Aircraft contrails do more to warm the planet than their exhaust fumes, a new study has found.
The contrails contribute to the creation of high-altitude cirrus clouds, which trap the Earth's heat - and have a far greater effect than the same planes' CO2 emissions.
"Aircraft condensation trails and the clouds that form from them may be causing more warming today than all the aircraft-emitted carbon dioxide (CO2) that has accumulated in the atmosphere since the start of aviation," say the authors in Nature Climate Change.
The scientists, at the German Aerospace Center (DLR), calculated that contrails and related cirrus clouds account for 31 milliwatts of warming per square meter, compared with 28 milliwatts from emitted CO2.
Contrails produce their warming effect primarily by trapping heat and preventing it from radiating back out to space. There's also a small effect from the fact that they limit slightly the amount of solar radiation hitting the Earth in the first place.
The effect comes not just when the contrails are visible as while lines across the sky, but, more significantly, when they spread out and merge into normal cirrus clouds. In this form, says the team, they end up covering 0.6 percent of the Earth's surface.
The authors point out that while CO2 remains in the atmosphere for hundreds of years, contrails don't - which means that cutting them somehow could have an immediate effect on global warming.
Possibilities might include making planes fly at lower altitudes, or developing engines that condense the emitted water into ice that would then fall to the ground.