In a 2015 study, researchers at Columbia University predicted that by 2050 there could be four times as many weight restriction days at the most at-risk airports in the United States. On the hottest days and longest flights, that could potentially mean dozens of passengers and their luggage are left waiting in the terminal.
To take off, a plane has to reach a certain minimum speed. On hot days and at high elevations, that minimum speed increases. “High elevation and high temperature mean less molecules of air for the plane to push off of,” explained Radley Horton, a climate scientist at Columbia who was involved in the study.
Longer runways can help, allowing pilots to attain a faster speed.
But when it’s too hot for a plane to take off, airlines must choose between canceling the flight, waiting until it’s cooler, or cutting back on weight. That’s why long flights out of the Middle East and parts of Central and South America are routinely scheduled for late in the evening or overnight.
The Columbia researchers found the number of weight restriction days has already increased since 1980. Some airports in the U.S. have adopted measures to deal with extreme heat. In Washington, D.C., those protocols are routine. Most summer days already have weight restrictions — because of temperatures, and also because the airport has a relatively short runway.
Those challenges rarely result in cancellations, explained Billy Nolen of Airlines for America, the airline trade association.
“You can normally accommodate weight issues by adjusting fuel levels,” he said.
Larger planes are better equipped to deal with weight restrictions. A standard 737 jet carries over 20 tons of fuel — and most flights do not require a full tank, Nolen said.
“These calculations happen every day, they’re computerized,” he said.
Smaller, older aircraft used for regional flights have an added challenge. Those models don’t stand up to extreme heat as well — and so, like the flights out of Phoenix last month — are much more likely to be cancelled.
Smaller planes like those in use on most regional flights are typically only certified to about 120 degrees — while more powerful aircraft can function up until 130 degrees, according to American Airlines spokeswoman Michelle Mohr.
And that, researchers say, means a higher likelihood of cancellations on short hops.
“In the future, we’ll see more planes unable to take off,” said Paul Williams, professor of meteorology at the University of Reading.
Using computer models, the Columbia researchers projected a dramatic increase in the number of days when extreme heat will necessitate weight adjustments. The study focused on four high-risk airports: Phoenix, because of its number of hot days; Denver, because of its elevation and thinner air; and New York’s LaGuardia and D.C.’s Reagan airports, because they have short runways and are highly trafficked.
“We can say with high confidence,” said Horton, “that the type of heat events that lead to weight limits are going to increase in the future.”
The researchers predicted the number of 10,000-pound restriction days — days when a standard 737 must be 10,000 pounds lighter than its maximum weight to take off — would increase from near zero to about 20 days per year in Phoenix.
The number of 15,000-pound restriction days is also expected to increase — and that tighter weight limit could lead to more bumping, Williams said.
“If there’s no more cargo to take off, then [airlines] start to bump off passengers,” said Williams.
In theory, that could mean offloading dozens of passengers when undertaking a cross-country journey on an extremely hot day. In cases where a full tank of fuel is necessary, 79 passengers plus their luggage would have to be removed to take off on a 15,000-pound restriction day.
Nolen insists that those kinds of weight restrictions are very unlikely — and that cancellations will also remain rare.
However, the International Civil Aviation Organization, the United Nations agency that oversees aviation, last year acknowledged rising temperatures as one of their biggest climate-related concerns.
Last month’s cancellations highlighted a poorly understood aspect of air travel and climate change. Many people understand air travel is bad for the climate, in terms of producing greenhouse gas emissions.
Yet climate change is also having an impact on air travel, in the form of shifting wind patterns, stronger jet streams, and increased instances of extreme weather and turbulence. Rising sea levels also affect siting airports.
“Climate change is causing jet streams to be less stable,” Williams, who helped prepare the ICAO report, explained. That could radically shake up flight schedules.
The jet stream that moves west to east over the Atlantic is getting stronger.
In 2015, the jet stream hit speeds over 200 mph. Riding the wave, an eastbound plane was propelled to nearly the speed of sound, delivering passengers from New York to London in just over five hours. In the opposite direction, planes bound for New York were forced to stop in Maine and refuel, because the head wind slowed them to a sluggish eight-hour haul.
Between the United States and Europe, “we’re going to see a lot more record-breaking fast eastbound flights and a lot more severely delayed westbound flights,” said Williams.
Changing wind patterns might also force airlines to reroute to avoid head winds causing more delays and using more fuel, both of which translate to higher costs, explained Stephen Munroe, a Bloomberg analyst who focuses on energy.
But while there are hundreds of studies on how air travel contributes to climate change, there are only half a dozen or so investigating the “reverse impact” of climate change on air travel.
“This is a new frontier,” said Columbia’s Horton. “To date we are not seeing much engagement from the aviation industry on this issue.”