Too hot to fly? Climate change may take a toll on 

air travel

Zach Wichter NYT NEWS SERVICE Cancellations, Bumpier Rides May Increase

In recent days, American Airlines has been forced to cancel more than 40 flights in Phoenix. The reason: With daytime highs hovering around 120 degrees, it was simply too hot for some smaller jets to take off. Hotter air is thinner air, which makes it more difficult -and sometimes impossible -for planes to generate enough lift.

As the global climate changes, disruptions like these are likely to become more frequent, researchers say , potentially making air travel costlier and less predictable with a greater risk of injury from increased turbulence.

Researchers are just beginning to explore how climate change affects aviation and planes' ability to fly . Because there is so little data available and so many factors at play -aircraft design, airport size and location, the weight of passengers and cargo, to name just a few -it can be hard to attribute any one service disruption to global warming.

Depending on their locations, airports may experience the effects differently. Highaltitude airports have thinner air by nature, so lift is even more affected by higher temperatures. Airports at sea levels could also be affected on particularly hot days if they have short runways: Planes might not have enough distance to achieve the speed and lift needed to get airborne.

As global temperatures continue to rise, some of the heaviest planes on the longest flights may eventually be unable to depart during the hottest part of summer days, Dr Radley Horton, a research scientist at Columbia University's Earth Institute, said.Like an ocean liner waiting for the right tide to leave port, airplanes may be grounded until the air is cool and dense enough for takeoff at full capacity .

He pointed out that a no-fly window of even a few hours at a particular airport could have a ripple effect across airli ne operations. Extreme heat on the ground also affects airport workers; loading and unloading luggage and servicing planes between flights could become more onerous.

One of the most important changes for global travel involves the jet stream, the powerful upper-atmosphere winds that aircraft must navigate. Jet stream patterns influence flight routes, travel time and airline fuel economy because long-distance air corridors are designed to take maximum advantage of prevailing weather patterns, which give a tailwind to eastbound flights and a headwind to westbound ones.

Paul D Williams, a professor at the University of Reading in Britain, suggested that incidents of turbulence could increase and become more severe. He said a stronger jet stream resulted in less stable air, so turbulence could become more likely even if there are no storms. Although modern aircraft are able to respond to turbulence, passengers may still notice bumpier rides in the future.