Of the varied conspiracy theories regarding contrails—you know, chemtrails—one stands out for being especially wrong: the belief that the plane-made clouds are chemicals the government is secretly spraying to battle climate change , to the peril of those on the ground. First, contrails are nothing but the incidental result of mixing hot, water-vapor-filled jet engine exhaust with cold air. Second, the government has nothing to do with them. Most important, they’re not battling climate change. They’re accelerating it.
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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 Stettler, the lead author on the paper, Mitigating the Climate Forcing of Aircraft Contrails by Small-Scale Diversions and Technology Adoption.The better news, Stettler 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. Often, though, that saturation is confined to a narrow band of altitude, and airplanes can find drier areas by moving up or down.
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Stettler and his fellow authors based their findings on flights over Japan in 2012, which offered the best data they could access on the exact positions of actual flights. They matched that to weather data and used a computer model that predicts when aircraft create contrails to see which flights would have done the most damage.
These mayors are members of C40, a network of 94 large cities—Paris, Los Angeles, Shanghai, Lagos, to name a few—committed to meeting the goals of the Paris Agreement of limiting global warming to 1.5 degrees Celsius over preindustrial levels and reducing global greenhouse gas emissions by 50 percent by 2030.That declaration didn’t just reaffirm these cities’ efforts to fight climate change .