Slowing down climate change is going to require big fixes, and many small ones, too.

Case in point: contrails, those wispy white lines that trail some airplanes flying high in the sky.

Contrails, short for condensation trails, are produced when exhaust from jets mixes with water vapor at extremely high altitudes, forming minuscule ice particles. Scientists have known for decades that in some cases, contrails spread out across huge areas, trapping heat in the atmosphere.

That may sound insignificant given the vastness of the sky. Yet studies have estimated that contrails are responsible for as much as 35 percent of all of the planetary warming attributable to aviation. By some measures, contrails account for upward of 1 percent of human-caused global warming.

“We now know enough about contrails and their impact to know we need to do something about it,” said Andrew Chen of RMI, a nonprofit that promotes sustainability.

Here’s the new part: A team from Google, Breakthrough Energy and American Airlines says it has demonstrated a relatively cheap and easy way to significantly reduce contrails.

Their research, which was shared exclusively with Climate Forward before being submitted to a scientific journal, found that adjusting a plane’s altitude by just a couple thousand feet reduced contrail formation by more than half.

The results, they say, suggest it could be relatively easy, quick and cheap to start reducing contrails at scale. “The opportunity here is twofold,” said Chen, who was not involved in the project. “It’s near-term, and it’s cost-effective.”

(And for the record: Contrails are not a bioweapon used to control the masses, as some conspiracy theorists would have you believe.)

Changing altitudes

The researchers used weather and satellite data, along with some machine learning, to develop a model that predicts where contrails are most likely to form.

Using the model created by Google and Breakthrough, American slightly altered the flight paths of 70 daytime flights. Pilots flew one leg of a round-trip journey at an altitude where contrails were expected to form. Going the other way, they flew at a slightly lower altitude, hoping to avoid contrail formation.

After the flights, the researchers looked at satellite imagery and determined that in total, the flights at the lower altitudes produced 54 percent fewer contrails.

“It’s a really exciting study,” said Chen. “I’m very pleased with the design, and it will help us answer a bunch of very high-priority questions right now.”

There was a cost: Flights avoiding contrails used 2 percent more fuel on average, because flying at lower altitudes expends more energy. That’s a big problem for an industry in which one of the biggest expenses is jet fuel — American spent nearly $14 billion on it last year.

“Burning more jet fuel is not inconsequential to American or to any other airline,” said Jill Blickstein, the airline’s vice president of sustainability. “We wouldn’t do this at any cost.”

Moreover, it wouldn’t make much sense for the broader climate challenges to have airlines burn more jet fuel just to prevent contrails.

But the researchers say they expect that only a small fraction of flights will need to change altitude to avoid contrails, and that in total, the additional fuel burn should be only 0.3 percent higher, resulting in a relatively cost-effective way to reduce global warming, even as the larger fight to reduce fossil-fuel consumption continues.

The challenges ahead

Beyond the potential costs, there are questions and challenges that need to be addressed before contrail avoidance becomes a routine part of commercial aviation.

The contrails that produce the most warming are those that form at night. They block heat that is rising from the ground, without any offsetting reflection of incoming sunlight. But the tests conducted by American took place during the day. The researchers say they are confident their approach will be just as successful during night flights, but it will take more tests to prove them right.

Then there’s the matter of making this technology widely accessible. The researchers say they’re talking with the Federal Aviation Administration about integrating forecasts for contrails into flight-management software, just like turbulence forecasts.

(A study published in Geophysical Research Letters in June found that climate change had increased the amount of turbulence on popular flight routes by more than 50 percent since the 1970s.)

“The airline industry knows how to avoid turbulence,” said Juliet Rothenberg, the lead product manager for climate A.I. at Google. “It’s a very straightforward extension to avoid contrails.”

Others are working on the problem, too, including Eurocontrol, a pan-Europe aviation organization; Delta, which is working with M.I.T.; and Satavia, a British start-up that is working with Etihad and KLM Royal Dutch Airlines.

There are also other ways to reduce contrails. Using low-carbon aviation fuel, which is in limited use but growing fast, can also help because its exhaust contains less soot.

Reducing contrails alone won’t solve aviation’s climate-change problem. Air travel is responsible for about 3.5 percent of human-caused warming, according to the National Oceanic and Atmospheric Administration. The much bigger challenge will be finding ways to power jets without fuels that spew planet-warming emissions into the atmosphere — or to simply fly less.

But the experiment by Google, Breakthrough and American is the most promising sign yet that contrail avoidance can work. It points to a relatively cheap, easy and quick way to reduce one source of global warming.

“You can’t just do contrail avoidance,” Rothenberg said. “But this is really unique among climate solutions in that it can scale in years, not decades.”

Forest countries unite for the Amazon

The countries of the Amazon Basin are holding the highest profile meeting in half a century to find common cause in protecting the world’s biggest rainforest.

Leaders from Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, Suriname and Venezuela are attending the summit in Belém, Brazil this week, along with attendees from other tropical forest countries, including Indonesia and the Republic of Congo.

Attendees criticized wealthy countries for not following through on a promise to deliver $100 billion in climate finance annually to poorer nations. The leaders of Norway, France and Germany were invited to attend the summit but are sending representatives instead. The United States was not invited.

It won’t be easy to reach a consensus. Colombia has been pushing for a ban on new oil development in the Amazon, but Brazil is still considering a huge offshore oil drilling project at the mouth of the Amazon River. And an attempt by Luiz Inácio Lula da Silva, the president of Brazil, to secure a pledge to end illegal deforestation by 2030 looks like a long shot.

Observers don’t expect any bold commitments in Belém, which will host the United Nations COP30 climate meetings in 2025. But just the fact that it’s happening is a step forward, said Alicia Guzmán of the environmental nonprofit Stand.earth.

“Whatever the result, it’s a historic summit,” she told me. “Now, governments won’t be able to go back to normal, without thinking of everything they are killing.”

— Manuela Andreoni

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