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Want to protect cities from the rising seas? Try coral reefs.

A school of manini fish pass over a coral reef at Hanauma Bay on January 15, 2005 in Honolulu, Hawaii.
A school of manini fish pass over a coral reef at Hanauma Bay on January 15, 2005 in Honolulu, Hawaii.
Donald Miralle/Getty Images

The world's coastal cities are likely to spend billions — if not trillions — of dollars this century protecting themselves from storms, floods, and rising sea levels. In many cases, that will mean building concrete seawalls and other artificial barriers.

But many countries may want to consider spending some of that money on restoring their coral reefs instead. A new study in Nature Communication finds that natural reefs often do a better job protecting tropical cities from storms and flooding and erosion than artificial seawalls do.

How reefs protect cities

When waves crash against coral reefs along the coast, they lose much of their energy before hitting the shore. Healthy reefs tend to offer more protection than degraded ones.

In this study, the researchers found that healthy coral reefs offer a surprising amount of protection against storms — reducing the height of waves by 51 to 75 percent. By comparison, the artificial breakwater systems used in places like Hawaii or Sri Lanka tend to reduce wave height by 30 to 70 percent, on average.

What's more, in places like the Caribbean, coral reefs seem to be the cheaper option. In a preliminary analysis, the researchers estimated that the average tropical breakwater project cost around $19,791 per meter. The average coral reef restoration, meanwhile, cost just $1,291 per meter. (This jibes with similar findings from the reinsurance industry, though the authors note that a more complete cost analysis was still needed.)

Now, granted, not every coastal region has a nearby coral reef to protect it. But many do. The study calculated that some 200 million people around the world could stand to benefit from reef protection or restoration projects. Many of those people live in Indonesia, India, and the Philippines — but it also includes about 3 million Americans.

Countries that receive storm-protection benefits from reefs:

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See a more complete map here. Nature Communications

Can coral reefs survive a warming world?

The search for coastal defenses will likely become far more urgent in the decades ahead as global warming pushes up sea levels around the world. That, in turn, will increase the risk of storm surges in many fast-growing coastal cities.

The Intergovernmental Panel on Climate Change currently projects that global average sea levels will rise 1 to 3 feet by the end of the century (although the recent news that West Antarctica's ice sheet is melting faster than expected could push that up).

As a result, the IPCC projects that spending on dikes alone will increase to up to $71 billion per year by 2100 — to say nothing of other defenses to protect against stronger storm surges.

At the same time, many of the world's coral reefs will come under severe pressure if greenhouse-gas emissions keep rising and the world's oceans get warmer and more acidic.

Still, the Nature Communication paper cites a few recent analyses suggesting that coral reefs aren't entirely doomed — and many reefs are more likely to survive in a warmer world if countries take steps to protect them. That might entail, for instance, curbing marine pollution or destructive fishing practices. That often costs money. But, in many cases, it may cost less money than erecting expensive sea walls.

"Although conservation efforts are most often directed to more remote reefs," the authors add, "our results suggest that there should also be a focus on reefs closer to the people who will directly benefit from reef restoration and management."

Further reading:

  • The Nature Communications study is here, and was published by an international team of researchers from the University of Bologna, The Nature Conservancy, United States Geological Survey, Stanford University and University of California Santa Cruz.
  • Coral reefs aren't the only natural barriers against storms and coastal flooding. There's plenty of research that mangroves and marshes and oyster reefs can protect some coastal regions.
  • Because coral reefs are so valuable — not just for storm protection — some scientists have been experimenting with "designer" reefs that could withstand a warmer world. The research is still quite new and potentially controversial, but it might be one of the few places to save reefs in some areas.