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U.S. government scientists have found a dramatic impact from the continuing decline of coral reefs: The seafloor around them is eroding and sinking, deepening coastal waters and exposing nearby communities to damaging waves that reefs used to weaken.

The new study, conducted by researchers with the U.S. Geological Survey, examined reefs in Hawaii, the Florida Keys, and the U.S. Virgin Islands, finding seafloor drops in all three locations. Near Maui, where the largest changes were observed, the researchers found that the sea floor had lost so much sand that, by volume, it would be the equivalent of 81 Empire State Buildings.

"We knew that coral reefs were degrading, but we didn't really know how much until we did this study," said USGS oceanographer Kimberly Yates, the lead study author. "We didn't really realize until now that they're degrading enough that it's actually affecting the rest of the seafloor as well."

Yates conducted the study with two other geological survey researchers and a researcher with Cherokee Nation Technologies. The work was published Thursday in the journal Biogeosciences.

Coral reefs naturally generate sand as hard coral skeletons die, and their calcium carbonate bodies become the next layer of the seafloor. Meanwhile, the living tops of coral columns grow taller and taller, which allows them to keep pace in eras of rising seas.

But as corals are subjected to more and more assaults from a combination of global climate change, local pollution, and direct human-caused damage, this natural dynamic appears to have been undermined, and seafloor accretion has swung to erosion.

"When corals stop growing fast enough, and when they stop making those big skeletons, you also lose that supply of sand to the rest of the seafloor, and you lose that supply of sand to the beaches," said Yates.

The exhaustive study started with old nautical charts, dating as far back as the 1930s in some cases, that listed seafloor depths in the vicinity of reefs. Then the scientists remeasured depths in the present.

And they found that averaged across the three areas that they studied, the seafloor dropped from between 0.3 feet and 2.62 feet. At extremes, losses in specific places exceeded 13 feet.

These numbers are particularly striking when considered in the context of today's rising seas. While the current rate of sea level rise is estimated at a little above 3 millimeters per year, the study calculated seafloor elevation loss rates that were sometimes double that - or even higher in Maui, which saw the greatest losses.

The upshot is that natural reef growth in these areas will not be able to keep up with sea level rise - rather, the reefs will fall well behind it, and coastal waters will grow deeper and deeper. In fact, they already have.

"Erosion of coral reefs and seafloor is happening much more and much faster than what was previously known or expected, enough so that it's affecting those local sea level rises," said Yates. "Enough so that it increases the risk to the coastlines from coastal hazards, storm waves, every day persistent waves, tsunamis and those kinds of things."

The authors caution that these findings only apply to their study areas for U.S. coral reefs in Florida, Hawaii, and the Virgin Islands. Globally, similar processes may well also be afoot - reefs across the world are generally threatened. But there could also be local processes that act very differently in other places.

They also warn that they can't attribute seafloor changes to any one cause - or even to multiple ones. From hurricane wave action to coral collapses, many factors change the elevation of the seafloor.

Still, the researchers are convinced that the findings represent a risk to coastal communities in regions that experience major hurricane strikes.

"Think of the reefs as kind of natural speed bumps," said David Zawada, one of the study's co-authors and also an oceanographer with the U.S. Geological Survey. "Take that away, this wave energy, more of it is going to be able to migrate in closer to shore."

"This is a critically important study, which shows that we are not only losing living corals but also that reefs are being eroded away around the world," said Michael Beck, lead marine scientist at the Nature Conservancy, which has a program to study and protect Florida's enormous reef tract, which is the third largest of its kind in the world. Beck was not involved in the study.

"Effectively, seas are not just rising but reefs are sinking and with them the many benefits they provide including flood protection to communities around the world. Their study points to why it is so urgent to act now to improve reef health through conservation and restoration."

There is a recurrent motif that you can now detect in climate change studies when researchers are delivering weighty findings. They often invoke the so-called "Anthropocene," or the idea that we're entering a new geologic epoch brought on by human alterations to the planet.

"The magnitude of reef volume lost due to erosion provides evidence for the onset of an Anthropocene reef crisis similar to ancient reef crises caused by climate change and marked in the geologic record by regional and global declines in reef volume," the authors write.