The white, cloudlike trails left in the wake of high-altitude jets are warming the earth more than the planes' carbon-dioxide emissions, a new study suggests. Although some so-called contrails, which are simply clouds of tiny ice crystals condensed from the moisture in aircraft exhaust, evaporate quickly, many linger for hours and spread across the sky. A new model that can estimate the formation and persistence of contrails suggests that as much as 6% of eastern North America can be covered with these humanmade clouds. In central Europe, coverage can reach 10%, researchers report today in Nature Climate Change. Just like natural cirrus clouds, contrails block infrared radiation emitted from Earth's surface, and the increased coverage provided by contrails boosts cloud-induced warming. On a worldwide average, contrail-induced cloudiness traps an extra 31 milliwatts of energy per square meter. Previous analyses suggest that the carbon dioxide emitted by aircraft since the beginning of the jet age traps 28 milliwatts per square meter.
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