PARIS – Coral reefs are as good as concrete defenses at protecting tropical coastal cities from rising seas, and it can be cheaper to conserve or restore damaged reefs than to erect new breakwaters, scientists said in a newly published study.
The paper, published in the science journal Nature Communications, coincides with findings that the West Antarctic ice sheet is starting to collapse — a phenomenon that some scientists warn may drive up sea levels over hundreds of years.
Researchers led by Michael Beck at the University of California at Santa Cruz estimated that coral reefs dissipate up to 97 percent of the energy that waves would otherwise deliver to a shoreline.
The seaward edge of the reef, called the crest, is by itself responsible for dissipating 86 percent of the energy.
Nearly two-thirds of the remaining energy is then buffered by the reef flat, the shallow part of the reef that extends outward from the shore.
Overall, this natural shield more than halves the height of waves. More precisely, the study said, it reduces them by an average of 64 percent, compared with a reduction of 30 to 70 percent by breakwaters.
Beck’s team also did a rough cost analysis: the average price for a tropical breakwater project was $197 million, compared with $129 million for restoring a reef.
“We find that restoring reefs is significantly cheaper than building artificial breakwaters in tropical environments,” they wrote.
Furthermore, they said an additional advantage comes in the long term: “Reefs have the potential for self-repair and thus lower maintenance costs as compared with artificial structures.”
Around 100 million people living on low-lying coastal areas 10 meters high (32 feet) or less could benefit from risk reduction by reefs, the study said.
Indonesia, India and the Philippines account for half, but the United States would also be among the top 10 beneficiary countries.
The U.N.’s Intergovernmental Panel on Climate Change estimated last September that sea levels would creep up by 26 to 82 cm (10 to 32 inches) by 2100, driven partly by ice melt and partly by expansion of the ocean as it warms.
But some experts say this estimate is too conservative, in the light of findings published Monday that the West Antarctic ice sheet is now melting at an unstoppable pace.
Sridhar Anandakrishnan, a professor of geoscience at Pennsylvania State University, said “something like 90 cm” would be a likelier figure.
Rising sea levels attack coastlines and increase the flooding impact of storm surges, or waves that are whipped higher by powerful cyclones that make landfall.
To protect vulnerable cities on coastlines and estuaries, many countries are eyeing plans to build concrete walls and breakwaters.
Spending on dikes alone is predicted to rise to a range of $12 billion to $71 billion by 2100, according to an estimate published in the U.S. journal Proceedings of the National Academy of Sciences earlier this year.
A 2011 report by the U.N. International Strategy for Disaster Risk Reduction said 3.6 percent of the world’s gross domestic product was annually exposed to tropical cyclones in the 1970s.
This rose to 4.3 percent of GDP in the first decade of the 21st century.
Previous studies have highlighted the usefulness of mangroves and marshes as natural protections against the ocean threat, but relatively little research has been done into coral reefs, a complex ecosystem threatened by overfishing and warming seas.
Beck’s team cautioned, though, that far more research is needed to get a clearer picture of the costs of reef restoration, as well as the benefits that could derive fishing and tourism.