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

Aviation emits pollutants that affect the 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 trade-off exists, which also translates to a trade-off between short-lived climate forcers and a long-lived greenhouse gas, CO2. Moreover, the NOx–O3–CH4 system responds in a nonlinear manner, according to both aviation emissions and background NOx. 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 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 that 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 time scales of the climate effects of NOx and CO2 are quite different, necessitating careful analysis of proposed emissions trade-offs.