Summer Coral Bleaching Forecast Brings Good News

The white tips on this coral indicate bleaching. Photo: Oregon State University / (CC BY-SA 2.0)

Coral reefs are home to millions of species, including octopi, parrotfish, eels, worms, and even parasitic crustaceans named after Bob Marley. But the bizarre and beautiful species that call coral home are at risk from bleaching—the creeping whitening of a reef usually caused by increased water temperatures. Thankfully, most reefs will escape bleaching this summer, according to a new update from NOAA.

Although the agency’s Coral Reef Watch doesn’t anticipate any major coral bleaching events through October, some areas are at risk for minor bleaching, including the northeast coat of the United States, the Philippine Sea, and the Mediterranean. Reefs at highest risk this summer are those off the coasts of Columbia and Panama.

Coral comes in a myriad of shapes, sizes, and colors, but these structures are actually exoskeletons of calcium carbonate. Within them live coral—soft, tentacled polyps only a few centimeters long. Many corals form a symbiotic relationship with a type of algae called zooxanthelle, which give them their brilliant colors. Corals supply the zooxanthelle with nutrients and shelter, while the algae provide energy for the coral.

When corals are stressed, often from high water temperatures, the algae can lose their energy-producing chlorophyll, making the structure appear lighter or faded. They can also expel their zooxanthelle, resulting in complete whitening.

Without their algae, corals are more susceptible to disease. Reef death can occur when bleaching is severe. A disastrous event in the Caribbean in 2005 bleached 80 percent of corals in some areas, NOAA reported. Subsequent mortality was highest in the U.S. Virgin Islands, where more than 50% percent of coral died from a combination of bleaching and disease.

Warm waters are not the only potential hazard to coral. Ocean acidification, changes in salinity, increased sediment in the water, and even cold water can all trigger bleaching. In 2010, the Florida Keys experienced a major bleaching event due to colder than usual temperatures.

To predict coral bleaching, NOAA uses sea surface temperature predictions from their climate forecast models, which also predict El Niño patterns, seasonal temperature, and precipitation. NOAA combines these predictions with real-time monitoring of sea-surface temperatures via satellites.

Increased sea surface temperatures and ocean acidification caused by global warming will cause more coral bleaching events in the future. Yet new research offers some hope that reefs might survive the initial effect of climate change.

Scientists studying reef cores from the coast of Panama discovered that the reef stopped growing about 4,000 years ago, potentially caused by ocean-wide climate changes in the Pacific. But the reef did recover, albeit after a lengthy 2,500 years.

“The hopeful part is that these reefs did prove to be resilient 1,500 years ago,” said Richard B. Aronson, lead author and biologist at the Florida Institute of Technology, to the New York Times. “Reefs today could recover, but only if we get a handle on the greenhouse gases causing climate change.”

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