Airplane contrails–those wispy, cotton-bud streaks in the sky–may look innocuous on a summer’s day, but they have a dark side.
Aviation is responsible for 3.5% of all manmade global warming, according to the Intergovernmental Panel on Climate Change. And some studies show contrails–which can trap heat in the atmosphere–could have as much warming impact as airplane CO2 emissions.
Researchers are looking at ways of reducing contrails, which occur when planes fly through very cold, moist air. By re-routing planes–either by having them fly lower in the sky, or by going on slightly longer trips–it’s possible to cut greenhouse impacts. Working out optimum routes from a climate point of view is complicated, though. For example, longer trips could lead to higher CO2 emissions, which would nullify the contrail-related benefits.
A new paper in the journal Environmental Research Letters calculates the maximum extra distance you can add to flights (thus minimizing contrails) before you add too much CO2 to the atmosphere. In some cases, it might be worth going long extra distances, the researchers find.
It depends on the size and type of plane. For example, a small aircraft that produces a 20-mile contrail could fly 200 miles further and still reduce its impact. For larger airliners, the possible deviation distance would be smaller–probably four times less far, the paper says.
There is a lot of uncertainty in the analysis, the authors, from the University of Reading, admit. To route a plane according to its contrail impact would require predicting the length of contrails before take-off and their eventual climate impact, neither of which is easier to do. Contrails, like clouds, are known to reflect heat back to the Earth, thus raising temperatures. But the effect varies with the contrail, and the weather conditions around it.
Still, the researchers think air traffic controllers should consider contrails in their planning. They say in most cases adding just 60 miles to a route to avoid a contrail would have benefits. The question, though, is whether they can persuade traffic controllers and, of course, airlines of the need for longer flights and more fuel expenditure. It could be a hard sell.