🛫 Airplane contrails warm the climate less than scientists thought

🛫 Airplane contrails warm the climate less than scientists thought

New research shows that contrails from airplanes cause warming equivalent to two-thirds of the carbon dioxide emissions from jet fuel, not three times as much as previous studies indicated. Airlines are now conducting trials to avoid creating climate-affecting contrails.

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  • New research shows that contrails from airplanes cause warming equivalent to two-thirds of the carbon dioxide emissions from jet fuel, not three times as much as previous studies indicated.
  • Over 80 percent of long-lived contrails form in already existing clouds, where they can increase the clouds' reflective capacity and reduce the warming effect.
  • Airlines and researchers are now conducting large-scale trials with AI-based tools to avoid creating climate-affecting contrails.

Contrails and their climate effect

Contrails form when water vapor from airplane engines condenses on soot particles and freezes into ice. Most contrails disappear quickly and have minimal climate impact. But when they appear in atmospheric layers with high humidity, the ice particles can grow and form so-called cirrus clouds.

These clouds let most of the sun's energy pass through but effectively absorb infrared heat from the Earth's surface. The effect is greatest at night, when the clouds reflect no sunlight and only trap heat.

New research provides revised picture

A study from German DLR, published in the Journal of Climate in April, used a climate model to measure the actual surface warming from contrails. The result showed that warming corresponded to two-thirds of the carbon dioxide effect, not three times as much as previous estimates.

David Lee, climate scientist at Manchester Metropolitan University who led an influential 2021 study, confirms the new findings. The model also identified a possible explanation: while increased carbon dioxide causes low, reflective clouds to dissipate, contrails do not have the same effect and even seem to be able to increase the total amount of reflective clouds.

Majority form in existing clouds

Another study, published in Nature Communications, analyzed seven years of data from the IAGOS project that equipped ten commercial aircraft with sensors. The researchers found that over 80 percent of cirrus clouds from contrails formed where clouds already existed.

When contrails thicken existing clouds, these become more reflective. This can reduce the warming effect or in some cases even tip the balance toward cooling. Andreas Petzold, atmospheric scientist at Jülich Research Center, points out that this must be considered in strategies for avoiding contrails.

Large-scale trials underway

Field trials to avoid contrails have expanded. Last year, DLR conducted a trial with 100 flights and four airlines on behalf of the German government. This year, Maastricht Upper Area Control Centre, which oversees air traffic over northwestern Germany and the Benelux countries, is testing Google's AI-based predictions.

Earlier small-scale trials, conducted by DLR in 2021 and by Google together with American Airlines in 2023, showed that pilots can adjust their flight paths to avoid the humid atmospheric layers where contrails form.

Citizen science provides new data

Denis Vida, planetary scientist at Western University, runs Global Meteor Network, a citizen science project with over 1,000 low-cost security cameras originally used to monitor fireballs. Last year, a participant discovered that the cameras were also excellent for monitoring contrails.

The network has now switched its cameras to run around the clock. These observations will form the foundation for ContrailBench, a database being developed by Google and others to train AI models to predict contrails.

Soot particles from contrails can eventually fall down and contribute to the formation of reflective clouds in lower atmospheric layers, which would further cool the surface.

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