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While sea levels worldwide have been going up an average of about an eighth of an inch (3 millimeters) a year for the past few decades, the rate has been accelerating in recent years. And along a stretch of Atlantic shore from the Outer Banks to Miami, the speed with which the seas have been rising is up to six times as fast as the global average, leaving some communities scrambling to beat back the higher tides.
“Throughout the southeastern US, you see increased erosion rates and sunny-day flooding,” said Arnoldo Valle-Levinson, a professor of civil and coastal engineering at the University of Florida. “There’s a big impact on the infrastructure, drainage, and roads that get flooded with salt water. Salt water is corrosive, so you also have utilities that could be affected by salt water, like sewage or street drainage.”
In addition, salt water pushing up into coastal waterways can alter those fresh water sources, damaging wetlands and natural habitat and affecting drinking water supplies for nearby communities.
Valle-Levinson and some of his colleagues recently studied nearly a century of records from tidal gauges along the East Coast. They found that in some reaches, such as south Florida, sea levels went up as much as an inch a year between 2011 and 2015 — while some parts saw numbers closer to or even below the global average.
Why the difference? Their conclusion suggests — at least in the Southeast — the size of the rise depends on the motion of the ocean.
The Atlantic is influenced not only by a cyclical change in air pressure over the ocean’s higher latitudes, but by conditions in the equatorial Pacific — the warming and cooling trend known as the El Niño-Southern Oscillation. When the cool phase known as La Niña combines with higher-pressure North Atlantic Oscillation, they produce a push-pull effect that drives Atlantic waters westward, Valle-Levinson said.
The inertia of all that water means the effect can linger for years. The “hot spot” the Florida study noted appears to be subsiding, the rate of increase to something closer to the global average — but it’s still laid on top of the increases already seen as a result of climate change.
Because it’s not a constant phenomenon, it’s hard to say what the long-term effect of the periodic “hot spots” may be for sea levels, Valle-Levinson said. “But I'd say that in 50 years, considering global warming and hot spots, sea level could be between 20 centimeters and 1 meter higher than what it is today,” he said.
Already, the increase in coastal flooding in the Miami area has drawn attention due to the pricey real estate at risk and the occasional octopus washing into a garage. Former Vice President-turned-climate evangelist Al Gore’s new movie highlights the problems there as well.
But it’s also causing problems up the coast in places like Charleston, South Carolina, a low-lying city full of historic buildings. Charleston had 50 days of tidal flooding in 2016; it’s expected to happen roughly every other day by the 2040s, said Mark Wilbert, the city’s emergency management director and chief resilience officer.
“The thing that we love about living here is having all this beautiful water around us,” Wilbert said. “Yet it is becoming more of a nuisance, and it can become dangerous very quickly.”
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A typical high tide in Charleston is five to six feet, Wilbert said. But when tides top 7 feet (2.1 meters), “We’ll start to see waters show up on the streets in low-lying areas.”
The city is also seeing more torrential downpours, another symptom of climate change, and those can cause flooding unrelated to the sea. And sometimes, they team up.
“The ones we are most concerned about is when you have a high tide with an extreme precipitation event,” Wilbert said. “Those can go from just a nuisance flooding event to something that can be much more serious in a very, very short period of time.”
Charleston has already started to rise to the challenge, publishing a strategy for managing sea-level rise in 2015. It’s embarked on a $238 million program to beef up its drainage systems, including a 140-foot-deep tunnel that will pump water up to rivers further inland. It’s adding two and a half feet to the seawall around the Battery, the historic district at the southern tip of town, and Charleston officials are meeting with counterparts up and down the coast to trade ideas.
Wilbert said the city is also considering raising the requirement for new buildings from one foot to two feet above current baseline flood elevations and encouraging builders and homeowners to take steps like building on stilts or raising existing structures.
Planners also will have to examine things like more physical barriers, restoring more wetlands that provide a “soft” coastal edge — “and maybe some areas, we just allow the sea to come in and take over some of those spots there, like it’s trying to do.”
That will likely mean relocating people and businesses along the coast. At least one community on the Gulf Coast, Louisiana’s Isle de Jean Charles, is already planning to move after decades of sinking land, erosion, and now rising seas eat away at their home. Without efforts to rein in the carbon emissions driving global warming, the real estate research firm Zillow estimates millions of homes could be flooded by 2100, including one in eight Florida properties.
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That’s likely to mean millions of people packing up and moving to inland cities like Atlanta, Houston, or Charlotte. A University of Georgia study recently estimated that unchecked climate change could uproot as many as 13 million nationwide. Many of them are expected to move to cities that have already been struggling to accommodate rapid growth and are “unprepared to accommodate this wave of coastal migrants,” the study found.
And even if emissions are curbed soon, the planet-warming carbon dioxide and other gases already pumped into the atmosphere are likely to mean a certain amount of sea-level rise is already baked in.
“The best we can hope for is not make it accelerate more,” Valle-Levinson said.
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