By Bryan Walsh
August 27, 2015

Scientists rank hurricanes according to the power of their winds–Category 5 hurricanes, the strongest possible, must have sustained winds greater than 155 m.p.h. (250 km/h). But the damage a storm does is a product not just of its strength but also where it hits. A powerful hurricane that never reaches land will be forgotten by everyone but meteorologists, while a relatively weak storm can wreak havoc if it strikes a heavily populated coastal city.

Hurricane Katrina–which made landfall in Louisiana 10 years ago on Aug. 29, 2005–was a dreaded double whammy: a Category 5 storm that scored a direct hit on what was perhaps the most vulnerable city in America. New Orleans’ flood protections proved woefully incapable of standing up to Katrina. Storm surges higher than 20 ft. (6 m) breached nearly every levee in the metro area, leaving whole sections of the city and adjoining suburbs underwater. Over 1,500 people died in Louisiana alone, and the final damage toll was more than $100 billion, making it the costliest hurricane in U.S. history. If Katrina was a warning of the hell that can be loosed when the wrong storm meets the wrong place, the question now is, What have we learned in the decade since?

Ten years and tens of billions of dollars later, including $14.5 billion for a new storm-protection system, a rebuilt New Orleans is likely safer from hurricanes than it has ever been before. Which isn’t to say that the Crescent City is safe. That’s because in an age of global warming, storm protection is a moving target.

Take the first part of the hurricane equation: the strength of the storm. A few weeks before Katrina struck, a professor of meteorology at Massachusetts Institute of Technology named Kerry Emanuel published a paper showing that hurricane power closely correlated with the temperature of the tropical Atlantic Ocean in hurricane seas–and that both had been increasing over the previous 30 years, thanks to a combination of natural climate variations and man-made global warming.

That quirk of academic timing led many to claim that climate change “caused” Katrina. Not true, but it’s likely that the planet’s warming has made hurricanes potentially stronger. The 2014 National Climate Assessment found that hurricane-associated storm intensity and rainfall rates are expected to increase as the climate continues to warm–which it will as long as greenhouse-gas emissions keep rising. That raises the chance that coastal cities like New Orleans will have to deal with stronger storms in the future.

But the strength of storms to come is less worrisome than our inability to defend against them. When Hurricane Sandy arrived in New Jersey on Oct. 29, 2012, it wasn’t technically a hurricane–its wind speed had fallen below the necessary sustained velocity of 74 m.p.h. (120 km/h). Yet Sandy caused nearly $70 billion in damage, not so much because of its winds but because of coastal storm surges, which led to the widespread flooding of some of the most valuable real estate in the world. And that flooding was worsened by higher sea levels–another result of climate change. In New York City alone, higher seas likely added an additional foot to Sandy’s storm surges. The Intergovernmental Panel on Climate Change projects that seas could rise as much as 30 more inches (75 cm) by 2100 if the world is unable to reduce greenhouse-gas emissions.

Given those twin dangers, you might think Americans would be less eager to live by the ocean. Yet more are moving there every day. Today, 123 million Americans–more than a third of the country–live in coastal counties, a figure that has grown by 39% since 1970. The insured value of residential and commercial properties in those counties now exceeds $10 trillion. That means more people and more money in harm’s way, which can make the storms that hit more catastrophic. In 1926, for example, a Category 4 hurricane hit Miami, then a city of 100,000 people. It caused devastation, but if the same storm were to hit the much richer and more populous city today, the damages would be in excess of $160 billion.

Properly shielding New Orleans or Miami or New York would cost far more than we’ve proved willing to pay. But failing to do so will mean entrusting the fate of our greatest cities to the luck of a storm, as the odds get worse with each passing year.

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Read next: New Orleans, Here & Now

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This appears in the September 07, 2015 issue of TIME.

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