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 three 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 might 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 recalculate 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 shrink reefs even more.

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 water measurements dating back to the 1930s and compared them to modern data, 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.

There were also many variables. Old nautical charts can be famously inaccurate. So the team turned to more precise readings, that until the 1950s were collected by dropping lines or dipping poles into water to measure depth. They then aligned them with modern readings and compared changes at the same location.

As they collected their data, a pattern began to emerge, and some astounding results.

"It was like a punch in the gut," said co-author David Zawada.

Where reefs had shrunk, the ocean floor nearby had grown deeper. In the Keys, elevations near Pacific, Elbow, Carysfort and Molasses reefs had dropped dramatically, sometimes over three feet. Nearby habitats like seagrass beds, which need shallow water to survive, were also deeper. Meanwhile sand was piling up down slope in deeper waters to the south toward Molasses Reef, where it does little to help the shoreline, Yates said.

In Biscayne National Park and the Florida Keys National Marine Sanctuary, they found losses in nine of 11 different habitats. Water had deepened the most — meaning elevation was lost — on patch and aggregate reefs or rocky bottom dominated by coral. The largest volumes of sand were lost on seagrass beds and other shallow sandy bottoms. The amount of sand lost at the five study sites ranged from 3.4 million cubic meters to 81 million cubic meters — the higher figure enough to fill 81 Empire State buildings, Yates said.

The problem is also a cascading one, she said. The more sand washes away, the worse erosion becomes. And the worse conditions get for life on the reef.

"We stand to lose a lot," she said.

Now that they know how much has been lost, Yates said the next step is re-calculating risks, starting in the Keys where most residents live just three to four feet above sea level. The team is now working with USGS engineers to model hazards.

"One of the things we want to know right off the bat is how much risk there is to coastlines," she said. "If nothing changes, if the rate of sea level rise doesn't change and the rate of coral reefs eroding doesn't change, what's the water depth going to look like in 2100?"