Primer on Aircraft Induced Clouds and Their Global Warming Mitigation Options

Pressure is increasing on all industrial sectors to address climate sustainability, not only for the welfare of the planet, but also to preserve the customer base and manage operating costs. The aviation industry has a unique opportunity to halve its global radiative forcing (RF) contribution by minimizing the generation of aircraft induced clouds (AIC). These anthropogenic (human-made) condensation trails create a greenhouse effect by absorbing or directing back to Earth approximately 33% of emitted outgoing thermal longwave radiation. The effect of AIC accounts for 2% of the Earth's total anthropogenic RF. The effect of reducing AIC on global warming is immediate (unlike CO2 emissions which have a 2-decade delay in affecting global warming). This paper describes the physics of AIC formation and RF to identify candidate interventions to reduce AIC RF: 1) reduce the quantity of soot generated, 2) reduce or eliminate ice crystal formation, and 3) modify RF properties of AIC. The highest utility and lowest costs is to reduce ice crystal formation by avoiding cruise flight levels in the atmospheric conditions that are conducive to AIC generation. Reducing soot through drop-in biofuels, and synthetic fuels, require significant investment to scale production. Options that require the redesign of jet engines or use of alternative fuels such as liquid natural gas and liquid hydrogen, require significant research and turn-over of the existing fleets. Fuel additives to suppress ice crystal formation, change the RF properties of ice crystals, or both, are still nascent research topics. The implications and limitations are discussed.