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.