Low-lying areas along the water have long been seen as ideal sites for building new runways and terminals, because there are fewer obstacles for the planes during takeoff and landing, and less potential for noise complaints. But coasts also provide few natural protections against flooding or high winds.

All told, extreme weather and rising sea levels today pose one of the most urgent threats to many of the world’s busiest airports, which often weren’t designed with global warming in mind.

Hurricane Sandy in 2012 inundated all three airports that serve New York City, crippling travel for days. Typhoon Goni closed runways at Hongqiao International Airport outside Shanghai in 2015, forcing passengers and crew members to teeter on improvised bridges of tables and chairs as they tried to reach dry ground. The worst floods in nearly a century in Kerala, India, killed more than 400 people last month, and the deluge caused Cochin Airport, a regional hub, to close for two weeks.

“We know that there are going to be impacts. And we expect those impacts to become serious,” said Michael Rossell, deputy director-general at Airports Council International, a group representing airports from across the world. “Recognizing the problem is the first step, and recognizing the severity is the second. The third is: What can we do about it?”

Many airports have started to bolster their defenses.

St. Paul Downtown Airport in Minnesota, which has been frequently flooded by the Mississippi, now has a portable flood wall that can be erected if the river starts to overflow. With the help of a $28 million federal grant, La Guardia Airport in New York is adding a flood wall, rainwater pumps and a new drainage system for the airfield, as well as upgrading its emergency electrical substations and generators.

Kansai airport, which serves the bustling cities of Osaka, Kyoto and Kobe and handled almost 28 million travelers last year, faces an additional predicament. A feat of modern engineering, Kansai sits on an island three miles offshore that was built over the course of a decade from two mountains’ worth of gravel and sand. The airport, which opened in 1994, was built in Osaka Bay partly to minimize noise problems but also to avoid the violent protests over land rights that are the legacy of older airports in Japan, like Narita, which serves Tokyo.

Signs of trouble came early. Engineers had expected the island to sink, on average, less than a foot a year over 50 years after the start of construction as the seabed settled under the airport’s weight. But the island sank more than 30 feet in its first seven years and has continued to descend, now losing 43 feet in elevation at the last measurement.