Temperatures rising above 118 degrees grounded some aircraft at the Phoenix Sky Harbor International Airport. The hot air was too thin to provide the lift they needed to take off successfully. Credit Patrick T. Fallon/Bloomberg


Excess heat in Phoenix grounded more than 40 flights in recent days, and scientists say a warming climate could also mean more turbulent rides...

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 to travelers from increased turbulence.

“We tend to ignore the atmosphere and just think that the plane is flying through empty space, but of course, it’s not,” said Paul D. Williams, a professor in the Department of Meteorology at the University of Reading in Britain who studies climate change and its effect on aviation. “Airplanes do not fly through a vacuum. The atmosphere is being modified by climate change.”

The problem in Phoenix primarily affected smaller jets operated by American’s regional partner airlines. “When you get in excess of 118 or higher, you’re not able to take off or land,” said Ross Feinstein, a spokesman for American Airlines, referring to the smaller aircraft.

Bigger jets like Boeing 737s and Airbus A320s have higher operating thresholds (126 and 127 degrees, respectively), he said. All three of those maximum temperatures are specific to the Phoenix airport; aircraft have different maximum operating temperatures depending on a variety of factors, including airport elevation.

But even though bigger planes weren’t affected, Mr. Feinstein said, American decided to give passengers on any flight to or from Phoenix between 3 p.m. and 6 p.m. — the hottest part of the day — the option to change their trips. Over all, more than 350 flights were potentially affected by the hot weather in Phoenix.

Robert Mann, the president of airline industry analysis firm R. W. Mann & Company, said that although airlines were working to become more efficient now, they were not doing much to prepare for the longer-term effects of climate change. “In a world where they’re focused on near-term issues, the glacial rate of environmental change is not within their fleet-planning horizon,” he said.

Mr. Feinstein of American Airlines referred questions about the effect of climate change on flying to an industry trade group, Airlines for America. The trade group provided its Earth Day statement describing its members’ efforts to become more environmentally friendly by using more fuel-efficient engines and modifying planes to be more aerodynamic.

How Higher Temperatures Affect Flying

As temperatures increase, air density decreases, which reduces lift and makes it harder for airplanes to take off. To address this, airlines could reduce airplane weight (by loading fewer passengers and less fuel or cargo) or schedule departures for cooler periods of the day.

Aviation is a major producer of carbon dioxide, responsible for about 2 percent of human-made emissions each year.

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. High-altitude airports like Denver have thinner air by nature, so lift is even more affected by higher temperatures.

La Guardia Airport in New York could also be affected, even though it is at sea level. La Guardia has a short runway relative to other major commercial airports, and on particularly hot days that can be a problem: Planes might not have enough distance to achieve the speed and lift needed to get airborne.

“Typically in the hotter days of the summer, you may have to bump payload, which includes cargo and/or passengers,” said David Wilhelm, a senior dispatch manager at Southwest Airlines. Reducing weight allows a plane to take off with less lift.

La Guardia, because of its short runway, already forces many planes to reduce their weight, regardless of the weather. A Boeing 737, for example, has to cut its maximum payload by a thousand pounds for a successful departure. That restriction increases on hotter days, up to 15,000 pounds when the temperature hits 91.4 degrees.

Restrictions like these are determined by individual airports and airlines, and not by a standardized industry regulation. American Airlines consults National Weather Service data and plugs it into a formula to calculate air density to determine if conditions at a given airport are suitable for takeoffs and landings.

In 2015, Radley Horton, a research scientist at Columbia University’s Earth Institute, published a joint study with a Ph.D. student, Ethan Coffel, on the effect of extreme heat on aviation. The conclusion: “We can say with confidence that there will be more weight-restricted days, and larger weight restrictions,” he said.

Already, since the 1980s, airports have seen an increasing number of weight-restricted summer days, their research found. “One thing that’s become abundantly clear,” Dr. Horton added, “is this is an underexplored area.”

How Stronger Winds Affect Flying

At cruising altitudes, winds are becoming stronger and more turbulent. Since the jet stream generally travels from west to east, this means that flight times could get longer heading westbound and shorter heading eastbound. This would probably increase overall travel time and fuel consumption across the industry.

