This story originally appeared on Yale Environment 360 and is part of the Climate Desk collaboration.

For cities in the US, the price of infrastructure projects to combat rising seas and intensifying storms is coming into focus—and so is the sticker shock.

In Boston, where many neighborhoods have been built and recently expanded in low-lying areas, an estimated $2.4 billion will be needed over the next several decades to protect the city from flooding, one study says. That report came as the city abandoned plans to build a harbor barrier that would have cost between $6 billion and $12 billion, which researchers concluded was economically unfeasible.

In Charleston, South Carolina, the mayor said last year that the city, which floods regularly during high tides, had an estimated $2 billion in needed drainage projects.

In Norfolk, Virginia, the Army Corps of Engineers has recommended a $1.4 billion series of seawalls and other infrastructure to protect part of the shoreline. As with many cities, that’s just the start.

In Harris County, home to Houston, planners say $30 billion is needed to provide protection against a 100-year flood. Hurricane Harvey, which in 2017 caused 68 deaths and $125 billion in damages in the state, was the city’s third 500-year-flood in three years.

And in New York City, Mayor Bill de Blasio has proposed a storm surge barrier and floodgates to shield parts of the city and New Jersey from rising waters. The estimated cost: $10 billion.

While the threats to these cities are growing as climate change intensifies, what is not clear is how to pay for projects needed to protect tens of millions of people and trillions of dollars of property. Conservative estimates of the capital investments needed to combat rising seas and worsening storms run into the hundreds of billions of dollars in the coming decades.

“The failure to face these costs is the next phase of climate denial,” says Richard Wiles, executive director of the Center for Climate Integrity, an environmental advocacy group that champions forcing polluters to pay for climate crisis costs. “We’ve got to look at this squarely and figure out how to pay for it.”

The center recently issued a study concluding that by 2040, building seawalls for storm surge protection for US coastal cities with more than 25,000 residents will require at least $42 billion. Expand that to include communities under 25,000 people and the cost skyrockets to $400 billion. That’s nearly the price of building the 47,000 miles of the interstate highway system, which took four decades and cost more than $500 billion in today’s dollars.

The research is a rough yardstick because it only considers seawalls, not other ways to mitigate flood risk including buying out homeowners and improving storm water systems. “It’s a deliberate underestimate,” Wiles adds. “We know it will cost more—a lot more.”

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The report says that Florida, with an estimated $76 billion in costs, is the state with the largest exposure, followed by Louisiana, North Carolina, and Virginia. A key issue as different jurisdictions and the federal government grapple with how to pay for this infrastructure is equity. Wiles and other experts say that while some of the bigger, richer cities may figure out how to finance their needs, smaller communities will face huge challenges funding resilience projects. “We think this is an emerging crisis for most of these communities,” Wiles says. “A very simple way of thinking about it is that it will be tens of thousands of dollars per resident in places that may not have a large tax base.”