The complete cessation of commercial air traffic offered a control sky without contrails for use in quantifying the environmental effects of contrails. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Get smart. Sign up for our email newsletter. Sign Up. Support science journalism. They do not accumulate and leave no lingering effect, whereas much of the CO2 emitted by aviation sticks around in the atmosphere for centuries.
As more and more is emitted, it keeps building up — even if aviation were halted, the CO2 would remain for a long time. So in the longer run, if we want to shut down global warming, there is no alternative to curbing CO2 emissions. But if the world wants a big short-term contribution from aircraft to keeping us below some specific temperature target, such as 1. Within days it would take some of the heat out of the atmosphere. Can it be done? Researchers spoken to for this article offer three approaches, though they disagree about which has the most potential.
One is to divert aircraft away from air where contrails are likely to form. This can be done vertically by changing altitude, or horizontally by detouring around the problem air. There is a potential trade-off. In retrospective studies of real flights, Banavar Sridhar, a senior scientist at the NASA Ames Research Center in California, found that changing altitude — usually by flying lower where the air is warmer — could produce a reduction of 35 percent in contrails and contrail cirrus for an extra fuel burn of only 0.
Even so, if done right, there could be a net overall reduction in warming. Volker Grewe of the Institute of Atmospheric Physics looked at real transatlantic flights across the seasons and how their flight paths could have been altered to cut their combined warming effect from both contrails and CO2.
He found that there could have been a 10 percent reduction in warming for only a 1 percent increase in operating costs. Contrails over Lisbon, Portugal, in February Of course, figuring out optimum flight paths after the event is different from doing it in a busy air-traffic control room.
Less attention has been given to the idea of altering the timing of flights by drastically reducing flying at night, when contrails have their greatest warming potential. This would not be entirely effective since daytime contrails produce cirrus clouds that can last long into the night. But it could help. A second approach to minimizing contrails is to change fuels — from kerosene-based fuels to biofuels, hydrogen, liquid natural gas, or even electricity.
Electric propulsion from batteries could one day end all contrails, but is far off, especially for long-haul flights. Even a mix of biofuel and kerosene-based fuel could halve soot. Reducing soot may not necessarily reduce contrails. There is potentially a third approach: changing engine design to generate more propulsive energy from a given fuel burn. This is already an overriding priority of aviation engineers seeking to cut costs, and it would also reduce CO2 emissions.
But there is problem as regards contrails, say Bock and Burkhardt. Ever see the lines that follow planes high up in the sky? They are called contrails and for Weather Whys viewer Jim from Whitefish Bay asked why sometimes planes have them and other times they don't. Contrails form when jet exhaust emits water vapor that condenses and freezes.
Contrails don't form for every airplane.
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