Australia’s Great Barrier Reef is the largest coral ecosystem in the world. The country’s leading marine scientists surveyed 911 of its reefs by air, and last month reported that an alarming 93 percent of them had been bleached to some extent and that more than one third of them had experienced severe bleaching. “Coral bleaching is caused by a breakdown between the coral animal and the algae living inside its tissues,” explained Northeastern’s Steven Vollmer, associate professor in the College of Science. “Bleached corals look white because the coral host has either digested or expelled the algae that gives the coral its brownish color.” Bleaching, which is typically due to rising sea temperatures, can kill coral. It could lead, said Vollmer, “to the ocean’s equivalent of a rainforest with no trees.”

We asked Vollmer, who teaches in the Panama segment of Northeastern’s Three Seas Program, how bleaching occurs, what it means for coral survival and the ecosystem as a whole, and why this year’s bleaching event—and its effect on the Great Barrier Reef—is so significant.

How does coral bleaching come about, and what does it mean for coral survival and, by extension, the ecosystem, including us?

The condition of bleaching can come from a variety of stressors, but it is typically associated with heat stress due to prolonged exposure to sea temperatures 2 to 3-plus degrees Celsius above seasonal highs. For example, where we work in Panama, warm is usually around 29 to 30.5 degrees Celsius, but we see corals become stressed and bleach when temperatures rise above 30.5 to 31 degrees Celsius or more for a couple of weeks. When the coral-algal symbiosis breaks down during bleaching, the coral’s survival is compromised because it gets up to 90 percent of its food—sugars—from its algae. So a bleached coral is basically starving to death. Some bleached corals can recover when conditions improve, but many die due to the stress. Coral reefs form when individual corals, called “polyps,” secrete calcium carbonate skeletons, so losing the corals means you basically lose the entire habitat. It would be the ocean’s equivalent of a rainforest with no trees. Coral reefs are important habitats throughout the tropics. Economically, they provide hundreds of millions of tourism dollars to communities worldwide, they sustain key fisheries and protect coastlines from storms, and their biodiversity is the source of hundreds of biopharmaceuticals.

What exactly is El Niño, and how does it contribute to coral bleaching?

El Niño was first described as a seawater warming event along the equator in the eastern to central Pacific. It is actually just one of three phases of a climate phenomenon called El Niño Southern Oscillation, or ENSO. We now know that El Niño impacts air and seawater temperatures as well as rainfall globally. During severe El Niño events like the one this year, as well as those in 1982-83 and 1997-98, we typically see much higher numbers of coral-bleaching events

Widespread bleaching has taken place before. Why is the situation now different, and what does it mean that the Great Barrier Reef is being affected?

Coral-reef scientists predicted that this severe El Niño was going to cause bleaching globally. Bleaching reports spanning the Caribbean, Pacific, and Indian oceans have been coming in steadily over the past year. My Three Seas class and I observed significant bleaching in our study sites on the Caribbean side of Panama this January. But the reports from marine biologist Terry Hughes’ group at Australia’s James Cook University documenting severe bleaching across the northern Great Barrier Reef caught many of us off guard because it is one of the world’s biggest and best reef ecosystems. We now are better able to predict severe El Niño events, and so more of us are looking for bleaching associated with it. But other factors make this year different as well. One of the most troubling predictions associated with global climate change is that we may be increasing the frequency and severity of El Niño —and thus bleaching—events beyond a point where corals could hope to recover.