After the storm, after the flooding, after the investigations, the US came to realize that what happened to New Orleans on August 29, 2005 was not a natural disaster. The levee system built by the US Army Corps of Engineers had structural flaws, and those flaws were awaiting the right circumstances. In that way, what happened was all but inevitable.

And just as the storm is not to blame, New Orleans is not unique in its vulnerability. The city endured a lot of tsk-tsking in the aftermath of Katrina, as if the storm was the climax to a parable about poor urban planning. Sure, the city sits below sea level, at the end of hurricane alley, and relies heavily on an elaborate (and delicate) system of infrastructure. But where the city's geography is unique, its vulnerability is anything but. Just about every coastal city, state, or region is sitting on a similar confluence of catastrophic conditions. The seas are rising, a storm is coming, and critical infrastructure is dangerously exposed.

The basic math of carbon dioxide is pretty simple: Generally, as CO 2 levels rise, the air will warm. Warmer air melts glaciers, which drip into the sea—even as the water itself warms, too. Both cause the oceans to rise. Even if the entire planet stopped emitting carbon dioxide, Earth would continue to suffer the effects of past emissions. "We’ve got at least 30 years of inertia in terms of sea level rise," says Trevor Houser, a Rhodium Group economist who studies climate risk. And even if the sea weren't rising, the rate of urban growth will more than double the area of urban land at high flood risk, according to a study Global Environmental Change published earlier this year.

But the sea is rising, at about .13 of an inch per year, for the past 20 years. (It was rising before then, too, but at about half the rate for the preceding 80 years.) Another recent study calculated that the world should expect about 4 feet of sea level rise for every degree Fahrenheit the global average temperature rises. This puts nearly every coastal city, in every coastal state, in danger of floods. Climate Central has an extensive project looking at sea level risk, if you're curious about your city's risk.

Warm air also holds more moisture, and moisture holds more energy, hence stronger (though not necessarily more frequent) storms. Those storms combine with high sea levels to create a danger greater than the sum of their parts. In a combined flooding event, a severe storm traps a city between rainfall and surging seas. Higher sea levels cause rivers to back up, water tables to saturate, shorelines to shorten. Storms—which are likely to be stronger than before—have fewer options to run off, so they pool and flood. And America built its coastal civilization oblivious to their threat.

The passive threat of infrastructure

Take Florida, the most climate-threatened swath of American soil. It's low, flat, built on porous limestone, and hurricane prone. According to a new analysis by disaster insurance agency Karen Clark and Co., Florida has four of the 10 US cities most vulnerable to combined flooding events.

Florida, knowing its place in the world, has copious levees and seawalls. But the levees are there mostly to protect against the Everglades. The seawalls are about as good at breaking a hurricane as a hood ornament is at breaking the wind. And all of that infrastructure is of little use in the face of combined flooding events—the sea will simply come up from below. Miami flooded last year when the storm sewers backed up because the water table was too high to drain them.

The Sunshine State's geography makes it an easy target for blame (not to mention hurricanes). But if there's anything the US should have learned in the decade since Katrina, it's that storms don't always hit where you expect them—because, you know, Sandy. "Florida is definitely the most vulnerable place, but you also have places like Norfolk that are built on the coastal floodplain, and parts of New England where there is a lot of sunk infrastructure very close to the increasingly vulnerable coast," says Houser. The pattern repeats itself all along the Atlantic coastal plain: Physical protections are largely insufficient to protect against a new class of climate threats.

And then, sometimes, that infrastructure falls apart entirely. Louisiana's levees couldn't have held off Katrina entirely, but it was their collapse, not the hurricane itself, that turned the Big Easy into a bathtub. "Some were improperly designed, some were improperly constructed, the rest were improperly maintained," says Sandy Rosenthal, the director of Levees.org, an infrastructure watchdog group.

That same sentence could apply to key infrastructure nationwide. A lot of the country's infrastructure—its bridges, transportation corridors, airports, seaports, water supply systems, electrical grids, flood control, and so one—were built poorly, hastily, or both. A lot of it is old and neglected. In a 2013 survey, the American Society of Civil Engineers gave US infrastructure a D+ grade.

