In the Miami area, the daily high-water mark has been rising almost an inch a year. Illustration by Jacob Escobedo

The city of Miami Beach floods on such a predictable basis that if, out of curiosity or sheer perversity, a person wants to she can plan a visit to coincide with an inundation. Knowing the tides would be high around the time of the “super blood moon,” in late September, I arranged to meet up with Hal Wanless, the chairman of the University of Miami’s geological-sciences department. Wanless, who is seventy-three, has spent nearly half a century studying how South Florida came into being. From this, he’s concluded that much of the region may have less than half a century more to go.

We had breakfast at a greasy spoon not far from Wanless’s office, then set off across the MacArthur Causeway. (Out-of-towners often assume that Miami Beach is part of Miami, but it’s situated on a separate island, a few miles off the coast.) It was a hot, breathless day, with a brilliant blue sky. Wanless turned onto a side street, and soon we were confronting a pond-sized puddle. Water gushed down the road and into an underground garage. We stopped in front of a four-story apartment building, which was surrounded by a groomed lawn. Water seemed to be bubbling out of the turf. Wanless took off his shoes and socks and pulled on a pair of polypropylene booties. As he stepped out of the car, a woman rushed over. She asked if he worked for the city. He said he did not, an answer that seemed to disappoint but not deter her. She gestured at a palm tree that was sticking out of the drowned grass.

“Look at our yard, at the landscaping,” she said. “That palm tree was super-expensive.” She went on, “It’s crazy—this is saltwater.”

“Welcome to rising sea levels,” Wanless told her.

According to the Intergovernmental Panel on Climate Change, sea levels could rise by more than three feet by the end of this century. The United States Army Corps of Engineers projects that they could rise by as much as five feet; the National Oceanic and Atmospheric Administration predicts up to six and a half feet. According to Wanless, all these projections are probably low. In his office, Wanless keeps a jar of meltwater he collected from the Greenland ice sheet. He likes to point out that there is plenty more where that came from.

“Many geologists, we’re looking at the possibility of a ten-to-thirty-foot range by the end of the century,” he told me.

We got back into the car. Driving with one hand, Wanless shot pictures out the window with the other. “Look at that,” he said. “Oh, my gosh!” We’d come to a neighborhood of multimillion-dollar homes where the water was creeping under the security gates and up the driveways. Porsches and Mercedeses sat flooded up to their chassis.

“This is today, you know,” Wanless said. “This isn’t with two feet of sea-level rise.” He wanted to get better photos, and pulled over onto another side street. He handed me the camera so that I could take a picture of him standing in the middle of the submerged road. Wanless stretched out his arms, like a magician who’d just conjured a rabbit. Some workmen came bouncing along in the back of a pickup. Every few feet, they stuck a depth gauge into the water. A truck from the Miami Beach Public Works Department pulled up. The driver asked if we had called City Hall. Apparently, one of the residents of the street had mistaken the high tide for a water-main break. As we were chatting with him, an elderly woman leaning on a walker rounded the corner. She looked at the lake the street had become and wailed, “What am I supposed to do?” The men in the pickup truck agreed to take her home. They folded up her walker and hoisted her into the cab.

To cope with its recurrent flooding, Miami Beach has already spent something like a hundred million dollars. It is planning on spending several hundred million more. Such efforts are, in Wanless’s view, so much money down the drain. Sooner or later—and probably sooner—the city will have too much water to deal with. Even before that happens, Wanless believes, insurers will stop selling policies on the luxury condos that line Biscayne Bay. Banks will stop writing mortgages.

“If we don’t plan for this,” he told me, once we were in the car again, driving toward the Fontainebleau hotel, “these are the new Okies.” I tried to imagine Ma and Pa Joad heading north, their golf bags and espresso machine strapped to the Range Rover.

The amount of water on the planet is fixed (and has been for billions of years). Its distribution, however, is subject to all sorts of rearrangements. In the coldest part of the last ice age, about twenty thousand years ago, so much water was tied up in ice sheets that sea levels were almost four hundred feet lower than they are today. At that point, Miami Beach, instead of being an island, was fifteen miles from the Atlantic Coast. Sarasota was a hundred miles inland from the Gulf of Mexico, and the outline of the Sunshine State looked less like a skinny finger than like a plump heel.

As the ice age ended and the planet warmed, the world’s coastlines assumed their present configuration. There’s a good deal of evidence—much of it now submerged—that this process did not take place slowly and steadily but, rather, in fits and starts. Beginning around 12,500 B.C., during an event known as meltwater pulse 1A, sea levels rose by roughly fifty feet in three or four centuries, a rate of more than a foot per decade. Meltwater pulse 1A, along with pulses 1B, 1C, and 1D, was, most probably, the result of ice-sheet collapse. One after another, the enormous glaciers disintegrated and dumped their contents into the oceans. It’s been speculated—though the evidence is sketchy—that a sudden flooding of the Black Sea toward the end of meltwater pulse 1C, around seventy-five hundred years ago, inspired the deluge story in Genesis.

As temperatures climb again, so, too, will sea levels. One reason for this is that water, as it heats up, expands. The process of thermal expansion follows well-known physical laws, and its impact is relatively easy to calculate. It is more difficult to predict how the earth’s remaining ice sheets will behave, and this difficulty accounts for the wide range in projections.

Low-end forecasts, like the I.P.C.C.’s, assume that the contribution from the ice sheets will remain relatively stable through the end of the century. High-end projections, like NOAA’s, assume that ice-melt will accelerate as the earth warms (as, under any remotely plausible scenario, the planet will continue to do at least through the end of this century, and probably beyond). Recent observations, meanwhile, tend to support the most worrisome scenarios.

The latest data from the Arctic, gathered by a pair of exquisitely sensitive satellites, show that in the past decade Greenland has been losing more ice each year. In August, NASA announced that, to supplement the satellites, it was launching a new monitoring program called—provocatively—Oceans Melting Greenland, or O.M.G. In November, researchers reported that, owing to the loss of an ice shelf off northeastern Greenland, a new “floodgate” on the ice sheet had opened. All told, Greenland’s ice holds enough water to raise global sea levels by twenty feet.