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Climate Change Has Killed Half of the Great Barrier Reef’s Corals

A new study finds corals on the Australian mega-reef declined 50 percent between 1995 and 2017

Fish and corals on Australia's Great Barrier Reef. Between a quarter and a third of all marine species spend some part of their life cycle in coral reefs. (Andreas Dietzel)
smithsonianmag.com

Australia’s Great Barrier Reef, the largest coral reef ecosystem in the world, lost half of its corals between 1995 and 2017, report Myles Houlbrook-Walk and Ollie Wykham for the Australian Broadcasting Corporation (ABC News). The new study, published this week in the journal Proceedings of the Royal Society B, found that nearly every coral species and size found on the Great Barrier Reef had declined, reports Maria Cramer for the New York Times.

Covering nearly 133,000 square miles and hosting more than 1,500 species of fish, 4,000 types of mollusk and 400 species of coral, the Great Barrier Reef is a vital marine habitat and a priceless crown jewel of the world’s ocean ecosystems.

For the new study, researchers with the Australian Research Council’s Center of Excellence for Coral Reef Studies measured changes in coral colony sizes along the Great Barrier Reef’s more than 1,400-mile length between 1995 and 2017.

“We found the number of small, medium and large corals on the Great Barrier Reef has declined by more than 50 percent since the 1990s,” Terry Hughes, a coral researcher at James Cook University and senior author of the study, says in a statement. “The decline occurred in both shallow and deeper water, and across virtually all species—but especially in branching and table-shaped corals.”

Losses among large corals are especially concerning, because reefs rely on these big, mature corals to produce vast numbers of offspring that help perpetuate or revive surrounding reefs. The researchers say their results suggest the Great Barrier Reef’s resilience, or ability to recover from damage, has been severely compromised since the 1990s.

“We used to think the Great Barrier Reef is protected by its sheer size—but our results show that even the world’s largest and relatively well-protected reef system is increasingly compromised and in decline,” says Hughes in the statement.

The staggering loss of corals was driven largely by marine heatwaves in 2016 and 2017—and to a lesser extent, 1998 and 2002—that resulted in massive coral die-offs, per ABC News. Elevated water temperatures stress the corals, and this strain can cause the photosynthetic algae corals rely on for much of their sustenance to either die or eject from their former symbionts. This has the effect of turning the corals bone-white, which is the genesis of the phenomenon’s name: coral bleaching. Without the algae, called zooxanthellae, the corals typically starve to death or succumb to disease.

Human-caused climate change is the primary driver of the rising ocean temperatures that have killed off half of the reef, Andreas Dietzel, a coral researcher at James Cook University and first author of the new research, tells ABC News. "We can clearly correlate the rising temperatures to coral mortality and bleaching on the Great Barrier Reef," he says.

Earth’s rising average temperatures are expected to continue the reef’s decline, Terry Hughes, a coral researcher at James Cook University and senior author of the study, tells Darryl Fears of the Washington Post. “The only effective way to improve the outcome for coral reefs is global action on greenhouse gasses. If global temperatures rise to 3 or 4 [degrees Celsius], the reef will be unrecognizable, so there is no time to lose.”

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