Contrails and Climate Impact

What Are Contrails?

Contrails (condensation trails) are line-shaped ice clouds formed when hot, humid exhaust gases from aircraft engines mix with cold, low-humidity air at altitude. The water vapour in exhaust rapidly freezes around soot particles from combustion, creating visible white streaks. Most contrails are short-lived, evaporating within seconds to minutes, but in humid air masses they can persist for hours and spread into diffuse cirrus clouds covering thousands of square kilometres.

Contrails form primarily above the tropopause (approximately 8–12 km altitude), where temperatures fall below −40°C. The Schmidt-Appleman criterion defines the thermodynamic conditions under which contrails form and persist — conditions that vary by flight level, season, and geographic region.

Warming Effect

Contrail cirrus traps outgoing longwave (infrared) radiation from Earth — the "blanket" effect — while also partially reflecting incoming solar radiation. At night, the warming effect dominates with no solar offset, making night flights disproportionately impactful. The net effect is warming, and it is significant.

A landmark 2020 study in Atmospheric Chemistry and Physics (Lee et al.) found that contrail cirrus contributes approximately 57 milliwatts per square metre of effective radiative forcing — compared to aviation CO2 at ~34 mW/m². The total non-CO2 aviation effect (contrails, NOx, water vapour) is estimated to be 2–4× larger than CO2 alone, which is why a radiative forcing multiplier of ~2× is applied in many carbon footprint calculators.

Critically, contrail warming is a short-term effect (days to weeks), unlike CO2 which persists for centuries. Reducing contrails provides immediate climate benefit; CO2 reduction has long-lasting cumulative impact.

Avoidance Strategies

Because contrails only form in specific "ice-supersaturated regions" (ISSRs), avoiding these atmospheric volumes could eliminate persistent contrail formation. Research projects are testing this approach:

• Google and American Airlines: A 2023 trial showed rerouting ~5% of flights (climbing or descending by 2,000 feet to avoid ISSRs identified using AI weather prediction) reduced contrail formation by 54% on test routes, with an average fuel penalty of just 0.3%
• SESAR: European initiative testing contrail-aware routing with Eurocontrol; trial flights with Lufthansa and Virgin Atlantic
• Breakthrough Energy / Project Contrails: Building open-source prediction models to identify ISSR flight segments in real-time planning

Research

Key uncertainties remain: ISSR prediction accuracy is currently around 60% at 12–24 hours' notice. The role of soot particle number in ice crystal formation means cleaner SAF combustion (fewer soot particles) may reduce contrail optical depth. The UK Met Office and DLR (German Aerospace Centre) operate the most active contrail research programmes. Satellite tracking using Sentinel-2 and GOES can now attribute specific contrail patterns to individual flights.

Contrails vs CO2 Impact

CO2 has a long atmospheric lifetime (hundreds of years) and a cumulative effect — every tonne emitted adds to the stock. Contrail forcing is instantaneous and transient. This means: if you want to minimise immediate climate impact, contrail avoidance matters greatly. If you are thinking about long-term atmospheric CO2 stabilisation, direct emissions reduction is more important. ICAO's CORSIA currently only covers CO2 — a limitation critics note understates aviation's true climate contribution.

Future Solutions

Beyond avoidance routing, approaches include SAF with lower aromatic content (fewer soot particles, potentially fewer ice nuclei), engine design changes to reduce non-volatile particle emissions, and hydrogen combustion (water vapour contrails may have lower optical depth than kerosene contrails). Preferring daytime over night-time flights where feasible — engines at low power during CDA descents also produce fewer particles — are near-zero cost levers airlines could deploy quickly.