realize an overall benefit. Conversely, to ensure that the fuel penalty for a 20% NO x emission reduction did not increase overall forcing, a 0.5% increase in CO2 was found to be the "break even" point. The time scales of the climate effects of NO x and CO2 are quite different, necessitating careful analysis of proposed emissions trade-offs.