Scientific Basis
The Unseen Climate Threat: Aviation’s Contrail Conundrum
The visual spectacle of aircraft contrails tracing white lines across the sky belies a complex and significant climatic impact. While aviation's carbon footprint has long been a focal point of climate policy, the non-CO2 effects – particularly those stemming from contrail formation – are increasingly recognised as a dominant factor in the sector's overall contribution to global warming:contentReference[oaicite:8]{index=8}. This section establishes the scientific basis for understanding contrails, quantifies their climatic forcing relative to CO2, and underscores the urgent need for comprehensive mitigation strategies within the United Kingdom and the European Union.
Understanding Contrails and Their Formation
Aircraft contrails, or condensation trails, are essentially human-made ice clouds. They form when hot, moist exhaust plumes from aircraft engines mix with cold, ambient air at high altitudes (typically between 8 and 13 kilometers). The exhaust contains water vapor, a primary product of hydrocarbon fuel combustion, and myriad tiny particles (predominantly soot/black carbon). In atmospheric regions characterized by high humidity and temperatures below approximately -40 °C – conditions often found in Ice Super-Saturated Regions (ISSRs) – the water vapor condenses and subsequently freezes onto these particles, forming a visible line-shaped cloud. If the surrounding atmosphere is near saturation, these contrails can persist and spread, sometimes evolving into broader contrail-cirrus cloud layers that linger for hours. During daytime, such contrail-cirrus may reflect incoming solar radiation (cooling effect), but they also trap outgoing infrared (heat) radiation from Earth, especially at night, leading to a net warming effect.
The Climatic Impact: Beyond CO2 – Contrails as a Dominant Forcing Agent
Recent assessments of aviation's climate impact reveal a striking reality: the effective radiative forcing (ERF) from aviation-induced contrail cirrus is significantly larger than that from the sector’s cumulative CO2 emissions. From 1940–2018, contrail cirrus had an ERF of about 57.4 mW/m2, substantially exceeding the 34.3 mW/m2 attributed to aviation CO2 emissions over the same period. This makes contrail cirrus the single largest climate forcing component of aviation, contributing approximately 57% of the net positive forcing by aviation. In other words, contrails currently exert a greater warming influence than all of aviation’s CO2 emissions to date. The European Union Aviation Safety Agency (EASA) has corroborated that historic non-CO2 effects (mainly contrails and NOx) accounted for more than half of aviation's net warming effect between 1940 and 2018.
| Forcing Source | Effective Radiative Forcing (ERF, mW/m2) | % Contribution to Net Aviation ERF |
|---|---|---|
| Contrail Cirrus | 57.4 | 57% |
| CO2 emissions | 34.3 | 34% |
| NOx emissions | 17.5 | 18% |
| Other (Aerosols, water vapor, etc.) | Minor negative / uncertain | Minor negative |
| Total Net Aviation ERF | +100.9 (range 55–145) | — |
The total net ERF from all aviation activities in 2018 was estimated at +100.9 mW/m2 (likely range 55–145 mW/m2), which is roughly 3% of the total ERF from all human activities. Crucially, the dominance of non-CO2 effects means that aviation's overall climate impact is significantly underestimated if only CO2 is considered. Using new metrics like CO2-warming-equivalent (e.g. GWP*), which account for the short atmospheric lifetime of contrails, recent data suggest aviation's current total climate impact may be around three times that of its CO2 emissions alone.
This scientific understanding underscores that policies narrowly targeting aviation CO2 are addressing less than half of the problem. To achieve truly sustainable aviation and meet climate neutrality goals, the warming impact of contrails must be tackled with the same commitment as CO2. The next sections explore how contrail-induced warming can be mitigated through operational and technological strategies, and how current policy frameworks can evolve to include contrails.