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Aviation's non-CO₂ climate forcing

The Lee et al. 2021 decomposition: contrail cirrus dominates aviation's effective radiative forcing, CO₂ is one-third, and why no single multiplier captures the full picture.

Updated 2026-06-018 min read
Primary sources · 5
  1. [1] Lee et al. (2021) — The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018 — the authoritative ERF decomposition · Atmospheric Environment 244, 117834 · January 2021 https://doi.org/10.1016/j.atmosenv.2020.117834
  2. [2] Lee et al. (2009) — Predecessor review on which DESNZ's 1.9 × multiplier was originally based · Atmospheric Environment 43, 3520–3537 · 2009 https://doi.org/10.1016/j.atmosenv.2009.04.024
  3. [3] IPCC AR6 WGI Chapter 7 — The Earth's energy budget, climate feedbacks and climate sensitivity — provides ERF framework · Cambridge University Press · 2021 https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-7/
  4. [4] DESNZ 2024 methodology — UK government source on the 1.9 × uplift as currently applied in corporate reporting · UK Department for Energy Security and Net Zero · June 2024 https://assets.publishing.service.gov.uk/media/66a9fe4ca3c2a28abb50da4a/2024-greenhouse-gas-conversion-factors-methodology.pdf
  5. [5] Teoh et al. (2024) — Global aviation contrail climate effects 2019–2021, ACP · Atmospheric Chemistry and Physics 24, 6071–6093 · 2024 https://acp.copernicus.org/articles/24/6071/2024/

Burning jet fuel at 35,000 feet does two things that make aviation uniquely warming: it releases CO₂, and it triggers contrails and NOₓ chemistry whose climate impact is at least as large as the CO₂ itself. A single multiplier cannot capture the temporal asymmetry, which is why picking the right number depends on what question you are answering.

100.9 mW m⁻²
Net aviation ERF in 2018
Lee et al. 2021
≈ 57 %
Contrail cirrus share of net aviation ERF
Lee et al. 2021
≈ 34 %
CO₂ share of net aviation ERF
Lee et al. 2021
3.5 %
Aviation share of all anthropogenic ERF (2018)
Lee et al. 2021

What "radiative forcing" means

Radiative forcing is the change in net energy flux at the top of the atmosphere caused by an external perturbation, in watts per square metre. A positive number warms the planet; negative cools. Aviation's 100.9 milliwatts per square metre means it adds about 0.1 W m⁻² to Earth's net energy budget — small in absolute terms next to total anthropogenic forcing of roughly 2.7 W m⁻², but a measurable 3.5 % share with rapid growth.

The Lee 2021 decomposition

Lee and co-authors decomposed aviation's net 2018 ERF into eight components, the three dominant ones plotted below. Contrail cirrus — ice clouds seeded by aircraft exhaust in cold, ice-supersaturated air — accounts for slightly more than half of the total. CO₂ is roughly one- third. The net NOₓ effect is positive (ozone production warms) but small because the methane-sink effect (cooling) and aerosol-induced cloud adjustments partly cancel the ozone warming.

Effective radiative forcing components of global aviation in 2018
Contrail cirrus57.4 mW m⁻²CO₂ (long-term cumulative)34.3 mW m⁻²NOₓ — ozone (warming)36 mW m⁻²NOₓ — methane (cooling)-21.2 mW m⁻²Water vapour (stratospheric)2 mW m⁻²Soot (aerosol direct)0.9 mW m⁻²Sulfate (aerosol direct)-7.4 mW m⁻²
Source: Lee et al. 2021, Atmospheric Environment 244:117834, Table 2

Why one multiplier is approximate

CO₂ accumulates in the atmosphere for centuries; one tonne emitted today keeps warming the planet for hundreds of years. A contrail forms in minutes, lives for a few hours, and disappears. Combining them into a single CO₂-equivalent depends on the time horizon you pick: GWP100 is the IPCC default, GWP20 weighs short-lived gases more heavily, GWP* tries to correct for the temporal mismatch directly.

