Airlines might be able to substantially reduce their impact on climate change simply by altering some flight paths, a recently completed study on contrail avoidance has found.
Contrails form when soot and water vapor emissions from jets mix with humid, cold air. They often look beautiful when framed against a blue sky, but they also trap heat in the atmosphere, just as clouds do. A 2021 study published in the peer-reviewed journal Atmospheric Science found that aviation is responsible for approximately 3.5% of global climate change, with contrails responsible for 57% of that impact.
Fortunately, scientists and airlines alike have a good idea of when and where contrails are most likely to form.
Furthermore, not all contrails are the same. Notably, some form and then quickly dissipate, while others can linger for several hours. The long-lasting ones often take on the appearance of ordinary cirrus clouds and are known as contrail cirrus. During the day, contrail cirrus, like clouds, can reflect the sun's radiation, cooling the atmosphere. But at night they trap heat in the atmosphere. Overall, contrails trap more heat than they reflect.
Contrail forecast mapping
In a recent study, American Airlines teamed with Google Research and the Bill Gates-founded nonprofit Breakthrough Energy to develop and test the effectiveness of contrail forecast mapping in enabling contrail avoidance.
Long-lasting contrails tend to form in cold and humid regions between 30,00 and 40,000 feet in elevation, and conditions for their formation are most ripe just ahead of incoming storm systems.
In their study, the Google Research, Breakthrough Energy and American teams brought together loads of data -- including satellite imagery, weather and flight-path data -- and then used AI to develop contrail forecast maps.
Armed with those forecasts, American pilots flew 70 test flights over six months. Analysis of satellite imagery from those flights showed that the pilots were able to reduce contrails by 54%.
"Together we've taken a significant step towards understanding a high-potential climate solution," Juliet Rothenberg, head of product for Climate AI at Google Research, said in a prepared statement.
The study also showed that pilots who attempted to avoid contrails by changing their flight course burned just 2% more fuel. Separately, a 2021 study conducted by the German Aerospace Center and the European air traffic control coordinator Eurocontrol, showed that as few as 10% of the commercial flights are typically in areas where warming contrails form.
Google Research said that the total fuel impact for contrail avoidance across an airline's flights could be as little as 0.3%, making the cost per ton of avoided C02 equivalents between $5 and $25.
"That would make it a cost-effective warming-reduction measure," Google Research said, adding that further improvements are expected.