"A lot of infrastructure went up in the midcentury," says Solomon Hsiang, a UC Berkeley economist who studies public policy. "Now we're reaching the end of the natural lifetime of that infrastructure, and we need to decide that we can no longer ride on all the investment that occurred 50 or 60 years ago." Much of this stuff is directly vulnerable to climate change. Earlier this year, the Army Corps of Engineers released two surveys describing hundreds of dams and thousands of levees vulnerable to rising seas and stronger storms. Threats identified—but not yet remedied.

The Gulf Intracoastal West Closure Complex is a part of the New Orleans Drainage System; it consists of a navigable floodgate, a pumping station, floodwalls, sluice gates, foreshore protection, and an earthern levee. The pump station complex is the largest of its type in the world and consists of 11 each, 5444 horsepower Caterpillar engines. The complex was designed to reduce risk for residences and businesses in the project area from a storm surge associated with a tropical event with an intensity that has a 1 percent chance of occuring in any given year. This projects was operated for the first time on August 29. 2012 in response to Hurricane Isaac. William Widmer

The psychology of protection

Why not? Structural vulnerability comes from financial neglect, which leads back to politics. "There's a lot of political incentive to fund new, shiny infrastructure, but less of an incentive to be a responsible caretaker of stuff that's already built," Hsiang says. After all, it's not particularly likely that a given piece of infrastructure will fail during a given politician's term. Easy enough to pass the bill on to the next guy.

Politicians also get things wrong when it comes to what happens after the storm. "People argue that after a storm we can build back better. That’s a very politically savvy statement to provide encouragement, but empirically it doesn’t hold up in the data," says Hsiang. According to research he co-authored for the National Bureau of Economic Research, places hit by a huge natural disaster typically take around 20 years to fully recover. In that time, unemployment rises, crime goes up, education goes down.

It's more than an entire generation of people affected—the larger economy takes a hit, too. In their research, Hsiang and the others found the cost to the economy is 10 times the cost of repairing the damage. "Let’s say a storm does $100 in damage," says Hsiang (imagining perhaps a wild pool party rather than a category 3 storm). "That actually costs the economy $1,000." The more influential investment happens long before the storm happens.

If American politicians are guilty of miscalculating flood risks, it's partly because they are a product of their environment. "The problem is, the way that most people learn about their own risk is by their insurance premium," says Hsiang. That signal has been decoupled from the real risk. That's because flooding typically is too risky for private insurance companies to cover without huge premiums. So, through FEMA, the federal government set up the National Flood Insurance Program.

The rates have been kept artificially low for decades. In fact, FEMA tried to raise rates last year and—via politicians—got a storm of opposition. "Some peoples' premiums went up by a factor of 10," says Hsiang. While it's true higher insurance rates will price out many of the people on the coast, keeping them low means the rest of the country is subsidizing the risk of them staying.

Don't think that affects you? It does. Sea level rise may not have a direct impact on much of the continent, but stronger storms will bring increased inland flooding. But let's say you don't get hit by a flood. Instead, the next big hurricane swamps Houston (or Tampa, or Charleston, or Norfolk, or Atlantic City, or Boston, or ...) and you're trying to start a business in Bismark. The number of people living on the coast is directly related to how much money banks must lend to rebuild. The loan for your high plains brewpub could be declined because all the bank's money is being sent to rebuild the coast.

The aftermath

Hurricane Katrina was a category 3 storm when it made landfall in NOLA. The levees broke in more than 50 places. When the Army Corps of Engineers did postmortem investigations, it discovered those breaks were destined to happen.

If you've been paying attention, you'll be quick to see solutions to all this tend to fall into two categories. One is to spend a bunch of money retrofitting, rebuilding, and renewing the country's critical flood infrastructure. Two is to get fewer people to live near the coasts and in flood plains. You don't need to be a politician to understand why those strategies don't figure into a lot of stump speeches.

The reality is these changes will be made incrementally. FEMA is getting set to make another go at flood insurance reform. Some cities, like Boston, Norfolk, and New York, are making huge efforts to protect themselves from climate change. And despite being held back by cuts imposed by congress, the Army Corps is plodding toward infrastructure renovations.

The next Katrina won't be named Katrina. It might not even be a hurricane. It might be a Nor'easter, a derecho, an atmospheric river after a long drought. It might hit on the coast, and it might hit in the middle of the continent. It might hit New Orleans, where the city's new levees fall well short of the protection Congress said the city deserved in Katrina's immediate aftermath. It might not. But every city is vulnerable to climate change, and every city's vulnerability is everybody's problem.