Coastal Cities at Risk as Rising Seas Trigger Dropping Sea Floor and Dying Coral

A study of five marine ecosystems shows that as waters are rising, coral reefs are dying and sea floors are eroding, exacerbating erosion along the shore.

Just when you thought rising sea level predictions couldn't get any scarier…

Research published today in the international open-access scientific journal Biogeosciences by the US Geological Survey and the European Geosciences Union shows that not only are sea levels rising in five important coral habitats, the sea floor below is quickly eroding as coral reefs degrade. The cause-and-effect situation is increasing the risk of flooding and storm damage for coastal communities in Florida, Hawaii, and the Caribbean.

By analyzing both new and historical data, US researchers have concluded that sea floor levels have dropped significantly at all five of the locations selected for the study — two sites in the Florida Keys, two in the US Virgin Islands, and one in the waters off of the Hawaiian island of Maui. The research team detected the elevation drop by comparing new readings to historical data sets going back to 1934.

It found that overall sea floor elevation had decreased at all five sites in amounts ranging from 0.09 meters to 0.8 meters (around 4 inches to 2 ½ feet), relative to the most recent depth measurements. All five locations had also lost large amounts of sand and other sea floor materials to erosion. In the waters around Maui, local reef tracts have lost 81 million cubic meters of sand and rock. Somebody at USGS crunched the numbers for a useful visualization: Maui alone has lost enough sand to fill up the Empire State Building 11 times.

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As to why the sea floor is dropping, that's where things get interesting — and by “interesting” we mean “scary.” It all has to do with the coral reefs, and this is where the cause-and-effect twists come in.

Researchers have known for a while now that rising sea levels, caused by climate change, have resulted in increased water depths for coral reefs. With healthy coral reefs, that's not necessarily a problem, according to lead researcher Kim Yates, speaking from her offices at the St. Petersburg Coastal and Marine Science Center in Florida.

“Rising sea levels don't necessarily kill coral reefs,” Yates said. “In fact, coral reefs are sort of designed by nature so that they grow upward toward sunlight and keep pace with rising sea levels.”

Unfortunately, coral reefs worldwide are declining due to multiple ecological threats including coastal development, overfishing, and pollution. They're also suffering from water acidification caused by elevated levels of atmospheric carbon dioxide. The decline of coral reefs, in turn, can trigger a cascade effect that impacts both the sea floor and nearby shorelines.

RELATED: Fla. Keys Coral Reefs Dissolving Faster Than Thought

When coral reefs disappear, they stop producing minerals that turn to sand on the sea floor and nearby beaches. Coral reefs also serve as underwater barriers that prevent the really big waves from crashing into the coastline and dragging sand back out the sea.

Fewer coral reefs leads to increased erosion of shorelines and the shallow-water sea floor bed. This erosion, in turn, results in even deeper waters for the reefs — as sea levels rise above them, the bottom is falling out below. The reefs eventually “drown,” Yates said. Not only is bad news for local marine animals, it directly threatens coastal habitats and seashore communities.

“When coral reefs aren't growing fast enough to keep up with sea level rise, or when they're degrading or dying off, they're no longer creating these big 3D underwater structures that protect coastlines,” she said. “And they're also no longer creating sand.”

That means trouble.

“So you know when you're standing on the shore, watching surfers, and the big waves are breaking way out there?” Yates explained. “When we lose that shallow sea floor and coral, the big waves can make it to the beach before they break up. There's obviously a big difference in the amount of erosion and damage that can take place.”

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The research team also found some additional evidence that human causes are behind the coral decline and sea floor elevation drops. "We saw lower rates of erosion, and even some localized increases in seafloor elevation, in areas that were protected, near refuges, or distant from human population centers," Yates said.

With the publication of the new study, researchers were finally able to put some hard numbers to the phenomenon of dropping sea floor levels around coral habitats. In press materials released with the new study, Yates cited still more scary numbers.

"Our measurements show that sea floor erosion has already caused water depths to increase to levels not predicted to occur until near the year 2100," she said. "At current rates, by 2100 sea floor erosion could increase water depths by two to eight times more than what has been predicted from sea level rise alone."

On the upside, Albuquerque will be a new surfing hotspot.

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