Florida is in the crosshairs of climate change. Rising seas, a population crowded along the coast, porous bedrock, and the relatively common occurrence of tropical storms put more real estate and people at risk from storm surges aggravated by sea level rise in Florida, than any other state by far.

Some 2.4 million people and 1.3 million homes, nearly half the risk nationwide, sit within 4 feet of the local high tide line. Sea-level rise is more than doubling the risk of a storm surge at this level in South Florida by 2030. For the hundreds of thousands of Floridians holding 30-year mortgages, that date is not far off in the future.

The world’s oceans are already rising, thanks to global warming. Global average sea level has gone up about eight inches since 1880. In South Florida, taxpayers are already paying the price for climate change as salt water pushes through porous bedrock into coastal drinking-water supplies, and rivers and canals choked by heavy rains have a harder time draining into the ocean. A recent Florida Atlantic University study estimated that just six more inches of sea level rise — very plausible within two decades — would cripple about half of South Florida’s flood control capacity.

It’s now, not later, for sea-level rise in South Florida.

That’s a big reason why Climate Central has worked for two years on a new analysis of this threat, blending storm surge, tides and more into the picture. Integrating storms and tides show that a small amount of sea level rise can make a big difference — multiplying the odds of extreme coastal floods around the United States, not just South Florida. Think of it like raising the floor at a Miami Heat game: you’d see a lot more dunks. Overall, sea-level rise is making the odds of a South Florida flood reaching more than four feet above high tide, by 2050, on par with the odds of losing at Russian roulette.

More than half the population of more than 100 Florida towns and cities lives on land below that four-foot line. Miami-Dade and Broward counties each have more people below four feet than any state, except Florida itself and Louisiana.

Just how vulnerable any area is depends on many elements. Our analysis factored in not only local sea-level rise projections, storm-surge patterns and tides, but also local topography and patterns of development. In an attempt to better inform people, businesses and planners who live and work near the coast, we have mapped and evaluated risk in 3,000 towns, cities and counties across the lower 48 states, including South Florida, and have created a free, ZIP code-searchable map with neighborhood views and risk information at SurgingSeas.org. Among our key national findings:

• Global warming has already doubled or tripled the odds of extreme high water events over widespread areas of the U.S. coast.

• Widespread areas are likely to see storm surges on top of sea level rise reaching at least four feet above high tide by 2030, and five feet by 2050.

• Almost five million U.S. residents currently live on land less than four feet above high tide, and more than six million on land less than five feet above.

Sea-level rise is already increasing flood threats everywhere. It’s set to become an even greater problem much sooner than most people expect. Swift cuts in greenhouse gas pollution can significantly reduce sea level increases, but past and present pollution already commit us to a good deal more rise.

It’s time we start preparing for higher seas and storms, if we want to avoid their worst effects. In South Florida, where the porous limestone makes building effective sea walls or levees almost impossible, the task is especially urgent.

Ben Strauss is director of the Program on Sea Level Rise at Climate Central, a nonprofit research and journalism group.