They may seem harmless, but their climate impact is similar to that of CO2 emissions. They trap heat in the Earth, contributing to global warming, and the CO2 emitted by burning jet fuel is generally considered the main environmental impact caused by airplanes. But another less-remembered culprit lurks: the so-called contrails: thin, linear clouds left by planes when they cross the sky. They may seem harmless, but their climate impact is as large as the sector's carbon dioxide emissions.
“I am now more concerned about the ban than about CO2 emissions, because it is an impact that the industry has not absorbed in any way,” said Jayant Mukhopadhyay, senior aviation researcher at the International Council for Clean Transportation (ICCT). ), an American environmental think tank.
According to a 2021 study, non-CO2 emissions could account for up to two-thirds of aviation's climate impact. The footprints left by airplanes represent 57% of this amount, similar to the CO2 emissions derived from the burning of fossil fuels.
Currently, the aviation sector is responsible for 2% to 3.5% of annual CO2 emissions worldwide. “But if we start to take into account these other pollutants responsible for aviation, the sector is contributing to global warming. [global] Very big,” Mukhopadhyay said, adding that it could be three times larger than previously estimated.
What are they and how are they formed?
Obstacles form when planes fly through pockets of very cold, moist air in the atmosphere. When airplane engines burn fuel, they emit carbon dioxide, soot, and steam. These water molecules stick to microscopic soot particles and freeze, forming ice crystals that form the linear patterns seen in the sky.
If the number of ice cubes is very large, elongated clouds called cirrus clouds are formed. “Some last a few seconds or minutes; Others can last for hours or even days, depending on the level of humidity and temperature,” said NASA senior scientist Patrick Minnis, who studies the climate impact of obstructions and the behavior of clouds. Cirrus clouds.
Cirrus clouds formed by passing airplanes filter sunlight, but on the other hand, they do not allow terrestrial radiation to return to space, which traps heat in the atmosphere. Its effects are worse at night, when sunlight is not reflected into space. “Creating contrails is basically like putting a blanket around the Earth, which traps heat and warms the planet,” Mukhopadhyay said.
How hot do they get?
Although scientists say these particle trajectories raise Earth's temperature, there is less consensus about the degree, timing, and what this means for climate change. “The exact magnitude of the impact of the contradictions is uncertain. Estimates range from 30% to four times the impact of CO2, so it is a very wide range,” Mukhopadhyay said.
This is because most studies are missing data and are therefore subject to uncertainty. According to Mukobadaya, unlike CO2, which has a long life, analyzing the impact of contrails over 100 years is meaningless because they dissipate quickly.
“We are interested in knowing how much the global mean surface temperature changes due to these pollutants,” Mukhopadhyay said.
How to reduce the climate impact?
Because condensation trails are short-lived compared to CO2 emissions, their impact fades quickly if efforts are made to reduce their formation, according to experts. One solution is to use “cleaner” fuels with less sulfur, such as hydrogen. This would reduce the amount of pollution emitted by airplanes and shorten the life of contrails, says NASA's Patrick Minnis.
Last year, the European Union passed legislation setting targets for sustainable aviation fuels. By 2050, 70% of all jet fuel sold in European countries should be “sustainable aviation fuels.”
Scientists have also discovered that not all aircraft create obstacles, which depend on the weather and flight path. For example, diverting less than 2% of flights in Japan could reduce the warming effect by almost 60%, according to a 2020 study.
But predicting in advance which routes will have the most clues is a difficult task.
“You have to treat the environment like a piece of cake. The top layer of the cake is the upper atmosphere, and within it are all kinds of layers of moisture. Therefore, it is relatively difficult to know exactly where these high humidity layers are. “Minnis said.
Adequate forecasting will require improvements in satellite operation and more data storage, which is expensive, the NASA researcher added.
Flying at a lower altitude would be an alternative because vapor condensation occurs at higher altitudes, but “the problem is that if you fly lower, you have more turbulence and you use more fuel,” Minnis said. That means more CO2 emissions.
“But we estimate that the impact of these additional CO2 emissions is significantly smaller than the impact of creating barriers,” said ICCT's Mukhopadhyay.
More data needed
A 2024 study by the International Air Transport Association (IATA), an industry trade body, suggests that more data should be collected to understand the non-CO2 impacts of aviation.
Airlines such as Lufthansa, Air France, KLM and American Airlines have already launched test flights to avoid creating obstacles by flying above or below dangerous areas with the help of satellite images, weather data, software models and powered forecasting tools. by artificial intelligence.
“This is a great first step, since 50% of the warming is occurring in the three regions with the most air activity: the United States, the EU and the North Atlantic,” said Mukhopadhyay.
A 2024 report from the University of Cambridge claims that accelerating the implementation of a global deterrence system could reduce aviation's climate impact by 40%.
The European Union agreed to require airlines to monitor and report on the climate impact of bans, which the industry opposes.
“The aviation industry has been delaying its actions for about 20 years due to uncertainty about the science,” Mukhopadhyay said, despite this, research is now being done in real life rather than computer simulations. “Very promising for future preventive measures.”
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