As Corals Die, the Sea Floor is Eroding — Leaving Coastal Communities Exposed

Elkhorn coral in the U.S. Virgin Islands that has died and collapsed. 

Elkhorn coral in the U.S. Virgin Islands that has died and collapsed.  Curt Storlazzi, USGS

The sea floor around five U.S. coral reef ecosystems is eroding far more rapidly than scientists thought, putting at risk coastal communities that rely on reefs for protection against storm surges and flooding.

The research, published this week in the journal Biogeosciences, found that the sea floor has dropped in elevation by as much as 2.5 feet in some coral reef systems in the Atlantic, Pacific, and Caribbean over the past century. Near reefs around Maui, for example, the losses equal 81 million cubic meters of sand, rock, and other material — enough to fill up the Empire State Building 81 times.

Coral reefs are declining across the globe due to coastal development, overfishing, pollution, ocean acidification, and coral bleaching. As these reefs die, the sea floor surrounding them no longer gets a constant source of new material and is no longer tethered in place, both of which increase the rate at which the sea bottom erodes. Reefs at lower depths are less effective at slowing or stopping intense waves and storm surges as they move toward coastal communities. Worldwide, more than 200 million people live in coastal areas that have historically depended on protection by coral reefs.

Scientists have long projected that future sea level rise would reduce reefs’ protective abilities, but the new research shows this process is happening far sooner than anticipated, the study authors said.

“Our measurements show that seafloor erosion has already caused water depths to increase to levels not predicted to occur until near the year 2100,” said biogeochemist Kimberly Yates of the U.S. Geological Survey, the study’s lead author. “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.”