It's become commonly accepted that the absence of aircraft contrails resulting from the grounding of all commercial airliners after 9/11 caused an immediate and noticeable change in the earth's temperature. But according to a piece in Flight International, the impact of contrails on the climate might be vastly overstated.
Contrails are the white, sometimes beautiful lines of clouds that flow from an airplane's engines as it streaks through the sky. They form high in the atmosphere when the mixture of water vapor in aircraft exhaust and air condenses and freezes. Contrails can spread out and turn into cirrus clouds that contribute to global warming by reflecting less sunlight than the amount of heat they trap.
Research into the impact of contrails on climate change has focused on daily temperature range, or DTR, which measures the difference between the hottest daytime temperature and coldest nighttime temperature on any given day. In one of the most widely cited contrail/climate studies (.pdf), scientist David Travis from the University of Wisconsin measured DTR in different locations around the country during the days immediately following 9/11, when all US commercial air traffic was grounded. He found that the DTR had reached up to 2-degrees Fahrenheit on those contrail-free days, more than double what had been seen in the previous 30 years. That supported the theory that contrails warm the earth by trapping gasses in the atmosphere.
"The assumption is a net warning," Travis said after his study was published.
But new research conducted at the UK's Leeds University seems to contradict or at least downplay Travis' findings. In the Leeds study, researchers incorporated contrails into a climate model and found that they do surpress DTR, but only by a tiny amount. Results from the Leeds study suggest that it would take 200 times as many flights over the USA as there are today to see the types of DTR changes suggested by Travis' research.
Others agree. "Aircraft are likely to be a significant factor in future climate, but probably not via their contrails," NASA's James Hansen told NOVA . "I think our main concern about aircraft will be their CO2 emissions, not contrails, which are a pretty small climate force."
Scientists speculate the real reason for the post-9/11 variation is something far less groundbreaking: a statistical quirk brought about by a change in wind direction that impacted low cloud cover or the fact that there were unusually clear conditions in the days following 9/11.
Photo: Flickr/FrancoiseRoache
