Hydrogen combustion mitigates contrail-induced climate forcing relative to kerosene for 2035-technology A320 aircraft

Abstract. Global-scale estimates of hydrogen combustion's contrail impact have relied on general circulation models with simplified contrail parameterizations or on gridded lifecycle simulations with generic aircraft, and no study has evaluated hydrogen against kerosene using real-world flight trajectories for a defined aircraft under directly comparable conditions. This study presents a trajectory-based comparison of contrail climate forcing from kerosene and hydrogen combustion for 2035-technology A320 aircraft, applying the Contrail Cirrus Prediction model (CoCiP) to 10,298 flights sampled from 2024 global air traffic, with 2024 ERA5 reanalysis meteorology and fuel-specific baseline aircraft designs simulated along identical routes. Hydrogen's higher water vapor emission expands the contrail formation envelope, more than doubling the persistent contrail fraction from 7.31% to 15.7% of fleet distance. Yet under baseline nucleation assumptions, fleet-level energy forcing per kilometer decreases by 86.8%, because eliminating soot lowers ice crystal numbers by 1 order of magnitude, yielding larger, faster-sedimenting crystals that form shorter-lived, narrower and optically thinner contrails. This reduction depends mainly on the assumed nucleation floor for soot-free hydrogen and on kerosene non-volatile particulate matter (nvPM) emissions: across the plausible hydrogen floor (10¹²–10¹⁴ ice crystals per kilogram of fuel burned) the reduction spans 99.9% to 86.8%; even the most conservative hydrogen floor remains 28.7% below the lowest-nvPM kerosene scenario (10% nvPM). A complementary surrogate model reproduces the same monotonic decline in forcing with decreasing kerosene nvPM, corroborating the trend. Hydrogen combustion thus offers considerable contrail mitigation potential exceeding that of soot reduction alone.