Estimating the climate and air quality benefits of aviation fuel and emissions reductions

In this study we consider the implications of our current understanding of aviation climate impacts as it relates to the ratio of non-CO₂ to CO₂ effects from aviation. We take as inputs recent estimates from the literature of the magnitude of the component aviation impacts and associated uncertainties. We then employ a simplified probabilistic impulse response function model for the climate and a range of damage functions to estimate the ratio of non-CO₂ to CO₂ impacts of aviation for a range of different metrics, scientific assumptions, future background emissions scenarios, economic growth scenarios, and discount rates. We take cost-benefit analysis as our primary context and thus focus on integral metrics that can be related to damages: the global warming potential, the time-integrated change in surface temperature, and the net present value of damages. We also present results based on an endpoint metric, the global temperature change potential. These latter results would be more appropriate for use in a cost-effectiveness framework (e.g., with a well-defined policy target for the anthropogenic change in surface temperature at a specified time in the future). We find that the parameter that most influences the ratio of non-CO₂ to CO₂ impacts of aviation is the discount rate, or analogously the time window used for physical metrics; both are expressions of the relative importance of long-lived versus short-lived impacts. Second to this is the influence of the radiative forcing values that are assumed for aviation-induced cloudiness effects. Given the large uncertainties in short-lived effects from aviation, and the dominating influence of discounting or time-windowing, we find that the choice of metric is relatively less influential. We express the ratios of non-CO₂ to CO₂ impacts on a per unit fuel burn basis so that they can be multiplied by a social cost of carbon to estimate the additional benefits of fuel burn reductions from aviation beyond those associated with CO₂ alone (all else being equal). For a non-CO₂ to CO₂ ratio based on economic damage costs, we find a central value of 1.8 at a 3% discount rate, with a range from 0.6 to 2.5 for the upper and lower bounds of scientific and scenario-based uncertainty. Since estimating the co-benefits in this way is an important requirement for cost-benefit analyses, we also provide estimates of the air quality benefits of aviation fuel burn reduction in a similar format. We find the marginal damage costs of aircraft emissions below 3000 feet to be of similar magnitude to the climate costs on a per unit fuel burn basis, or an order of magnitude smaller on a per flight basis since we take no account of the air quality impacts of emissions above 3000 feet where the majority of fuel is consumed for the fleet.