BEFORE Hurricane Sandy tore through New York and New Jersey, it stopped in Florida. Huge waves covered beaches, swept over Fort Lauderdale’s concrete sea wall and spilled onto A1A, Florida’s coastal highway. A month later another series of violent storms hit south Florida, severely eroding Fort Lauderdale’s beaches and a chunk of A1A. Workers are building a new sea wall, mending the highway and adding a couple of pedestrian bridges. Beach erosion forced Fort Lauderdale to buy sand from an inland mine in central Florida; the mine’s soft, white sand stands out against the darker, grittier native variety.

Hurricanes and storms are nothing new for Florida. But as the oceans warm, hurricanes are growing more intense. To make matters worse, this is happening against a backdrop of sharply rising sea levels, turning what has been a seasonal annoyance into an existential threat.

For around 2,000 years sea levels remained relatively constant. Between 1880 and 2011, however, they rose by an average of 0.07 inches (1.8mm) a year, and between 1993 and 2011 the average was between 0.11 and 0.13 inches a year. In 2007 the Intergovernmental Panel on Climate Change (IPCC) forecast that seas could rise by as much as 23 inches by 2100, though since then many scientists have called that forecast conservative. Seas are also expected to warm up, which may make hurricanes and tropical storms more intense.

Even as seas have risen over the past century, Americans have rushed to build homes near the beach. Storms that lash the modern American coastline cause more economic damage than their predecessors because there is more to destroy. The Great Miami Hurricane of 1926, a Category 4 storm, caused $1 billion-worth of damage in current dollars. Were it to strike today the insured losses would be $125 billion, reckons AIR Worldwide, a catastrophe-modelling firm. In 1992 Hurricane Andrew, a Category 5 storm, caused $23 billion in damage; today it would be twice that.

Most Floridians live in coastal counties. Buildings cluster on low ground; more people than in any other state live on land less than four feet (1.2 metres) above the high-tide line. Florida’s limestone bedrock makes it easy for salt water from surging seas to contaminate its freshwater aquifers. And it relies heavily on canals for flood control, which a sea-level rise of just six inches would devastate. South Florida is not the only region threatened by climate change and hurricanes. Increased rain, violent storms and rising sea-levels could inundate low-lying areas around San Francisco and Seattle, or burst the levees that protect swathes of Sacramento and California’s Central Valley from the Sacramento and San Joaquin river delta. Houston, the centre of America’s petrochemical industry, and Norfolk, Virginia, home to its largest naval base, could also be in trouble. So could some of the barrier islands along the Atlantic coast, such as North Carolina’s Outer Banks, and traditional Atlantic maritime regions such as Maryland’s Eastern Shore. These two areas, like South Florida, have seen sharp rises in population and development. New York is also at risk, as Hurricane Sandy showed last autumn (see charts). Manhattan is vulnerable to rising sea levels: the districts flooded by Sandy corresponded almost perfectly to land reclaimed since the 17th century (see map). That land is far more valuable now than it was then: Jeroen Aerts and Wouter Botzen of the Netherlands’ VU University reckon the value of structures threatened by storms and floods has increased four- to sevenfold in the past century. Since the flood map was last updated in 1983, floor space inside the city’s flood plain has risen 40%, to 535m square feet.

However, traditional flood-mitigation schemes, such as buying out householders or raising existing buildings, are impractical in New York. Seth Pinsky, who spearheaded the city’s post-Sandy adaptation plan, notes that New York now has 400,000 people, 270,000 jobs and 68,000 buildings inside the 100-year flood plain. Ground floors in New York are built for shops. Raising buildings would either be too costly, too destructive to neighbourhoods, or both.

Turning the shoreline over to beaches, dunes or wetlands will not work in crowded Manhattan, which like many cities wants more development along its waterfront, not less. Some have proposed protecting the city with massive storm barriers at the mouth of the Atlantic Ocean, similar to the gates that protect London, Rotterdam and St Petersburg. But aside from the steep price tag (as much as $29 billion), such barriers could worsen flood risk for areas outside them.

In 2007 Michael Bloomberg, New York’s mayor, released PlaNYC, a scheme for adapting to climate change, which could be called “ambitious” or “dictatorial”, depending on one’s view of the mayor. It called for, among other things, protecting wetlands and planting more trees, which will keep the city cooler and capture more stormwater run-off. It also demands changes in building codes.

Mike’s dyke

Many of its ideas were incorporated into a more sweeping post-Sandy plan released on June 11th, which calls for floodwalls and levees to protect vital infrastructure, such as a food-distribution centre in the Bronx and hospitals on Manhattan’s East Side, and coastal communities on Staten Island. It recommends storm-surge barriers to prevent creeks and rivers from backing up into residential areas; a new lower Manhattan district, modelled on Battery Park City, protected by a multi-purpose levee; and new or repaired natural barriers such as sand dunes, beaches and wetlands around the outer boroughs.

The city would offer incentives to building owners to move important stuff like electrical equipment higher off the ground. It would amend zoning and building codes to encourage new buildings to be raised higher, and require hospitals, telecoms and other utilities to meet tougher resilience standards. Mr Bloomberg put the price tag at nearly $20 billion, with city and federal sources for only $15 billion identified so far. But put beside New York’s extraordinarily high economic output, the price is hardly outlandish.

New York’s plans illustrate that although climate change is global, adaptation is local. In America such things as land-use, zoning, construction and transport are typically under state or local control. That sets America apart from more centralised countries like the Netherlands. As Rohit Aggarwala, a former adviser to Mr Bloomberg, says: “It’s not clear the federal government is the leader on this issue, even if they wanted to be in charge.” During disasters, the Federal Emergency Management Agency (FEMA) may come in to clean up, but evacuation orders come from state and local authorities, and police, fire and medical teams also tend to be employed locally.

The federal government does play a supporting role, not least because it brings extra money. For example, FEMA buys up houses that are repeatedly flooded. Since 2009 the Army Corps of Engineers has incorporated forecasts of sea-level rise into all its civil-works programmes. The Department of Housing and Urban Development offers grants to encourage cities and regions to work together on climate-change-adaptation plans and studies. And a separate federal post-Sandy task force has required that any structure rebuilt with any of the $50 billion in disaster funds should be raised to one foot above the most recent federal flood guidance.

Last year Congress required the insurance subsidy that the federal government has long offered to householders who live and build on flood plains to be phased out. Such subsidies, in effect, pay people to live in dangerous places.

A region’s preparedness depends in part on how seriously its leaders take climate change. Proactively minded cities have joined forces; New York and ten others are among the 61 cities around the world that, in partnership with the Clinton Climate Initiative, share plans and information to reduce greenhouse-gas emissions and adapt to a changing climate. In Florida, four of the southernmost counties—which include the state’s three most-populous ones, accounting for more than a quarter of its total population—have formed the Southeast Florida Regional Climate Change Compact. These counties share data, work together on legislation and seek funding in concert.

The federal government has limited sway over regions where people are less convinced of climate change. North Carolina’s legislators, for instance, have outlawed “scenarios of accelerated sea-level rise unless such rates are...consistent with historic trends.” (As one angry North Carolinian noted, this is like ordering meteorologists to predict the weather not by looking at the radar image of a hurricane barrelling towards the coast, but by consulting the “Farmer’s Almanac”.) A survey by the Massachusetts Institute of Technology found cities in America among the least likely, globally, to have plans for adapting to changing weather. But some, at least, are starting.