News•May 14, 2014

Great Barriers: Reefs First Line of Coastal Defense

A new study quantifies what many surfers already know: coral reefs help diffuse incoming waves and create huge breaks.

While that helps determine where to hang ten, it also hints at something much more important: reefs are a first line of defense for coastal communities when it comes to storm surge protection and that in many cases, they are much more efficient and cost effective than building sea walls.

Coral reef scenery off the coast of Egypt.

In a warming world where sea levels are rising even as more and more people move to coastal communities, coral reefs could offer a solution to help reduce coastal flooding and minimize damage.

Roughly 40 percent of the world’s population lives within 60 miles of a coastline. While major storms such as Sandy or Super Typhoon Haiyan can wreak massive damage, most coastal communities also have to grapple with regular, smaller-scale flood events.

Sea level rise is already exacerbating flooding and storm surge and that will only worsen in the future. Oceans have risen an average of 8 inches since the start of the 20th century. Though that may sound like a small number, a recent study found that the risk of New York flooding has increased 20-fold due in part to sea level rise. The recent National Climate Assessment report said sea levels could be 6 feet higher by 2100.RELATEDBreathtaking Coral Reef Panoramas Help Scientists
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Adapting to future sea level rise could save the world trillions of dollars. Most studies have explored the benefit of installing sea walls — “gray infrastructure” — as a means of reducing risks. However, an analysis published in Nature Communications on Tuesday points to the value of using natural infrastructure already found throughout the tropics.

Coral reefs have the potential to provide benefits to nearly 200 million. Michael Beck, lead marine scientist at The Nature Conservancy who co-authored the study, said you only need to look at one of the world’s most famous surf breaks to understand the benefits of reefs.

“Imagine on the North Shore of Hawaii where you have 15-20 foot waves crashing on the reefs. If you get caught under that lip, you feel like you’ve been torn apart,” Beck said. “ But after that, it’s a bunch of rolling white wash. We talk about ecosystem service and benefits all the time and it’s really vague. This is something that’s a service you can see just sitting at your desk using Google Earth.”

A view of Teahupo'o, a famed surf break in French Polynesia.
Credit: Google Earth

Beck and his colleagues looked at 250 previous studies of coral reefs. They found that reefs were able to dissipate up to 97 percent of a waves’ energy as it approached shore. Most of that happened right where the reef meets the open ocean. Just how much energy reefs take out of waves and storm surge depends on two factors: how close reefs are to the surface and how rough they are.

Shallow, jagged reefs tend to slow waves the most. However, climate change could reduce both the qualities that make reefs so effective at protecting coasts.

Coral bleaching occurs when waters get too warm or acidic, both of which are occurring and will continue to worsen over the next century, due in large part to climate change. Staghorn and elkhorn coral, two of the most jagged corals in the Caribbean, have already died off across a large portion of their range and are being considered for listing on the Endangered Species List.

Sea level rise also poses a hazard to coral reefs. While they can keep pace with up to 3 feet of sea level rise, anything greater would sink them according to Steve Palumbi, a coral researcher at Stanford who wasn’t involved in the study.

“Although corals are capable of keeping up with projected sea level rise, they currently can’t grow at their peak performance,” Palumbi said.

That’s because in addition to shifts in global climate, there are local factors that are hurting coral reefs. Pollution and overfishing have substantially damaged reefs around the world. Since 1950, 19 percent of the world’s coral reefs have been lost to these pressures and 35 percent of all reefs are threatened today.

However, Beck said there are reasons for optimism that reefs can be saved and continue to play a vital role in protecting coastal communities.

“Because they’re so visible and iconic, we’ve done a lot to conserve them,” he said. “They’re in better shape than salt marshes, mangroves, and a lot better off than oyster reefs. There were all these dire predictions following the big global bleaching in 1998. But it turns out a lot of those reefs are recovering, and we know they recover faster in places that are better managed.”

A sea turtle swims over a reef that's been through a coral bleaching episode.

Recent studies have also pointed to some coral that are more resistant to acidic or warm waters. Those could be used as building blocks for reinforcing reefs around the globe or building what some have termed “designer reefs.”

Researchers point to an economic argument for using reefs to slow storm surge around the world. Beck and his colleagues found the costs of reef restoration and stabilization were significantly less expensive than installing new sea walls, though the results could be slightly skewed given the higher number of sea walls installed in developed countries where construction costs tend to be higher.

“The living, growing seawall of the reef is a better investment than a dead constructed seawall of concrete,” Palumbi said. “This should come as little surprise. Who wouldn't want a self-repairing seawall barrier that grew and expanded over time as sea level rose?”

Palumbi and Beck both cautioned restraint in building reefs pell mell around the world. In fact, Beck said that’s not the point of the research. He said the value was in focusing on how to restore and protect existing reefs or ones that were around until recently. That'll make more than Kelly Slater and Mick Fanning happy.

“That’s still relevant and will benefit 200 million people,” he said.