The coral bleaching that has devastated Australia's iconic Great Barrier Reef — and been described as the most extreme event in the reef's history — would have been almost impossible without human influence, according to a new study.
A recent aerial survey found that 93 percent of the smaller coral reefs that make up the Great Barrier Reef showed signs of bleaching, which is due to abnormally warm ocean temperatures.
A group of Australian researchers say the record-setting bleaching event was made 175 times more likely by human-caused climate change. Essentially, without all of our pollution-spewing smokestacks and automobile emissions, the Great Barrier Reef would probably have remained the teeming, colorful ecosystem that's made it a Unesco World Heritage Site and a premier tourist destination.
The researchers found that record-warm sea surface temperatures in March in the Coral Sea, which caused the bleaching, were "dramatically increased" by the effects of climate change.
The results were so concerning that the scientists chose the unusual step of releasing their findings without peer review in order to alert the public to the fact that climate change was the primary cause.
"The methods have been peer reviewed, so we have high confidence in the methods and fairly high confidence in the results as well," said Dr. Andrew King, one of the study's authors and a climate extremes research fellow at the University of Melbourne.
"We thought it was important that we got those results out there so the public is aware of the fact that climate change is the major driver of these bleaching events," he said.
The preliminary findings released last week are the first attempt to quantify the impact of climate change on the reef.
In order to assess how climate change was altering the likelihood of bleaching, the scientists looked at the impact of atmospheric warming on ocean temperatures at — or close to — the surface. Researchers compared computer simulations of Earth's climate that included the influence of humans and omitted any human-generated greenhouse gases.
"We looked at the likelihood of hot March sea surface temperatures in those two different worlds and we found a very big increase in likelihood in the world with humans — at least a 175 fold increase in likelihood," King said.
King and his team estimated that human-generated greenhouse gases increased March temperatures — the hottest on record — by just over 1 degree Celsius.
"So without humans it would have been roughly a degree Celsius cooler in that region," King said. "That would have meant there wouldn't have been the conditions for bleaching."
The study also found that the impact of El Niño was minimal.
"Overall, the influence is quite weak," King said.
More worryingly, the researchers's modeling simulations estimated that temperatures like those in March could become the new normal by 2034.
Coral is a symbiotic organism that host a type of algae called"zooxanthellae, which provides coral with its colorful appearance. But under heat stress, coral expels the algae, causing it to turn white. Mildly bleached coral reefs can recover if temperatures cool and the algae that inhabits them returns. But if bleaching events become more frequent, more severe, as the researchers project, the reef's ability to recover is seriously compromised.
"Particularly in the northern parts of the reef, it means it is going to be damaged and not be able to repair itself," King said. "In the southern parts of the reef, it might not be so bad. There is potentially more time to try and save that portion of the reef by reducing greenhouse gases."
The study's authors say their results are consistent with previous investigations that have looked at the likelihood of bleaching under climate change.
Associate Professor Jason P. Evans, a senior research fellow at the Climate Change Research Centre at the University of New South Wales, said the methods used by the scientists were among the best currently used by researchers today and the results were likely "robust."
Professor Terry Hughes, the leader of Australia's Coral Bleaching Task Force, said the results of the study were not particularly surprising. He said it was more evidence to show Australia had a narrowing window of opportunity to deal with greenhouse gas emissions.
"The Barrier Reef isn't dead, but it's certainly impacted by these increasing bleaching events," he said. "We need to make the connection between policies around developing new coal mines and coral bleaching."
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