Climate impact of aviation NOx̳ emissions : radiative forcing, temperature, and temporal heterogeneity

Abstract

Aviation NOx emissions are byproducts of combustion in the presence of molecular nitrogen. In the upper troposphere, NOx emissions result in the formation of O₃ but also reduce the lifetime of CH4 , causing an indirect reduction in the formation of O₃. Meta studies by Lee et al. and Prather et al. concluded that the short-lived O₃ radiative forcing (RF) was greater than the combined long-lived CH₄ and O₃ RFs, leading to a net positive RF (4.5 to 14.3 mW/m² per Tg of NOx emissions). However, few simulations assess the surface air temperature (SAT) response, or conduct a large ensemble simulation with climate feedback in the cases where SAT is predicted. We aim to quantify the climate forcing and temperature response of aviation NOx emissions. Eight 400-member ensemble simulations are conducted with an earth system model of intermediate complexity. Inter-scenario comparisons between emissions starting in 1991, 2016 and 2036 with mid-range and high anthropogenic emissions are performed. We then determine the existence of long-term temporal heterogeneity of climate forcing and impact. The global net RF of an aviation NO, emissions inventory is positive from 1991 to 2100 while leading to a global average SAT responses of -0.068 K in 2100. Despite the positive zonal RF in the Northern Hemisphere of up to 413.9 mW/m² at 45°N, all latitudes experience cooling after 2075. In another scenario, constant aviation NOx emissions at 4.1 Tg/year cause a global net RF of near zero while leading to a SAT response of -0.020 K in 2100. The unexpected temperature behavior in both scenarios is attributed to the forcing from CH₄ destruction being 64% more effective in generating a SAT response than the O₃ forcing. Despite the positive net RF, the probability of aviation NOx emissions being cooling is 67% because of the relative difference in O₃ and CH₄ efficacies.