Although they're really just ice crystals, the contrails left by high-flying airliners are nonetheless not entirely innocuous. According to a new study, however, changing the altitude of flights by just 2,000 feet could greatly reduce their effect on the environment.
Condensational trails (aka contrails) form when moisture in cold air condenses on the black carbon particles found in hot aircraft exhaust, forming ice particles. The resulting ice-particle trails appear white and fluffy from the ground, and they typically only last for a few minutes.
Sometimes, however, they spread and mix with neighboring contrails – or with naturally-occurring cirrus clouds – forming what are known as "contrail cirrus" clouds. These may stick around for up to 18 hours.
As they do so, they trap heat emitted by the earth, keeping it from leaving the planet's atmosphere. This creates an imbalance called "radiative forcing," in which the amount of heat-causing radiation reaching the earth from the sun exceeds the amount of heat that escapes back into outer space.
According to previous research, aircraft-caused radiative forcing may have as much of a warming effect on the climate as the cumulative carbon dioxide (CO2) emissions of the entire aviation industry.
With that in mind, scientists from Imperial College London developed a computer model based on data from the airspace over Japan. Among other things, they determined that 80 percent of the radiative forcing in the region was caused by just two percent of the flights.
This was because those particular planes were flying through particularly humid layers of air, that were very conducive to the formation of contrails. By moving most of the offending aircraft 2,000 feet (610 m) higher or lower, though – thus getting them out of those layers – the model indicated that radiative forcing in the area could be reduced by 59 percent.
It was also found that altering the flights in this manner would cause the planes to use slightly under 10 percent more fuel, thus producing more CO2 emissions. Nonetheless, the reduction in radiative forcing was said to far outweigh the slight increase in greenhouse gases – especially if only select flights were moved.
"According to our study, changing the altitude of a small number of flights could significantly reduce the climate effects of aviation contrails," says Dr. Marc Stettler, lead author of a paper on the research. "This new method could very quickly reduce the overall climate impact of the aviation industry."
The paper was recently published in the journal Environmental Science & Technology.
