Stanford, California - Coral reefs are widely regarded as one of the most beautiful, diverse and delicate ecosystems on the planet. A new study by an international team of scientists reveals that reefs also play the tough guy role in protecting hundreds of millions of people from rising sea levels and damaging wave action.

The researchers performed a meta-analysis of 27 previous studies of how coral reefs around the world dissipate wave energy, conducted in conditions ranging from normal surf to hurricane-level waves. They found that coral reefs reduce wave energy by an average of 97 percent, dissipating disproportionately more wave energy as wave energy increased.

"It's obvious to the eye that waves inside a protected lagoon are much mellower than those crashing on the outer reef crest, but the extent and generality of the energy dissipation revealed by the data analysis for different locations and reef settings was surprising," said study co-author Fiorenza Micheli, a professor of biological sciences at Stanford and Stanford's Hopkins Marine Station, as well as an affiliate of the Stanford Woods Institute for the Environment. "It is a huge reduction. The majority of wave energy is lost on the reef crest."

The effects were comparable to artificial breakwaters that are engineered specifically to dissipate wave energy, Micheli said.

Human activities that directly damage coral, in combination with increasing ocean temperatures and acidification, have already degraded or are posing serious threats to at least two-thirds of the world's coral reefs. The new analysis, published this week in Nature Communications, raises the stakes for conservation efforts, the researchers said, and could help focus those efforts toward reefs in high-risk areas.

The researchers estimate that there are between 100 million and 200 million people around the world living fewer than 10 meters above sea level and within 50 kilometers of coral reefs, putting them at risk of wave action and rising sea levels. The bulk of those people live in or around the Pacific Ocean; the top five countries that could be protected by reefs touch the Pacific. The United States has the seventh most people at risk, with 7 million people spread across Florida, Hawaii, and other coastal and island areas.

Restoring and recovering reefs could entail reducing local impacts from pollution and coastal development, or may require active habitat restoration if reef degradation is too severe for natural recovery, Micheli said. Both options provide a less expensive means of reducing the impacts of waves on the shore than artificial seawalls: the authors report that the median cost for building artificial breakwaters is $19,791 per meter, compared to $1,290 per meter for coral restoration projects. Restored reefs could reduce wave energy immediately, becoming more valuable through the years as they grow, keeping pace with rising sea levels.

"Reef restoration can also provide additional benefits," Micheli said. "While reducing risk, coral reefs also support biodiversity, improve water quality, and support fisheries and tourism."

The research was conducted by scientists from the University of Bologna, the Nature Conservancy, the U.S. Geological Survey, Stanford and the University of California, Santa Cruz.