THERE are few more delightful antidotes to stress than to lie on your back in warm grass and watch the clouds go by. As children, we love finding the outlines of animals and castles in the billowing shapes. As adults, there is something calming and comforting about those fluffy tufts of white drifting slowly past. Clouds are beautiful. Clouds are innocent.
With one exception. The streaky smears of cloud that crisscross the sky in the wake of airplanes may look too wispy to cause any harm. But we now know that these condensation trails, or contrails, make an outsized contribution to global warming by trapping heat like a downy jacket. “They are one of the few manifestations of man-made climate change agents that you can actually observe,” says David Lee, an atmospheric scientist at Manchester Metropolitan University in the UK. As the evidence mounts to show how harmful contrails are, some engineers are reaching for an audacious solution: scrub them from the sky altogether.
Contrails are created when water condenses to form ice crystals around tiny particles of soot from aircraft exhausts. Yet there is no fundamental reason why this has to happen. Decades of experiments with spy planes, alternative engines and, most recently, with artificial intelligence have shown that it is possible to stop them forming. It won’t be easy: wiping the atmosphere clean of contrails may require nothing less than a wholesale reimagining of the traffic in our skies.
You may think of contrails as tiny lines against a vast sky, but in certain conditions, they can be a lot more. They can stretch tens of miles. Wind can spread them out. They can linger for hours. So as plane after plane runs the same route through the air, new and old contrails mingle and accumulate, forming airborne mosh pits of ice cloud. Scientists call these “contrail cirrus”—high-altitude clouds that can spread over hundreds of square miles. And they’re likely to become more of a problem: One study found that as air traffic increases, the heat-trapping effect of contrail cirrus in 2050 could be three times greater than it was in 2006. Clouds trap heat coming off the earth that would otherwise head for space, making them the biggest variable in the planet’s temperature and climate, according to NASA.
New research out of the UK has found two silver linings in the cloudy conundrum. The first is that just 2.2 percent of flights create 80 percent of contrail-related warming. According to a January paper in the journal Environmental Science & Technology, the troublemakers are mostly flights that take off in the late afternoon and early evening, whose contrails live mostly during the night—when they still trap some heat but can’t deflect any sunshine (which can balance out their impact). “The effects at night are purely warming,” says Marc Settler, the lead author on the paper, Mitigating the Climate Forcing of Aircraft Contrails by Small-Scale Diversions and Technology Adoption.
The better news, Settler and his coauthors report, is that those planes can avoid making most contrails with minor altitude changes. Contrails form only when the air is heavily saturated with water molecules that can attach to soot particles from engine exhaust and condense, grow into ice crystals, and join into clouds.
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