Australia's Great Barrier Reef suffered a "catastrophic die-off" of coral during an extended heatwave in 2016, threatening a broader range of reef life than previously feared, a new report has revealed.
Scientists said around 30% of the reef's coral died in the heatwave from March to November 2016, the first of an unprecedented two successive years of coral bleaching along the 2,300km reef off Australia's north-eastern coast.
The study published in the journal Nature found that corals, which serve as habitats for other creatures, were particularly hard hit by the rising sea temperatures caused by global warming.
Coral reefs make up less than 1% of Earth's marine environment, but are home to an estimated 25% of ocean life, acting as nurseries for many species of fish.
Professor Terry Hughes, a report co-author and head of the Centre of Excellence for Coral Reef Studies at Australia's James Cook University, said the most susceptible to heat exposure were branching corals - table-shaped creatures that provide nooks and crannies for fish nurseries and fisheries.
Corals that were more likely to survive the warmer sea temperatures were smooth and melon-shaped, he said. While they were good calcifiers that add volume to reefs, they were "not very useful as habitable providers".
"So there is a shift in the mix of species and the overall loss of corals has a broader impact on all the creatures that depend on the corals for food and habitats," Prof Hughes said.
His research centre said the new study showed that corals on the northern Great Barrier Reef "experienced a catastrophic die-off" following the 2016 heatwave.
"The coral die-off has caused radical changes in the mix of coral species on hundreds of individual reefs, where mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining," said Andrew Baird, another of the report's authors.
The scientists said the focus should be on protecting the surviving corals, which number about a billion after the two bleaching events in 2016 and 2017.
"They are the ones that are going to refeed and repopulate an altered reef into the future," Prof Hughes said, adding that one way to maintain their health was through improving water quality by reducing coastal pollution.
Efforts to curb climate change, such as through the Paris Agreement to cap global warming at under 2.0 degrees below pre-industrial levels, were also critical.
"We've now seen four bleaching events (1998, 2002, 2016 and 2017) on the Great Barrier Reef with one degree of global average warming," he said.
"We are on a pathway where we are committed to a different Barrier Reef. If we continue with business-as-usual emissions, then I don't think the reef can survive."