Global warming kills a third of Great Barrier Reef’s corals

Similar mortality expected in other tropical oceans

Dead and dying staghorn co ral , central Great Barrier Reef in May 2016. Credit: Johanna  Leonhardt
Dead and dying staghorn coral, central Great Barrier Reef in May 2016. Photo by Johanna Leonhardt.
Great Barrier Reef mortality map
Map of mortality estimates on coral reefs along 1100km of the Great Barrier Reef. Map courtesy ARC Centre of Excellence for Coral Reef Studies at James Cook University.

Staff Report

For years, scientists have warned that global warming threatens to decimate the world’s coral reefs within our lifetimes and this week, the dire warnings played out in Australia, where new surveys showed that more than a third of the corals along the Great Barrier Reef died in the past few months after an extensive coral bleaching episode.

“We found, on average, that 35 percent of the corals are now dead or dying on 84 reefs that we surveyed along the northern and central sections of the Great Barrier Reef, between Townsville and Papua New Guinea,” said Professor Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University.

Australian scientists have closely tracked the status of reefs along their coastline for the past few months as it became evident that this year’s strong El Niño would raise ocean temperatures above the limit of what most corals species can survive, and the latest survey results confirm their worst fears. In a press release, the researchers said the impacts are still unfolding along the 2,300-long reef, with the worst damage to the central and northern sections.

“Some reefs are in much better shape, especially from Cairns southwards, where the average mortality is estimated at only 5 percent,” Hughes said.

As fish nurseries, coral reefs nurture ocean ecosystems, and can also help protect coastlines from storm surges. Warm water disrupts the relationship between the coral and its symbiotic algae. Reefs can survive a certain level of bleaching, but the intensity and duration of this year’s event is unprecedented.

coral bleaching infographic
Coral bleaching is likely to devastate many of the world’s reefs in the next few decades. Graphic courtesy NOAA.

More serious reef degradation is likely this year in all the globe’s tropical ocean regions. According to warnings from NOAA’s coral reef watch program, up to 95 percent of U.S. corals, including Florida’s and Hawaii’s, will take a hit from this years conditions.

“This year is the third time in 18 years that the Great Barrier Reef has experienced mass bleaching due to global warming, and the current event is much more extreme than we’ve measured before,” Hughes said.  “These three events have all occurred while global temperatures have risen by just 1 degree C above the pre-industrial period. We’re rapidly running out of time to reduce greenhouse gas emissions.”

“Fortunately, on reefs south of Cairns, our underwater surveys are also revealing that more than 95 percent of the corals have survived, and we expect these more mildly bleached corals to regain their normal colour over the next few months,” said Dr. Mia Hoogenboom, also from JCU.

Although substantially fewer corals have died to the south, the stress from bleaching is likely to temporarily slow down their reproduction and growth rates. According to the scientists, the reefs further south have escaped damage because water temperatures there, were closer to the normal summer conditions.

“It is critically important now to bolster the resilience of the Reef, and to maximise its natural capacity to recover,” says Professor John Pandolfi from the ARC Centre of Excellence for Coral Reef Studies at The University of Queensland. “But the reef is no longer as resilient as it once was, and it’s struggling to cope with three bleaching events in just 18 years. Many coastal reefs in particular are now severely degraded,” he said.

“In Western Australia, bleaching and mortality is also extensive and patchy,” said Dr, Verena Schoepf from The University of Western Australia. “On the Kimberley coast where I work, up to 80 percent of the corals are severely bleached, and at least 15 percent have died already.

The researchers plan to re-visit the same reefs over coming months to measure the final loss of corals from bleaching. The recovery of coral cover is expected to take a decade or longer, but it will take much longer to regain the largest and oldest corals that have died.

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