Airplane Contrails Have Surprising Effect on the Atmosphere

The climate impact of flying isn’t just about carbon emissions. The contrails that airplanes create also influence the temperature of our atmosphere—and a new study finds that impact is set to grow in a big way.

As planes cruise through the upper reaches of the troposphere. spewing exhaust, they also leave behind trails of water vapor that can form streaky cirrus clouds. Most of these contrail cirrus clouds dissipate quickly, but under the right conditions they can linger for hours, and when that happens they warm the atmosphere by absorbing thermal radiation emitted by the Earth.

Scientists have known about the greenhouse effect of contrail cirrus for years—in fact, there’s an entire niche field of research devoted to it. And it’s important: Globally, the atmospheric warming associated with these clouds is estimated to be larger than that caused by aviation’s carbon emissions. That surprising fact has some scientists curious about whether the effect will grow as the skies continue to get more trafficked into the future.

Now, a pair of researchers from the German Aerospace Center (DLR) has attempted to answer that question. Using a previously developed climate model that includes contrail cirrus clouds and an aviation emissions database developed by the U.S. Department of Transportation (which projects future air traffic out to mid-century), the authors looked at how the atmospheric warming effect of contrails will change. Their findings, published today in Atmospheric Chemistry and Physics, show that by 2050, contrail-induced warming could be three times higher than it was in 2006. In fact, this type of warming will likely outpace warming from rising carbon dioxide emissions, thanks to concurrent improvements in fuel efficiency.

Senior study author Ulrike Burkhardt of the DLR wasn’t too surprised by the results, noting that air traffic itself is projected to increase fourfold over the same time period. “It’s incredible how air traffic is increasing,” Burkhardt told Earther. She added that the expected rise in contrail cirrus warming is also, partly, due to modern planes flying slightly higher than their predecessors, which is likely to lead to more contrail cloud formation over the tropics.

Now, this effect is by no means a planetary catastrophe. The authors’ models indicate cirrus clouds will contribute some 160 milliwatts of additional ‘radiative forcing’—extra energy flowing back toward the Earth’s surface— by mid-century. Ethan Coffel, an atmospheric scientist at Dartmouth College who wasn’t involved with the paper, noted that for comparison, under the climate change scenario the authors use, heating from greenhouse gas emissions will be around 6,000 milliwatts per square meter by the end of the century.

“So while the contrail forcing is certainly significant, it’s a relatively small contributor to overall warming,” Coffel told Earther via email.

The warming effect of cirrus clouds is also a short-lived, and because it occurs in the upper atmosphere, it’s not clear how much of a difference it actually makes for temperatures at Earth’s surface, something Burkhardt called an “open research topic.” Regardless, it’s an important effect to understand, in part because there are obvious ways to mitigate it.

Tightening pollution controls to reduce the number of soot particles airplanes spew would help decrease the amount of water vapor condensing in their wake. However, soot would have to be decreased a lot to have a significant effect—even if it was reduced by 90 percent, the authors’ models show contrail cirrus clouds producing more warming in 2050 than they did in 2006. Ultimately, the best option for reducing this effect and aviation’s carbon footprint is to fly less.

As a final note to head off anyone planning to take this study wildly out of context: Airplane contrails are not part of a secret government plot to control your behavior or the weather, or to sterilize the population. They never were. And yes, there’s actually research to back that up.

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