Current aviation roadmaps are not within planetary boundaries

Aviation decarbonization roadmaps predominantly rely on replacing fossil kerosene with alternative jet fuels, yet the systemic impact of these alternatives on planetary boundaries other than climate change has not been quantified. Here we integrate prospective life-cycle inventories with the planetary boundaries framework to evaluate the absolute impacts of aviation roadmaps as time-dependent trajectories, rather than endpoint snapshots. Across scenarios to 2050, we find that fuel-switch-centered roadmaps do not achieve sustainability, persistently transgress the climate change and biosphere integrity core boundaries, and exert large demands for energy, land, freshwater and CO₂ capture that challenge their feasibility. Slower traffic growth brings aviation closer to its share of the safe operating space in terms of lowering energy demand, but respecting additional planetary boundaries requires air traffic to decline by at least 7.2% yr^-1. Our results show that aligning aviation with Earth's safe operating space cannot be achieved through fuel substitution alone and requires governance strategies that combine supply-side solutions with explicit demand-side measures.