Great Barrier Reef: Can Iconic Coral System Survive After Heat Wave Kills One-Third?

Enormous and unique, the iconic Great Barrier Reef is an ecosystem hosting nearly 3,900 coral reefs. The vibrant corals sprout near the coast of Australia across a stretch of about 133,000 square miles—the largest such system in the world.

Home to hundreds of diverse animals—fish, turtles, birds and even crocodiles—the reef is recognized as a UNESCO World Heritage Site. But, ravaged by increasing water temperatures, the system’s corals are dying in droves. An extended heat wave in 2016, researchers report in the journal Nature, has killed nearly a third of the Great Barrier Reef's corals.

Water warmer than should be can bleach and even kill coral polyps—the tiny invertebrate creatures that make up the sprawling mounds and branches of reefs. They shed the vibrant algae that gives them their colors and, as their algae dwindles, corals fade until they're completely white.

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"When corals bleach from a heat wave, they can either survive and regain their color slowly, as the temperature drops, or they can die,” Terry Hughes, study author and director of the Australian Research Council Centre of Excellence for Coral Reef Studies (Coral CoE), said in a statement. “Averaged across the whole Great Barrier Reef, we lost 30 percent of the corals in the nine month period between March and November 2016.”

Researchers used satellites to track heat exposure across the 1,400-mile length of the Great Barrier Reef after an extreme marine heat wave struck the area in 2016. They saw that 29 percent of individual reefs had lost two-thirds or more of their coral, with the northern third most affected. The team linked this wave of death to heat exposure and bleaching.

4_19_Great Barrier Reef A Hawksbill sea turtle swims near Lady Elliot Island, a resort in the Great Barrier Reef Marine Park, in Australia, on January 14, 2012. Home to hundreds of diverse animals—fish, turtles, birds and even crocodiles—the reef is recognized as a UNESCO World Heritage Site. Mark Kolbe/Getty Images

The 2016 heat wave was not an isolated event. Rising seawater temperatures are an ongoing threat to coral reefs around the world. This wave was part of “a global heat and coral bleaching event spanning 2014 to 2017,” co-author Mark Eakin of the U.S. National Oceanic and Atmospheric Administration explained. Further heat stress and bleaching hit the Great Barrier Reef again in 2017, this time across its central region.

The events have radically changed the cocktail of coral species living on hundreds of reefs within the greater system, said Andrew Baird, another study author from Coral CoE at Australia’s James Cook University. “Mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining," he said.

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"We're now at a point where we've lost close to half of the corals in shallow-water habitats across the northern two-thirds of the Great Barrier Reef due to back-to-back bleaching over two consecutive years," study author Sean Connolly of Coral CoE at James Cook University added.

But, in the face of this widespread devastation, Hughes is hopeful that human action can protect the billion or so remaining coral at the Great Barrier Reef. The corals that survived are “tougher,” he said, than those that perished. “We need to focus urgently on protecting the glass that’s still half full, by helping these survivors to recover,” he added.

Corals aren’t just beautiful—they are a key part of an ecological system that supports both animals and humans. Understanding how temperature rises of more than 2 degrees above pre-industrial levels is crucial to protecting these diverse ecosystems, the researchers say.

The collapse of coral reef populations will have significant implications for marine life and the humans who rely on tropical reef ecosystems for fish for food. The hundreds of millions of people who benefit from coral reefs, the authors say, mostly live in poorer, rapidly developing communities.

"The Great Barrier Reef is certainly threatened by climate change, but it is not doomed if we deal very quickly with greenhouse gas emissions. Our study shows that coral reefs are already shifting radically in response to unprecedented heat waves," said Hughes.