How aviation's non-CO₂ uplift changes with the chosen metric
MetricAviation non-CO₂ uplift on CO₂Use case
ERF ratio (instantaneous, 2018)≈ 2.94 × (net / CO₂)Snapshot of current warming
GWP100 (DESNZ standard)≈ 1.9 ×UK SECR, corporate disclosure
GWP100 (post-Lee-2021 update)≈ 1.7 ×Some 2024 methodologies
GWP20≈ 2.5 – 3.0 ×Near-term temperature focus
GWP* (Allen / Forster)Depends on emission trajectoryPathway-based long-term targets
Source: Derived from Lee et al. 2021 components; GWP horizons per IPCC AR6

Contrails — why they matter so much

A contrail forms when hot, water-saturated engine exhaust mixes with cold, ambient air and the resulting plume reaches ice supersaturation. The contrail then traps outgoing longwave radiation (warming) while reflecting some incoming shortwave (cooling) — net warming on average, because most contrails form at night or at high latitudes where the longwave trapping dominates.

Why DESNZ still uses 1.9 ×

DESNZ acknowledges in its 2024 methodology paper that the underlying science has moved on from 1.9 × — Lee et al. 2021 implies a higher instantaneous ratio, ICAO is studying a more granular per-route approach, and contrail science is uncertain. DESNZ holds at 1.9 × because changing the multiplier annually would destabilise corporate disclosure — the number is intentionally conservative and stable rather than scientifically optimal.

How the multiplier has evolved across major methodologies
Year / sourceMultiplierNotes
Lee et al. 2009 base1.9 ×Adopted by UK CCC, then DEFRA/DESNZ
DESNZ / DEFRA 2010 – 20241.9 ×Held stable for reporting continuity
Atmosfair (Germany)≈ 2.0 ×Per-route model, slightly higher long-haul uplift
Lee et al. 2021 implied (ERF)≈ 2.94 × instantaneousInstantaneous ratio, not GWP100
Some 2024 industry methodologies≈ 1.7 ×Tightened GWP100 reading of Lee 2021
Source: DESNZ methodology papers 2009–2024; ATAG; ICAO; Atmosfair documentation

Frequently asked

Should I trust 1.9 × or use Lee 2021's higher number?▾
Both are right for different questions. 1.9 × is the right number for UK SECR disclosure and any reporting framework that anchors on DESNZ. Lee 2021's ~2.94 × ratio is the right number for instantaneous climate-impact comparisons and for sensitivity analysis. Our calculator publishes the underlying CO₂ value so you can apply whichever multiplier your framework requires.
Are contrails really half of aviation's warming?▾
Yes, in the central Lee 2021 estimate for 2018 — 57.4 mW m⁻² out of 100.9 mW m⁻² net ERF. The uncertainty range is wide (the paper's 90 % CI spans roughly 17 to 98 mW m⁻²), but the conclusion that contrails are large is robust.
Why is the NOx-derived forcing so small if NOx is a known greenhouse gas?▾
NOx itself is not a greenhouse gas, but it catalyses ozone production (warming) and also breaks down atmospheric methane (cooling). The two effects partly cancel — net ≈ +17.5 mW m⁻² — and aerosol-cloud interactions add further uncertainty.
Will sustainable aviation fuel reduce non-CO₂ effects too?▾
Partially. SAF burns with lower soot, which produces fewer ice nucleation sites and may reduce contrail persistence. Several studies estimate SAF can cut contrail-driven warming by 50–70 % independent of its lifecycle CO₂ benefit.
What happens at high latitudes versus the tropics?▾
Contrail effects are more strongly warming over Arctic and mid-latitude routes (especially night flights) and weaker — sometimes net-cooling — in the tropics. Per-route contrail-aware climate impact modelling is an active research area and is not yet in production calculators.

Continue reading

Methodology umbrella →
How we apply the 1.9 × uplift in our CO₂e number.
DEFRA 2024 emission factors →
Where the 1.9 × multiplier lives in the published numbers.
Sustainable aviation fuel →
How SAF reduces both CO₂ and contrail formation.
Cabin class and your CO₂ footprint →
What 1.9 × means for an economy versus business ticket.