DATA SHEET // NOx
NOx (net, ~17%)
At cruise altitude, NOx speeds up ozone formation, a warming effect, but also raises hydroxyl radicals that break down methane, a cooling effect. The two partially offset, leaving a net warming that's smaller than CO2 or contrail cirrus but still genuinely debated in size.
90% confidence range — Lee et al. 2021
The shaded wings show how much wider this slice could be at the high estimate (29 mW/m²). At the low estimate (0.6), the slice shrinks to almost nothing — the ozone warming and methane cooling nearly cancel out — but on this ERF basis the range stays just on the warming side throughout, it doesn't cross into net cooling.
DATA SHEET // CONTRAIL CIRRUS
Contrail cirrus (~57%)
Water vapour in engine exhaust freezes onto soot particles at cruise altitude, forming ice crystal trails that can spread into thin cirrus cloud cover. Like natural cirrus, they let sunlight through but trap outgoing heat, giving a net warming effect.
The 57% figure is a fleet-wide average: Teoh et al. (2020, 2024) find that roughly 2-3% of flights, mostly long-haul, high-altitude, night-time routes, drive about 80% of total contrail warming.
90% confidence range — Lee et al. 2021
The widest wings of any term here: at the high estimate (98 mW/m²) the slice would be so much bigger that its two wings meet and overlap on the far side of the ring — the darker patch where they cross. At the low estimate (17 mW/m²) the tick mark shows it could shrink to a fraction of what's shown. Contrail cirrus is the biggest slice of aviation's warming and the least precisely known.
DATA SHEET // CO2
CO2 (~34%)
Burning jet fuel releases CO2, which traps outgoing heat the way a blanket traps body heat. It persists in the atmosphere for centuries, so its warming effect accumulates and lasts long after the flight itself.
90% confidence range — Lee et al. 2021
The narrowest wings of any term here: 31 to 38 mW/m². CO2's physics are well understood, so the slice barely moves either side of the solid wedge. This is the number regulators build policy on.