Planes struggle to lift off when hotter temperatures make for less dense air.
It's going to take some unloading for planes to fly in a hotter world. Patrick Semansky/AP Photo

New research piles on the unpleasant effects of hotter air on plane travel.

Aviation is responsible for about two percent of the world’s annual carbon emissions—which would make it the world’s seventh largest emitter, if the industry were a nation. There’s also evidence that the “waste vapor” of airliners may have a pronounced planet-warming impact that goes beyond their CO2 content.

One could say the climate is now coming back with a vengeance.

As the atmosphere warms, and heat waves blister the planet more often, flights will have a harder time taking off, resulting in lower payloads, rescheduled flights, and higher costs.

In a new paper published Thursday in Climatic Change, researchers from Columbia University and Virginia’s Logistics Management Institute modeled how aircraft departures will be affected by hotter days driven by climate change.

As rule of thumb, as air molecules heat up, they becomes less dense, and provide less lift for flying things. The video embedded below provides a concise explanation of the basic physics.  

That means that many aircrafts traveling at their regulation lift-off weights will have to drop people, packages, or fuel when temperatures swelter—or stay grounded, as more than 40 flights leaving Phoenix, Arizona, were forced to last month in 120 degree F weather.

In the paper, the researchers studied the most common jet sizes at 19 of the world’s busiest airports, where average temperatures vary dramatically. Assuming global emissions rise unimpeded, airports with hotter average temperatures, shorter runways (where it’s harder to achieve fast take-off speeds), and higher elevations (where thin air is more vulnerable to weather changes) will have the most problems. All jet sizes will be impacted.

For example, a packed Boeing 737-800 (the smallest of the five jet sizes) may have to drop passengers, cargo, or fuel about half of the time when taking off from New York City’s LaGuardia International in the hottest parts of the day, due to LGA’s truncated runways. Those flights might need to drop up to 3.5 percent of their load.

Similarly, a larger Boeing 777-300 departing from scorching Dubai at the hottest times of the day may have to shed weight about 55 percent of the time, with fuel, cargo, or passenger weight reductions of up to 6.5 percent.

The chart below shows how RCP 8.5—that’s the worst-case scenario for global warming, where temperatures rise by as much as 5 degrees C—will restrict take-offs across the 19 airports, for four different jet sizes. The blue lines show the historic relationships between take-off weights (TOW), restricted flights, and the percent of payload restricted, between 1985 and 2005. The red lines project into the future of 2060-2080.

(Climatic Change)

Average effects across the 19 airports are less dramatic; temperate-weather airports, such as London’s Heathrow, Paris’s Charles de Gaulle, and New York’s John F. Kennedy, are forecasted for little change. Larger airports and jet models designed to operate exclusively in those settings tend to see less impact, too, since longer runways can compensate for hot air’s lack of support.

Still, small weight reductions on a flight-by-flight basis can have a big effect on overall capacity. If a Boeing 737-800 needs to skim .5 percent of its maximum take-off weight, that could mean three fewer passengers get seats on that flight. If it needed to drop 4 percent, that could mean dropping a dozen or more passengers.

By the end of the century, 10 to 30 percent of packed planes might need to shed some weight, or wait for cooler hours to fly. For an industry that scrapes by on skimpy profits, that could easily translate to rising costs.

Air travelers may feel the effects of climate change physically, as well as financially, as jet stream winds grow stronger. Since they usually travel west to east, this could stretch westbound flights longer and contract eastbound flight times, leading to higher overall travel times and fuel consumption. Stronger winds also makes for less-stable air at cruising altitudes. That means passengers are in for more turbulent rides, even in the absence of a storm.

All of this assumes, of course, that airlines are still flying 2017-era equipment decades into the future—and that commercial planes are flying at all, in what could be a far less habitable earth. By 2080, as this week’s terrifying New York magazine cover story reminded us, civilization may have engineered a hellscape of unbreathable air, poisonous seas, coastal inundation, famine, and war. Air travel inconveniences—the kind we love to complain about today—would be the least of our problems.

But let’s say that humanity survives, frequent-flyer miles intact. Revamped airports and aircraft designs could mitigate some of heat’s drag on takeoffs; many flights could be also rescheduled for cooler times of the day. Unless we collectively decide to cease plane travel altogether—that, along with avoiding meat, driving, and child-making, would be the best lifestyle plan of action, from a climate perspective—airlines should start planning now. “This fact is true of all climate impacts,” the new paper’s authors write. “Even if they can be adapted to, they still have a cost.”

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