“Essentially, every coastal flood today is made deeper and more damaging by sea-level rise caused by climate change,” said Benjamin Strauss, chief executive and chief scientist at the research organization Climate Central. “The principle is simple: If you fill a bathtub higher, it’s easier for splashes to get out of the tub.”
That means we have our own species to thank for at least some fraction of the dangerous storm surge for which the Carolinas are bracing. All storm surges, everything else being equal, can reach farther inland today than they could have before humans started heating up the atmosphere.
“If you start with a higher sea level, the same surge obviously will go up higher than without that sea-level rise,” said climate scientist Stefan Rahmstorf, a sea-level expert at the Potsdam Institute for Climate Impact Research. “It’s a no-brainer. The only question is how much.”
Scientists can at least begin to answer that question, too: In the Carolinas, floodwaters could be half a foot or more higher than they otherwise might have been.
Seas have risen about a foot along the Carolina coast since 1900, according to Rahmstorf and his colleague Andrew Kemp, a Tufts University sea-level rise expert who has studied the Carolinas, in particular. (You can also see this in the tide gauge at Wilmington, N.C.) About a third of that change is due to land sinking, or subsidence, which cannot be blamed on humans or climate change. But the remaining amount — about eight inches, or 20 centimeters — is the result of sea-level rise, a substantial part of which is attributable to humans.
“All things being equal, an identical storm 100 years ago and an identical storm today, that water’s going to be about 30 centimeters higher today than it was 100 years ago, of which about two-thirds of that is anthropogenic, one-third is subsidence that’s occurred over the last 100 years,” Kemp said.
Kemp’s colleague Robert Kopp, a climate expert at Rutgers University, differed only slightly with this assessment, calculating that some percentage of the past century’s sea-level rise could have happened even without humans. So Kopp puts the number closer to six or seven inches of rise since 1900 that is attributable to humans and, therefore, will be present in the storm surge of Hurricane Florence or any other storm that hits land.
“It’s a real amount,” Kopp said of the ocean change along the coastal Carolinas. “It’s clearly tie-able to sea-level change. And a portion of that is clearly tie-able to human activity.”
Kopp also said that this is only the beginning — climate change will directly add more and more water to the seas, and the risk that that causes will only increase.
“The sea-level rise we’ve experienced so far is really just a prelude to what we’re expecting to see in the half-century and century to come,” he said.
Sea-level rise is driven by climate change in multiple ways. First, warmer water expands and takes up more space, and seas are warming as well as rising. Second, melting ice from Greenland, Antarctica and smaller glaciers around the world (Alaska, Patagonia and many other places) adds water to the already expanding ocean.
This does not mean that climate change and sea-level rise are the biggest factors determining how much damage Florence will cause at landfall. Many other factors — the storm’s speed and size, the shape of the seafloor and coastline, whether it’s high or low tide, and more — loom larger in determining that.
It’s also important to underscore, as Kemp noted, that 100 years ago, not only would the seas have been lower, but there also would have been a lot less property in harm’s way.
Six or so inches may not sound like much. Still, the climate change component of the hurricane’s storm surge should not be considered trivial when it comes to effects, Strauss said.
Strauss, who with colleagues has estimated that human-fueled sea-level rise accounted for 10 percent of the damage from Hurricane Sandy in 2012, said that even small amounts of additional water can exacerbate flooding. In fact, preliminary research from the group finds that sea-level rise caused an extra $2 billion in damage from Sandy, Strauss said, though that work has not been published yet.
“If you own a home near the coast and the water gets just above your electrical outlets, those six inches could have cost you $10,000,” he said. “There’s going to be a lot of property that gets damaged because of those extra six inches. Those inches may not be the difference between life and death, but they will make a difference everywhere there is flooding.”
Higher sea levels mean that any storm will have what Strauss calls a larger “floodprint,” affecting more homes and businesses and land.
“On the margin, flooding reaches places it wouldn’t otherwise have reached,” he said. But the bigger problem? “Everywhere that floods is deeper,” he said.
One key question is whether, after Florence hits, scientists will be able to portion out the climate-change component by showing which areas were flooded or in other ways affected that might not have been otherwise, as was done with Sandy.
As of Wednesday, according to an analysis by the Union of Concerned Scientists, some 110,000 residential and commercial properties worth $29 billion were in the projected storm-surge zone for Florence. But Kristina Dahl, a climate scientist who heads the group’s sea-level-rise analyses, said her team is not yet able to say which of these properties might not have been in that zone if not for climate change.
She also underscored that the analysis is preliminary and based on storm-surge models that could change anytime up to landfall.
The issues of sea-level rise and how — or whether — to deal with them have been contentious in North Carolina. In one of the nation’s most notorious battles over climate change, the Republican-controlled state legislature voted in 2012 to prohibit the government from basing coastal policies on findings from the state’s Coastal Resources Commission, which forecast that the ocean is likely to rise 39 inches by the end of the century.
That prediction, along with the development of a website where people could check by street address to see how their property might be affected, triggered a wave of alarm among some coastal residents and the real estate industry, which worried that the dire predictions could harm home values and result in skyrocketing insurance costs. They and others helped persuade Republican lawmakers to pass legislation that looked ahead only 30 years rather than to the end of the century, a forecast that would show no more than eight inches of sea-level rise.
The episode brought disdain from environmental advocates, who argued the state was merely choosing to ignore science, as well as much mocking on national television.
"This is a brilliant solution,” comedian Stephen Colbert joked at the time. “If your science gives you a result that you don’t like, pass a law saying the result is illegal. Problem solved.”