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

Abstract

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.

Climate Change Mitigation Pathways for the Aviation Sector

“…NOx emissions from aircraft engines are likely to alter the atmospheric composition and have impacts on radiative forces [21]. Emissions of NOx generate ozone (O 3 ) (warming) on a time scale of weeks to months [22,23].…”

Accounting for the carbon emissions from domestic air routes in China

“…Increasing aviation carbon emissions is the direct inducing factor of global warming, the greenhouse effect, and extreme weather (such as typhoons, high temperatures, heavy rains, mudslides, droughts, and other natural disasters), which have severely damaged the ecological environment [5,6]. In addition, aviation pollutants stay in the atmosphere for a long time and transport distances, which have a more significant impact on human health and the quality of the atmosphere.…”

Recent and Projected Trends in Global Civil Aviation Fleet Average NOxEmissions Indices

“…If the full environmental impacts of aviation are to be addressed, design and policy decisions have to consider the air quality degradation associated with non-CO2 emissions in addition to their short-term and long-term climate forcing. Despite technological improvements, it is expected that the growth of civil aviation will cause the amount of NOx emissions to increase between the years 2005 and 2050 [7,8]. Reducing these emissions and their associated environmental effects is complicated by the existence of tradeoffs in engine design, in which higher pressure ratios and turbine inlet temperatures are beneficial in terms of thermodynamic efficiency, specific thrust, and combustion completeness, but detrimental in terms of increasing NOx formation [9,10].…”