As the planet heats up so do the world's waters, and that means more coral bleaching. But now a new study reveals that some corals can bounce back from such near death experiences.



The heat death of a reef reveals itself as whitening, dubbed coral bleaching, which results when corals expel the tiny plants that provide food and are responsible for the rainbow of reef colors. In 2014, coral bleaching happened in the northern Mariana Islands, the Marshall Islands, the Hawaiian Islands and even the Florida Keys. Severe bleaching has now happened two years in a row off Guam and overheated waters have now appeared off the Pacific island nations of Kiribati and Nauru and are also pooling near the Solomon Islands.



"The odds seem good for 2014 to be only the third recorded global scale mass bleaching," says Mark Eakin, coordinator of the Coral Reef Watch at the U.S. National Oceanic and Atmospheric Administration. This event may not be as bad as 2010 or the worst year ever—1998—but "any global-scale mass bleaching is a big issue."



A new study offers hope. Looking at reefs off two of the central Seychelles isles in the Indian Ocean, scientists from Australia found that reefs could rebound even from severe bleaching events, such as those that whitened more than 90 percent of a given reef in 1998. "This is perhaps the most severe coral bleaching event on record," notes Nicholas Graham, a coral researcher at James Cook University and lead author of the new study, published January 14 by Nature. (Scientific American is part of Nature Publishing Group.)



The team has monitored 21 reefs in the Seychelles since 1994, taking a range of measurements that include the total number of plant-eating fish and the amount of nutrients reaching the reefs. The majority of these reefs—12 out of 21—were able to recover after bleaching in warming waters in 1998. The other nine became seaweed-covered ruins.



The scientists have teased out the factors that most strongly predicted a resilient or doomed reef: water depth, the complexity of its shape, nutrient levels, amount of grazing by fish and survival rates for young coral. In fact, using just two of those—growth in waters 6.6 meters or more in depth and complex, branching shapes at least 30 centimeters high atop the reef—the team could predict which reefs would or would not recover 98 percent of the time.



The factors in reef resilience may not seem surprising; for instance, corals at greater depths may better resist heating waters because the warmest waters are closest to the surface. And this doesn’t mean that simple reefs in shallow, warm waters are necessarily doomed. Cutting down on the amount of nutrient and sediment pollution can boost the resiliency of shallow reefs as can cutting back on fishing for seaweed grazers. "Reducing local impacts as much as possible will give them the best chance of survival," Graham notes. "Managing the impacts to reefs is really about understanding and managing human actions."



Reefs that have survived one bleaching event may even be more resistant to future trouble, as reefs that weathered 1998 proved even more resilient in the 2010 bleaching event off Indonesia. "Many reef corals just might be capable of adapting fast enough to survive current rates of global environmental change," wrote marine biologist John Pandolfi of the University of Queensland in a commentary on the new research.



On the other hand, marine reserves did not seem to offer any extra protection to coral reefs, at least off the Seychelles islands of Mahe and Praslin, even though more seaweed-eating fish were present in these no-take reserves. Complex reefs in deeper water that are not deluged with pollution recover best, according to the new study, and may serve as coral refuges.



One factor that could complicate this resilience analysis is seawater acidity, or pH. The oceans also absorb the atmospheric carbon dioxide that accumulates from rampant fossil-fuel burning, which renders the water more acidic. The corals that form Australia’s Great Barrier Reef are now growing half as fast as in the 1970s, largely because much of that new growth is dissolving away at night, according to a 2012 paper in the Journal of Geophysical Research. And although corals can adapt to warming waters and resist more acidic ones, climate change adds to the stress caused by water pollution, overfishing and other threats to reefs, which shelter coastlines from the impacts of extreme weather and sea level rise as well as provide food for hundreds of millions of people worldwide.



If coral reefs are to thrive in the Anthropocene—the current epoch in which human activity has become a driving force on the planet—CO2 levels in the atmosphere will have to drop. As for 2015: "my next big concerns are the reefs on these South Pacific islands, the Indian Ocean and perhaps Southeast Asia later this year," Eakin says. "If it continues, then we will have to worry about the western North Pacific, again, and Caribbean for next fall."

