American Airlines demonstrated what could be the world’s cheapest way to fight global warming

Aviation is a significant contributor to climate change, with emissions from airplanes responsible for about 3.5% of human-caused global warming. It’s not simply the byproducts of burning jet fuel, however. Just look up: Airliners can leave behind contrails — condensation trails — of ice crystals that form artificial clouds around particles in the planes’ exhaust.

These clouds trap heat in the Earth’s atmosphere, and if we could reduce them, scientists believe that it could slow global warming. And good news, according to researchers at Breakthrough Energy and Google: This is low-hanging fruit.

Marc Shapiro, the head of contrails at Breakthrough Energy, calls reducing contrails the “highest-leverage climate opportunity that we know of,” the equivalent of removing carbon from the atmosphere at a cost of 10 dollars per ton or less.

The science of contrail control began in the 1940s, with efforts to build stealthier military aircraft, but by the 1990s scientists had realized that cirrus clouds play a heating role in the atmosphere — and cirrus clouds are similar to (and indeed can be seeded by) aircraft contrails. Last year, the Intergovernmental Panel on Climate Change (IPCC) estimated that contrails represent about 35% of the aviation industry’s contribution to global warming.

But with know-how from satellite observation, greased by machine learning, airlines can fly their aircraft to avoid creating contrails. The key is avoiding humid “ice supersaturated regions” of the atmosphere where long-lasting contrails can form. Typically, this requires changes in altitude similar to those pilots already perform to avoid regions of turbulence.

Breakthrough Energy, an organization founded by Bill Gates to pursue climate solutions, and Google’s Research division separately developed models to predict where contrails will form, and in 2022 they began collaborating to develop one solution. “Relative humidity predictions are notoriously inaccurate,” explains Juliet Rothenburg, who leads Google’s work at the intersection of climate change and “artificial intelligence.”

Their approach uses data from NASA’s GOES-16 weather satellite, along with other weather indicators. The developers had to train machine-learning software to recognize contrails in their data set, and then train it to predict where contrail-conducive regions would occur in the future.

A visualization of contrails detected in the atmosphere using satellite data and machine learning.Gif: Google

To test the product in the wild, the two organizations brought in American Airlines. The predictions were integrated into the pilots’ flight planning software, and over the course of 70 flights between January and March of 2023, pilots would fly both modified and standard routes to create useful comparisons. By examining satellite data after the flights took place, the researchers were able to conclude that their software reduced contrail creation by 54%.

“All airlines are going to have to do this eventually — in years, not decades,” Jill Blickstein, American’s VP of sustainability says.

Still, there are wrinkles to iron out. Notably, the additional maneuvering increased American’s fuel usage by about 2%, which Blickstein calls a “meaningful number. We don’t take additional expense lightly.” Beyond buying more fuel, it’s important to make sure the climate benefits of avoiding contrails also outweigh the additional emissions.

The three organizations are planning to expand these experiments and involve other airlines and air traffic control organizations, in particular with night flights in low-density airspace, like transatlantic routes. (Nighttime contrails have a much bigger warming effect.)

While deploying this technology across airlines is fairly straightforward — it’s another “weather data layer” to add to existing flight planning software, Shapiro says — the impact of those new maneuvers on air traffic would need to be reckoned with. Plans to use satellite data to optimize flight paths to burn less fuel, for example, have run into challenges coordinating with air traffic controllers.

It’s not yet clear how this service will be deployed commercially. Google and Breakthrough view it as research underlying their commitments to reducing carbon footprints. Blickstein says American is pleased to contribute to this research, but that any reduction in climate impact wouldn’t affect the airline’s pledge to reach net zero emissions by 2050 because the data is not granular enough to precisely calculate a single airline’s contrail contributions. Higher resolution satellites being launched in the years ahead could change that, Rothstein says.

Breakthrough’s hope is to develop an independent organization that can collect this contrail data and provide verification. Eventually, the vision is that the organization becomes an airline-supported clearinghouse for this data, like the International Airline Trade Association, a venue for global coordination in the air travel business.

Still, “we can do a lot with a coarse estimate,” Blickstein says. The potential is there to reduce about 1% of human contribution to global warming, if we can just fly the planes in the right places.

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