Abstract
<title>Abstract</title> <p>Contrail cirrus clouds are a major non-CO2 contributor to aviation climate forcing. Current models link contrail ice particle numbers to climate impact, yet the upstream link to fuel composition remains poorly explored. Here we show that fuel hydrogen content governs soot number emissions and thereby provides a mechanistically grounded predictor of contrail ice particle numbers, with the first in-flight validation of this relationship showing agreement within ~±20%. Increasing fuel hydrogen content by 1.4 percentage points, from 13.8% to 15.2%, substantially reduces soot and ice particle numbers, contrail optical depth, lifetime and coverage, resulting in a calculated 52 [35, 67] % reduction in contrail energy forcing. Global deployment scenarios reflecting alternative fuel policies indicate substantial long-term mitigation potential from high-hydrogen content fuels. By 2050, their deployment could reduce the aviation-induced increase in global surface temperature by 21 [16, 27] %, offsetting a substantial proportion of the climate effects of projected aviation growth. Our results can establish fuel hydrogen content as a physically based metric linking fuel composition to aviation climate impacts, informing future fuel standards and mitigation strategies for climate-compatible aviation.</p>