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Vanishing ocean floor in the Florida Keys worsens risks from sea rise, study finds

Jenny Staletovich, Miami Herald on

Published in News & Features

MIAMI -- If sea rise weren't scary enough, scientists have now found another phenomenon threatening the Florida Keys and other coasts protected by reefs: a vanishing ocean floor.

In a study published Thursday in the journal Biogeosciences, a team from the U.S. Geological Survey documented a dramatic erosion of the sea floor around coral reefs, ranging from a few inches to nearly 3 feet since the 1930s. Combined with sea rise, the disappearing bottom means the hazards facing coasts -- storm surge from hurricanes and even erosion from everyday waves -- will likely be worse than now projected, especially for the low-lying Keys.

"We worked very hard to try to prove ourselves wrong because the change was so striking," said lead author Kimberly Yates. "And we just could not do that."

The study, which focused on the Upper and Lower Keys, the Virgin Islands and Maui, found water now at depths that had not been predicted to occur for another eight decades. That's because with sea floor loss factored in, sea rise occurred at a far faster rate than previously thought. Other reefs are likely experiencing the same losses, Yates said, meaning many more coastal communities armored by reefs may face higher threats.

"We knew sea rise was happening," she said. "But the loss of the sea floor makes this worse."

The study focused on the changes -- comparing historic water measurements to modern ones -- and did not address causes or re-calculate future sea rise. The team suspects the loss is tied to ailing reefs, which generate sand, and have been in sharp decline since the 1970s, pounded by pollution, increasing coastal development and overfishing, among other things.

Among climate scientists, reefs have become a focus of research because they play such a vital role in protecting coasts. Sand they generate keeps shores shallow to absorb waves while the reefs themselves can buffer pounding storm surge from hurricanes. Erosion is a particular concern, with some fearing new impacts could overtake healthy sand-producers, like parrotfish that eat coral or boring sponges.

Until now, Yates said no one ever took a look at the net loss from erosion to assess widespread impacts.

"It's like an elephant and you've got one scientist measuring the tail and another measures the ear or length of the trunk and those things are really important to the elephant. But we found a way to measure the whole elephant," she said.

To calculate the change, the team looked at nautical charts dating back to the 1930s and compared them to modern charts, using complicated calculations to assess change. Engineers have done similar assessments to track shifts in shipping channels. But no one had ever applied them to reefs at such a large scale, Yates said.

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