Heat waves associated with rising global temperatures will dramatically affect air travel later this century, occasionally triggering flight delays and bumping passengers and cargo, a new study suggests. When air warms, its density drops—which, in turn, affects the amount of lift air can generate as it rushes across aircraft wings. Less lift means an aircraft can carry less weight, but it also means an airplane—especially a weighty one—needs a longer runway in hot weather, a restriction that can lead to flight delays or cancellations like those caused by record-breaking heat in Phoenix last month. To assess how future heat waves might affect air travel, researchers used climate models to estimate hour-by-hour temperatures throughout the year at 19 particularly busy airports in the United States, Europe, the Middle East, China, and South Asia for the period between 2060 and 2080. At some airports—especially those with long runways in temperate regions and at low altitude where the air is relatively dense, like New York City’s John F. Kennedy, London’s Heathrow, and Paris’s Charles de Gaulle airports—impacts should be minimal, the researchers report today in Climatic Change. But at another New York City airport, La Guardia, shorter runways would trigger weight restrictions on fully laden Boeing 737-800 aircraft more than half the time on the hottest days. Similarly, at Dubai International Airport, a fully booked Boeing 777-300 could be weight-restricted during the hottest part of the day about 55% of the time. Overall, weight restrictions on hot days worldwide could range as high as 4% or more, the researchers say. But even a weight restriction of only 0.5% would result in the bumping of three passengers from an aircraft designed to carry 160 people, the team notes.