Trading off Aircraft Fuel Burn and NO_<i>x</i> Emissions for Optimal Climate Policy

Aviation emits pollutants that affect the climate, including CO₂ and NO_<i>x</i>, NO_<i>x</i> 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_<i>x</i> emissions. Conversely, combustor modifications to reduce NO_<i>x</i> 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_<i>x</i>–O₃–CH₄ system responds in a nonlinear manner, according to both aviation emissions and background NO_<i>x</i>. 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_<i>x</i> emissions the consequential CO₂ penalty of 2% actually increased the total radiative forcing (RF). For a 2% fuel penalty, NO_<i>x</i> emissions needed to be reduced by >43% to realize an overall benefit. Conversely, to ensure that the fuel penalty for a 20% NO_<i>x</i> 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_<i>x</i> and CO₂ are quite different, necessitating careful analysis of proposed emissions trade-offs