Air travel is about to become even more frustrating. Warmer global temperatures will make it tougher for planes to take off, tightening restrictions on just how much luggage or how many people can come aboard, a new study suggests. Higher temperatures make air less dense, reducing the lift force on planes’ wings. Because heavier planes are harder to speed up, they’ll need more runway distance to reach their minimum takeoff speed in warmer weather. But when runways aren’t long enough, those flights will need to reduce their loads. To estimate how much of a problem this could be, researchers used a climate model to project future summer temperatures at four major U.S. airports, assuming minimal action to cut global warming emissions. The scientists compared the findings with the temperature thresholds that prompt various weight restrictions at those airports for a Boeing 737-800 commercial plane. All four airports would have 50% to 200% more days with weight restrictions by the 2050s to 2070s than they do now, the team concludes online this month in Weather, Climate, and Society. Phoenix’s airport would go from almost no days to 20 or more per year. Meanwhile, airports in New York City, Washington, D.C., and Denver that now do have a few dozen weight-restriction days every year could each have a few dozen more per year in the future, the researchers report. To compensate, airlines will have to reduce passengers or cargo, the team says, unless the aviation industry lengthens runways or designs more aerodynamic planes.