Trading Off Aircraft Fuel Burn and NOx Emissions for Optimal Climate Policy

Aviation emits pollutants that affect climate, including CO2 and NOx; NOx 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 NOx emissions. Conversely, combustor modifications to reduce NOx may increase CO2. Hence, a technology tradeoff exists, which also translates to a tradeoff between short lived climate forcers and a long-lived greenhouse gas, CO2. Moreover, the NOx-O3-CH4 system responds in a non-linear manner, according to both aviation emissions and background NOx. A simple climate model was modified to incorporate non-linearities parameterized from a complex chemistry model. Case studies showed that for a scenario of a 20% reduction in NOx emissions the consequential CO2 penalty of 2% actually increased the total radiative forcing (RF). For a 2% fuel penalty, NOx emissions needed to be reduced by >43% to realize an overall benefit. Conversely, to ensure the fuel penalty for a 20% NOx emission reduction did not increase overall forcing, a 0.5% increase in CO2 was found to be the ‘break even’ point. The timescales of the climate effects of NOx and CO2 are quite different, necessitating careful analysis of proposed emissions tradeoffs