When airplanes move through the atmosphere, their engines burn fuel at high temperatures and produce water vapor. As this vapor encounters the cold upper atmosphere, it can form visible trails in the sky. These contrails have become a focal point for scientists and industry alike, prompting concerns about their potential impact on the planet.
Environmental activists and various nonprofits have long critiqued these trails, arguing that they contribute more to warming than the emissions alone. The debate centers on a 2021 study led by David Lee, head of the aviation working group for the United Nations Intergovernmental Panel on Climate Change, which suggested that contrails can have a warming effect comparable to, or greater than, the CO2 emitted by engines. The initial conclusions have been widely cited, with estimates suggesting contrails account for a small but meaningful portion of climate change.
An aspect that has been reevaluated
Public opinion on the harm of vapor trails has shifted in unexpected ways. David Lee later published a new review detailing the uncertainties involved and urging calm in the discussion. The piece argues that the claim contrails should be reduced lacks definitive scientific proof. Beyond the straightforward greenhouse effect, other potential effects are rarely considered. They might even have a cooling effect in some situations by reflecting sunlight, though this is only a theory and not a settled fact. At the same time, high, cold clouds can trap heat at the surface, contributing to the greenhouse effect in a different way.
This remains a clear demonstration of how research on contrails and their climate influence is unclear and complex, with no single consensus.
Lee has also warned against simple misinterpretations of his data. The interaction of aerosols with clouds is complex and varied, with numerous studies yielding different results. The 2021 assessment does not venture a precise prediction on many of these interactions.
Further testing
Despite ongoing uncertainties, the debate has intensified media scrutiny and spurred expanded scientific programs to better understand contrails’ role in climate change. In October, Boeing and NASA conducted flight tests from Everett, Washington. A NASA DC-8 and an Airbus A737 MAX 10 flew in formation to sample exhaust and analyze the signatures of potential aerosols. The aim was to determine whether sustainable aviation fuels (SAFs) could reduce aerosol formation.
In parallel, Google and the Breakthrough Energy consortium—an environmental research initiative founded by Bill Gates—undertook a small study with American Airlines to explore whether pilots could avoid air regions more prone to contrail formation. In the first test of the year, Google integrated satellite data, weather information, and flight data to build contrail-prediction maps for 35 American Airlines routes. Pilots were asked to test two routes: one avoiding high-contrail zones and one following the usual path. The route avoiding high-contrail zones reduced contrail formation by about half.
On the issue of exaggeration
Some researchers urge calm to avoid measures that might cause more harm than good in the long run. Others challenge Lee’s latest conclusions, arguing that the issue is overstated. For example, Marc Sapiro, director of the Breakthrough Energy project, expressed disagreement in a recent interview.
The policy landscape has also been affected in Europe. European airlines are expected to monitor, report, and verify climate impacts beyond CO2 on flights starting in 2025. By 2028, the European Commission plans to propose measures to address these effects, with contracts and related policies commonly identified as key considerations.
Reference article: phys.org news 2023-12 NASA Boeing jet contrails science
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