Technology · March 12, 2024

How rerouting planes to produce fewer contrails could help cool the planet

A handful of studies have concluded that making minor adjustments to the routes of a small fraction of airplane flights could meaningfully reduce global warming. Now a new paper finds that these changes could be pretty cheap to pull off as well.

The common climate concern when it comes to airlines is that planes produce a lot of carbon dioxide emissions as they burn fuel. But jets also release heat, water vapor, and particulate matter that can produce thin clouds in the sky, known as “contrails,” in particularly cold, humid, icy parts of the atmosphere.

When numerous flights pass through such areas, these condensation trails can form cirrus clouds that absorb radiation escaping from the surface, acting as blankets floating above the Earth. 

This cirrus-forming phenomenon could account for around 35% of aviation’s total contribution to climate change—or about 1% to 2% of overall global warming, according to some estimates.

A small fraction of overall flights, between 2% and 10%, create about 80% of the contrails. So the growing hope is that simply rerouting those flights could significantly reduce the effect, presenting a potentially high leverage, low cost and fast way of easing warming. 

Last summer, Breakthrough Energy, Google Research, and American Airlines announced some promising results from a research collaboration, as first reported in the New York Times. They employed satellite imagery, weather data, software models, and AI prediction tools to steer pilots over or under areas where their planes would be likely to produce contrails. American Airlines used these tools in 70 test flights over six months, and subsequent satellite data indicated that they reduced the total length of contrails by 54%, relative to flights that weren’t rerouted.

There would, of course, be costs to implementing such a strategy. It generally requires more fuel to steer clear of these areas, which also means the flights would produce more greenhouse-gas emissions (more on that wrinkle in a moment).

More fuel also means greater expenses, and airlines aren’t likely to voluntarily implement such measures if it’s not relatively affordable. 

A new study published in Environmental Research: Infrastructure and Sustainability explored this issue by coupling commercial tools for optimizing flight trajectories with models that simulated nearly 85,000 American Airlines flights, both domestic and international, under various weather conditions last summer and this winter.

In those simulations, the researchers found that reducing the warming effect of contrails by 73% increased fuel costs by just 0.11% and overall costs by 0.08%, when averaged across those tens of thousands of flights. (Only about 14% of the flights needed to be adjusted to avoid forming warming contrails in the simulations.)

“Obviously there’s a trade-off between added fuel and reductions in harmful contrails; that’s real, and it’s one of the biggest challenges to this climate solution,” says Marc Shapiro, a coauthor of the paper and director of the contrails team at Breakthrough Energy, an organization founded by Bill Gates to spur innovation in clean energy and address climate change. “But what we’re showing in this paper is that the added fuel burn is a lot less than we expected.”

Airlines could also use such a commercial trajectory tool to make decisions that balance their financial and climate goals, he says. For example, they could allow some contrail-forming flights when the cost of adjusting the routes would be especially high.

Other research groups and airlines are also evaluating this concept through projects, including a collaboration between Delta and MIT’s Department of Aeronautics and Astronautics. (MIT Technology Review is owned by MIT but is editorially independent.)

There are other approaches to reducing contrail formation, including switching to different types of fuels or continuing to develop more capable electric or hydrogen-powered aircraft. 

But the studies to date suggest that rerouting flights could be one of the simplest ways of substantially reducing contrail-related warming. 

“So far, it’s looking very promising that it will be the cheapest, fastest way to reduce the climate impacts of aviation,” says Steven Barrett, head of the MIT department. 

Finding any way to make near-term progress on aviation is all the more important since it’s still likely to take a long time to develop and implement scalable, affordable ways of addressing the emissions from heavy fuel use, he adds.

But it will take more modeling studies and real-world experiments to demonstrate that “contrails avoidance,” as the approach is known, works as effectively as hoped.

For one thing, Barrett says, researchers still need to test, refine, and engineer systems that can reliably predict, with enough time to reroute planes, when and where contrails will form—all amid shifting weather conditions.

There are also some thorny complications that still need to be resolved, like the fact that cirrus clouds can also reduce warming by reflecting away short-wave radiation from the sun.

The loss of this cooling effect would have to be tallied into any calculation of the net benefit—or, perhaps, avoided. For instance, Shapiro says the initial strategy might be to reroute flights only during the early evening and night, which would eliminate the sunlight-reflecting complication. 

In addition, any decreased warming from contrail avoidance must more than offset the added warming from increased greenhouse-gas pollution. This becomes a trickier question when we weigh whether we care more about short-term or long-term warming: not producing contrails delivers an immediate benefit, but any added carbon dioxide can take decades to exert its full warming effect and may persist for hundreds to thousands of years.

The new study, at least, found that even when additional greenhouse gases are taken into account, reducing contrails cuts net warming over both a 20-year and a 100-year timeline, though less so in the latter scenario. But that, too, would need to be evaluated further through additional studies.

Yet another open question is whether airspace constraints and traffic bottlenecks might limit airlines’ ability to regularly reroute the necessary flights.

As a next step, Breakthrough Energy hopes to work with airlines to explore some of these questions by scaling up real-world flights and observations. 

But even if subsequent studies do continue to indicate that this is a fast, affordable way to ease warming, it’s still not clear whether airlines will do it if regulators don’t force them to. While the fuel costs to make this work may be tiny in percentage terms, they could add up quickly across a fleet and over time.

Still, the study’s authors assert that they’ve shown contrail avoidance could deliver “massive immediate climate benefits at a lower price than most other climate interventions.” In their view, this approach “should become one of aviation’s primary focuses in the coming years.”

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