The study examined conditions at four airports: La Guardia; Ronald Reagan Washington National Airport, which also has relatively shorter runways; Phoenix Sky Harbor International Airport; and Denver International Airport. Some airports, like Denver, could counteract rising temperatures by extending their runways. That option is less workable for a location like La Guardia, however, as it is hemmed in by the East River.

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. Horton 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 also pointed out that a no-fly window of even a few hours at a particular airport could have a ripple effect across airline operations while further squeezing airlines’ already tight profit margins.Extreme heat on the ground also affects airport workers; loading and unloading luggage and servicing planes between flights could become more onerous. In Phoenix this week, American Airlines set up cooling stations — air-conditioned tents on the tarmac — for its employees.

Places like Phoenix, already known for summer heat, are measurably warming up. Data from the National Centers for Environmental Information show that every year since 1976 has been hotter than the city’s historical average. Seven of the 10 hottest years on record there have been in the past decade.

With forecasts predicting record-breaking temperatures in Phoenix on Tuesday — and some flights being canceled pre-emptively — many passengers stayed away from the airport entirely. Security lines had almost no wait. The terminals were so empty, one traveler was spotted riding his bike through the airport.

Allison Thomas, a 28-year-old college student, said she had endured three flight cancellations as she tried to return home to Seattle. “I’m so tired!” she said, but added that she felt lucky that she had family in town. “Other people had to go to hotels.”

After her third cancellation, Ms. Thomas finally gave up. She decided to drive to San Diego to try to catch a flight there — despite the risk that planes might not take off there, either.

Longer runways can help aircraft build up the speed they need to take off even on hot days, but that is of little help to airports like La Guardia in New York, which is hemmed in by New York City and the East River. Credit Timothy Fadek/Bloomberg

Jet stream winds at high altitudes are getting more intense, researchers say, which not only makes flights bumpier but also potentially affects travel times. Dr. Williams pointed out that from Jan. 8 to Jan. 12, 2015, a strong jet stream forced some flights from Europe to the United States to make unscheduled refueling stops on the East Coast before reaching their final destinations, even though they should have had enough fuel to make it all the way.

On the other hand, flights in the other direction during that time with the jet stream at their backs made the trip faster than usual. In fact, a British Airways flight broke the conventional passenger jet trans-Atlantic speed record, making the trip in 5 hours and 16 minutes, Dr. Williams pointed out. Only the supersonic Concorde has done it faster.

It may seem counterintuitive, but the gains flying east aren’t enough to offset the losses flying west. Dr. Williams has researched this, too, and he found that fighting the headwind slowed a flight more than flying with a stronger tailwind sped it up. This would eventually produce longer round-trip flight times, and higher fuel consumption.

Transcontinental flights over the United States may in the future have to make midroute stops more frequently, too. On a calm-weather day, a nonstop flight from New York to Los Angeles already approaches the maximum range of a Boeing 737 or Airbus A320, common aircraft on that route and others like it.

Airlines, of course, could also use jets with longer ranges on such routes. But as Dr. Williams points out, “It might be preferential to break it up into two shorter segments.”

Dr. Williams published another paper, in May, which 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, something that is known as “clear air” turbulence. Eastbound flights would be more likely to be affected because they tend to fly more directly in the jet stream to take advantage of the tailwind.

Although modern aircraft are better able to respond to turbulence than their predecessors, passengers may still notice bumpier rides in the future. Since the 1980s, Dr. Williams said, “the number of serious injuries being caused by turbulence has a clear upward trend.”

Manufacturers are working to make aircraft more adaptable to bumpy rides, and to provide technology that can better predict and detect clear-air turbulence.

Laura Einsetler, a captain for a major domestic airline who runs her own aviation blog, said in her experience, flying conditions had become more extreme in the last four or five years. “We always used to course-deviate, but it didn’t feel like this type of anger in the world with global climate change. It definitely feels like we need to be more on our toes.”

Correction: June 20, 2017

An earlier version of this article misstated the recent policy of American Airlines to allow passengers traveling to or from Phoenix to change their trips. It applied to flights arriving or departing the airport between 3 p.m. and 6 p.m., not all flights.

Rebekah Zemansky contributed reporting.


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