reef diving

The Nature Conservancy's Kydd Pollock and Brandon Bourke dive under a breaking wave at the reef crest near Penguin Spit in Palmyra Atoll in the equatorial North Pacific. (Photo: © Tim Calver/The Nature Conservancy)

Sea levels are rising and tropical cyclones are intensifying, which is bad news for about 200 million people who live along Earth's coasts. If only evolution had spent millions of years fine-tuning some kind of sea creature to build and maintain giant barriers that can soften the ocean's fury for us.

It did: corals. The reefs these animals build are well-known to scientists and surfers for absorbing the blow of incoming waves and creating big, dramatic breaks. But now, thanks to a new study, we have a fresh appreciation for just how vital these ecological construction crews have become. Published in the journal Nature Communications, the study offers "the first global synthesis of the contributions of coral reefs to risk reduction and adaptation," according to a statement from the Nature Conservancy, which helped produce the study along with an international team of researchers.

Coral reefs reduce a wave's energy by up to 97 percent, the study shows. The reef crest alone — the shallowest area where waves break first — dissipates most of the energy, absorbing about 86 percent of a wave's power before it reaches the reef flat or lagoon. Without such a buffer, coastal residents must face the full brunt of rising seas and stronger storm surges driven by climate change.

"Coral reefs serve as an effective first line of defense to incoming waves, storms and rising seas," says Michael Beck, lead marine scientist for the Nature Conservancy and a co-author of the study. "200 million people across more than 80 nations are at risk if coral reefs are not protected and restored."

About 44 percent of all humans live within 60 miles of an ocean coastline, according to the United Nations. And since global warming is rapidly raising sea levels and encouraging more severe coastal flooding, coral reefs could present a natural solution to an enormous man-made problem.

Not only that, but they can do it better and more economically than even the best human engineers. The median cost for building an artificial breakwater is $19,791 per meter, the study's authors report, while the median cost for coral-reef restoration projects is about $1,290 per meter.

In other words, preserving coral reefs is 15 times cheaper than trying to mimic them with concrete.

"Coral reefs are wonderful natural features that, when healthy, can provide comparable wave reduction benefits to many artificial coastal defenses and adapt to sea-level rise," says Curt Storlazzi, an oceanographer with the U.S. Geological Survey and co-author of the new study. "This research shows that coral reef restoration can be a cost-effective way to decrease the hazards coastal communities face due to the combination of storms and sea-level rise."

coral reef

The lagoon coral reef and shore at Ant Atoll, Pohnpei, Federated States of Micronesia. (Photo: © Nick Hall/The Nature Conservancy)

The researchers analyzed 250 previous studies on coral reefs to quantify their wave-breaking abilities. On average, only 3 percent of a wave's energy made it past a reef, with most of that energy erupting where the reef crest meets the open ocean. The exact amount of energy disruption depends on a few variables, though, including the depth of a reef and the roughness of its texture.

Shallow and jagged reefs are the most effective barriers, the study shows, making them invaluable natural resources as man-made climate change swells sea levels by up to 3 feet and boosts the number of category 4 and 5 hurricanes over the next century. These reefs can only save us from ourselves if we let them, though. Corals around the world are endangered by a variety of human activities, including water pollution, invasive species and, ironically, climate change. Warm, acidic waters in the Caribbean have been especially hard on jagged species like staghorn and elkhorn coral, for example, both of which are now candidates to join the U.S. endangered species list.

But while ocean acidification and rising water temperatures can be deadly to corals, there's also some evidence these animals can endure such sudden changes — with a little human assistance.

"While there are many concerns about the future of coral reefs in the face of climate change, there are still many reasons for optimism about the future of coral reefs, particularly if we manage other local stressors such as pollution and development," says Fiorenza Micheli, a biology professor with Stanford University's Hopkins Marine Station and a co-author of the new study.

wave on coral reef

Brandon Bourke of the Nature Conservancy dives under a breaking wave at a reef crest in Palmyra Atoll. (Photo: © Tim Calver/TNC)

Conservation efforts often focus on remote coral reefs, but the study's authors suggest reefs near people should be at least as high a priority. Not only are those reefs often at greatest risk from pollution, development and overfishing, but they also have the most potential to directly protect civilization. About 197 million people worldwide live less than 10 meters above sea level and within 50 kilometers of a coral reef, and they'll face much higher costs from natural disasters if those reefs die.

"This study illustrates that the restoration and conservation of coral reefs is an important and cost-effective solution to reduce risks from coastal hazards and climate change," says Filippo Ferrario, a biologist at the University of Bologna and lead author of the new study.

Here's a list of the 15 most coral-protected countries on Earth, ranked in terms of the number of people who receive risk-reduction benefits from coral reefs:

1. Indonesia: 41 million

2. India: 36 million

3. Philippines: 23 million

4. China: 16 million

5. Vietnam: 9 million

6. Brazil: 8 million

7. United States: 7 million

8. Malaysia: 5 million

9. Sri Lanka: 4 million

10. Taiwan: 3 million

11. Singapore: 3 million

12. Cuba: 3 million

13. Hong Kong: 2 million

14. Tanzania: 2 million

15. Saudi Arabia: 2 million

coral reef

A wave breaks over a healthy coral reef in Tumon Bay Marine Preserve, Guam. (Photo: Curt Storlazzi/USGS)

coral reef

Corals grow near a mangrove swamp in the coastal waters at Kofiau, Indonesia. (Photo: © Jeff Jonover/TNC)

Russell McLendon is science editor at MNN. Follow him on Twitter and Google+.

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