Cutting contrail warming

The cloud streaks that form behind planes in the sky, known as contrails, contribute to climate warming. Now, using computer modelling, scientists have identified which flights are most likely to form contrails that have a big warming impact, and suggested ways that contrail formation can be avoided...

“We were able to use a high resolution data set of aircraft trajectories in the Japanese airspace,” explains Marc Stettler, one of the studies authors from Imperial College, London. “What that showed was that 2% of the flights contributed to 80% of the climate warming.”

Contrails contribute to climate warming by reflecting heat emitted from the Earth’s surface and preventing it from leaving the atmosphere. During the day, they also reflect incoming heat from space, but at night only the heat-trapping effect operates. “It’s really the contrails that persist for a long period of time into the night that have a warming effect,” says Stettler.

Contrails form when water in the air crystallises on soot particles ejected from a plane’s engine, which happens when a plane passes through a particularly humid patch of sky. However, the regions of the atmosphere that are sufficiently humid for contrail formation are relatively thin layers.

“Small altitude changes, either up or down by 2000 feet, would mean that an aircraft can fly outside that region,” explains Stettler. “This effect of contrails is something that we could almost switch off.”

The researchers were careful to avoid suggesting diversions that increased carbon dioxide emissions, even if they reduced contrail formation. Even with this very strict constraint, the model calculated that, by changing the altitude of 1.7% of flights, the contrail warming effect can be reduced by 20%.

The scientists combined a six week snapshot of aircraft trajectories and high resolution weather data with a model of aircraft soot emissions and contrail formation. This allowed them to model contrail behaviour of each flight. It was then simply a matter of ranking each flight by its contrail warming contribution to identify which flights were the worst offenders.

However, the researchers had the benefit of hindsight when it came to the weather data. Current flight plans are made based on weather forecasts. “One of the other pieces of work we are looking at now is how accurate the forecasts are in terms of predicting the high humidity regions of the atmosphere that drive contrail formation”, says Stettler.

For the aviation industry, safety is of course the most important factor. With that in mind, Stettler is cautiously optimistic about the next steps. “We are looking at engaging with air traffic management service providers to understand how they work , and how this strategy might be implemented within their processes.”