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The Great Barrier Reef has lost half its corals in just 20 years, study says

The UNESCO World Heritage site has suffered three mass bleaching events in the last five years as the planet continues to warm.
Image: Vlassof Cay in the Great Barrier Reef in Cairns, Australia.
The Great Barrier Reef, off the coast of Queensland, Australia, lost half its coral between 1995 and 2017, a new study has found. Mark Kolbe / Getty Images file

The population of corals within Australia’s Great Barrier Reef has plummeted by 50 percent in the last two decades, according to a new study published on Wednesday.

Researchers from the ARC Centre of Excellence for Coral Reef Studies in Queensland, Australia, assessed the colony size of corals in the reef — the world’s largest — between 1995 and 2017, and found a drastic depletion in the population of small, medium and large coral.

"The decline occurred in both shallow and deeper water, and across virtually all species, but especially in branching and table-shaped corals,” study co-author Professor Terry Hughes said of the findings, published in the Royal Society journal.

These specific corals are especially important in providing a habitat for marine life such as fish that inhabit the reef, the researchers said, meaning their loss also results in a decline in reef biodiversity. Despite covering less than 0.1 percent of the ocean floor, reefs host more than 25 percent of all marine fish species, according to the International Union for the Conservation of Nature, a network of government and civil society organizations.

“These were the worst affected by record-breaking temperatures that triggered mass bleaching in 2016 and 2017," Hughes added.

Image: Coral on Lady Elliot Island on the Great Barrier Reef in Queensland, Australia.
Coral reefs contain remarkable biodiversity, but the vast majority face being wiped out as the planet continues to warm. David Gray / Reuters file

Coral bleaching occurs as a result of the reef experiencing warmer than usual sea temperatures. Climate change has dramatically increased the frequency of these events which inhibits the reef’s ability to recover. A 2019 report by the U.N.'s Intergovernmental Panel on Climate Change warned that reefs can take more than 15 years to recover, yet further mass bleaching — the third event in just five years — occurred again this year.

"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," Hughes said.

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Coral reefs — which harbor the highest biodiversity of any ecosystem on earth — are some of the most vulnerable environments to the climate crisis. The U.N. report warned that the vast majority of all tropical reefs on earth will disappear even if global warming is limited 2.7 degrees Fahrenheit; an aspiration that looks increasingly unlikely to be achieved.

The earth has already warmed over 1.8 degrees above pre-industrial levels with current estimates suggesting that the world is on track for between 3.6 and 4.4 degrees of warming.

"There is no time to lose, we must sharply decrease greenhouse gas emissions as soon as possible," the study authors concluded.