Altitude-Dependent Climate Effects of Hydrogen Combustion in Turbofan Aircraft: A Comparative Literature Synthesis

Liquid hydrogen combustion eliminates direct carbon dioxide emissions from turbofan aircraft but releases roughly 2.6 times more water vapour per unit of released combustion energy than conventional Jet-A fuel, raising the question of whether the resulting non-CO2 effects offset the carbon benefit. This study addresses that question through a comparative literature synthesis of 24 peer-reviewed sources, asking under what flight altitudes and atmospheric conditions hydrogen combustion yields a net climate benefit relative to kerosene. A two-stage search protocol combined a date-restricted systematic screen with a targeted search for foundational work published under earlier terminology. Nine standardised variables were extracted from the primary studies and the supplementary evidence was tabulated separately. Altitude was treated as a categorical variable across three bands, with ambient temperature and relative humidity with respect to ice recorded as the underlying mechanism. Hydrogen contrails form at threshold temperatures approximately 10 K higher than kerosene contrails while containing more than 80 to 90 percent fewer ice crystals, which reduces their optical thickness. The altitude dependence is established but is jointly determined by latitude, season and aircraft design, and the direction of the contrail-frequency response reverses between equatorial and non-equatorial regions. The principal gap is not an absence of altitude-resolved evidence but an absence of harmonised comparison under common metrics.