As Hurricane Irma bore down on Florida, the water between the storm's center and the shoreline bulged. Mighty winds whipped the Atlantic Ocean into a mound. Extraordinarily low pressures allowed it to rise even higher. This bulge, the storm surge, took up so much water that long stretches of the Caribbean coast went dry. The hurricane had flailed the ocean into an unfamiliar shape.

A storm surge is “literally just the brute force of the winds pushing the water into the land,” said Karthik Balaguru, who studies hurricanes at the Pacific Northwest National Laboratory.

And where a hurricane dumps its surge can be the site of catastrophe. On Monday, the water flooding downtown Jacksonville reached a record five feet, according to the National Weather Service. That was more than a foot over the mark that had stood for more than half a century.

Locals were told to flee. The Jacksonville sheriff's department tweeted, “ATTENTION: Evacuation Zones A/B along the river. Get out NOW.”

The surge's reach was long. More than 200 miles up the coast from Jacksonville, officials in Charleston, S.C., issued a flash flood warning as a swell of water spilled into that city.

Surges can reach incredible heights, too. During a Monday news conference, Florida Gov. Rick Scott (R) said Monroe County was hit by a 10-foot surge. In Miami, spared the brunt of the storm, the surge reached four feet. (Given hurricanes' destructive wet power, it may be easy to confuse a surge with other elements of an oceanic storm. A surge is not a wave or tide, though a rising tide can lift a surge higher.)

Because so much water is involved, surges are frequently a hurricane's deadliest symptom. Many of the estimated 1,500 deaths from Hurricane Katrina could be attributed to the storm surge along the Gulf Coast, according to the National Hurricane Center. In Mississippi, the surge achieved “historical proportions,” with the highest elevation marked at more than 28 feet.

Two major factors control how a storm surges: the strength of a hurricane's wind and the shape of the coastline. The Atlantic continental shelf is relatively flat, affording a hurricane there more shallow water that it can push into a surge. What's more, the counterclockwise rotation of the hurricane's northeast quadrant shoves the water toward land instead of away from it, Balaguru said.

Predicting a storm surge is difficult. Even slight changes in the hurricane's center and strongest winds can influence its path, explained Rebecca E. Morss, a scientist at the National Center for Atmospheric Research in Boulder, Colo.

Plus the way surges move can take communities by surprise. “If the water has a path to get to you and there's not a hill in the way, it's going to get to you,” Morss said. “If you’re in the wrong place, you don't want to be there.” During Hurricane Ike in 2008, the surge did not go over the sea wall in Galveston, Tex., but instead came around from the other side, devastating the city.

Scientists expect storm surges to crest even higher in the future because of global warming.

Even if carbon emissions peak in the next few decades — a very conservative scenario — surges at the end of the 21st century will be severe, Balaguru and his colleagues reported in a 2016 article in Climactic Change. They compared surges at the end of the 20th century with those predicted at the end of the 21st and calculated that the average storm surge decades from now will have increased by 25 percent to 47 percent over those 100 years prior.

This finding reflects a combination of sea-level rise and more intense storms. It is generally accepted among climate researchers that future hurricanes will not be more numerous — but they will be more intense, Balaguru said.

As oceans and gulfs warm, that's more energy for hurricanes to extract from the upper layers of water. More energetic storms mean fiercer winds, which in turn mean more powerful surges. What's more, given the structure of the Atlantic coast's continental shelf, “sea level rise tends to matter a lot more” than along steeper sea edges.

“The oceans are changing in a way that will intensify hurricanes,” Balaguru said. “If you couple that with sea level rise, you have a perfect recipe for damage along the coastline.”

Read more:

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