Aviation

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Around two to three per cent of global carbon dioxide emissions, and a considerably larger share of the warming effect, because aviation does something no other sector does: it injects water vapour, nitrogen oxides and particles into the upper troposphere, where the resulting contrail cirrus traps outgoing radiation. Lee and colleagues estimated in 2021 that aviation's total effective radiative forcing was of order three and a half per cent of the anthropogenic total for the period they assessed, with the non-carbon-dioxide contributions — contrail cirrus foremost — larger than the carbon dioxide contribution itself and far more uncertain. That asymmetry is peculiar to aviation and it cuts both ways: it makes the sector worse than its carbon accounting suggests, and it opens an avoidance route that no other sector has, since contrails form in thin ice-supersaturated layers that a small number of flights could in principle be routed around. The reason the sector is hard is separately physical, and is treated under energy density. Aviation is also the sector where demand growth has most consistently outrun efficiency gains: airframe and engine fuel burn per passenger-kilometre has improved steadily for decades and total fuel use has risen anyway. Not a place.