This story has been updated.
This year, we’ve seen alarming bleaching of the Great Barrier Reef, caused by warm sea temperatures. A recently completed aerial survey of the reef found that 93 percent of the smaller reefs that comprise it showed at least some bleaching, and in the northern sector of the reef, the large majority of reefs saw bleaching that was severe — meaning many of these corals could die.
There was already considerable murmuring that this event, which damages a famous World Heritage site and could deal a blow to a highly valuable tourism industry, did not simply happen by chance. And now, a near real-time analysis by a group of Australian climate and coral reef researchers has affirmed that the extremely warm March sea temperatures in the Coral Sea, which are responsible for the event, were hardly “natural.”
“Human caused climate change made the extreme ocean temperatures that led to the massive bleaching events along the Great Barrier Reef this year at least 175 times more likely,” finds the analysis, which was led by Andrew King, a researcher studying climate extremes at the University of Melbourne.
King and his four colleagues freely confess that their analysis has not yet been peer-reviewed — science doesn’t move that fast — and admit to adopting the “unusual approach of releasing the results before publication.” But they defend the move in light of the situation.
“Because we have confidence in the methods, the methods have been peer reviewed, and because the results are so strong, we decided we needed to release them almost immediately. It’s important for the public to know that climate change is making these bleaching events far more likely,” said King in an interview.
Indeed, when it comes to the overall methods, we have reported previously on peer reviewed research conducted by King and a colleague, demonstrating how climate change greatly upped the odds of record breaking Australian temperatures in recent years. The new research follows a similar methodology, King said.
The researchers have also posted their data and methods online. King added in an interview that the researchers were sure to include a coral reef expert, Ove Hoegh-Guldberg, who directs the Global Change Institute at the University of Queensland, in the analysis.
The situation itself is rather extraordinary in a scientific sense — we know the extent of the bleaching event itself due to an aerial survey that scientists released online, also in near real time.
At the heart of the new analysis is the warm March sea temperatures of the Coral Sea region, which the researchers say set a new record for that month, at more than 1 degree Celsius above the long-term average. The scientists then proceeded to conduct what is called an “attribution” study, which involves running large numbers of climate model iterations, with and without the human global warming influence included, to determine how likely such a warm departure is to occur in each set of model runs.
The result was that the warm Coral Sea extremes were far more likely in the runs that included human-induced climate change than in those that did not. This is where the statement that the event is 175 times more likely to occur due to climate change comes from.
“The likelihood of bleaching in the normal climate is just under .1 percent, a one in a thousand year event, roughly,” said King. “But in the modern world, it’s much more likely.” The scientists also wrote about the analysis at the website The Conversation, where they made a similar point, noting that “Surface temperatures like those in March 2016 would be extremely unlikely to occur in a world without humans.”
Reacting to the research in an interview with Climate Central, David Kline, a researcher who studies corals at the Scripps Institution of Oceanography but was not involved with the study, similarly commented, “This is the smoking gun.”
The researchers also noted that they did not detect much of a role of the recent strong El Nino event in March 2016’s warm Coral Sea temperatures. Rather, they say human-caused global warming is what set the stage. Granted, in other parts of the world, that may not be the case, notes Kim Cobb, a coral reef expert whose research has focused on Christmas Island in the central Pacific, which saw massive coral death this year.
“There is no doubt that anthropogenic climate change played a key role in the coral bleaching and mortality epidemic this year,” says Cobb. “Ocean temperatures have been warming around the world, bringing reefs closer to the bleaching threshold even in normal years. But this year’s record-breaking El Nino was the nail in the coffin for many of the world’s coral reefs, including my remote research site.”
The new Great Barrier Reef analysis also looks forward in time — and the analysis, unfortunately, gets even worse. The study also found that while March was scorchingly hot in the Coral Sea and well out of the ordinary, these kinds of temperatures will become, more and more, the norm. Warm temperatures like those of March would tend to occur every other year by 2034, the analysis found.
“The future of coral dominated reef systems like the Great Barrier Reef is now in serious doubt,” says a press statement about the work.
So what do other researchers think of conducting a real-time attribution study like this one? The Post gathered a number of responses from researchers familiar with the kinds of scientific questions involved.
Heidi Cullen, the chief scientist at Climate Central and a researcher who has favored this approach, strongly supported the analysis in a statement.
“The methods used in this analysis have been peer reviewed and are completely defensible,” Cullen said by email. “Our understanding of the physical mechanism — the connection between higher ocean temperature and coral bleaching — is solid. I wish this result was not so devastating. But we must face the facts.”
Adam Sobel, a climate researcher at the Lamont-Doherty Earth Observatory at Columbia University, expressed a similar view.
“It should go through peer review as they say, but they’re right that the method is essentially the same as that in many previous studies and so it doesn’t seem out of line to put this out there provisionally now,” he commented. “When we see a disaster unfolding in real time — and this is a disaster of the highest magnitude — and there’s good evidence that it’s largely a result of the human influence on climate, I think it’s appropriate to put that evidence before the public.”
David Titley of Penn State University, who led a recent National Academies report on extreme event attribution — which said that we can now say, with at least some events, that climate change made them more likely to occur — was somewhat more circumspect, however. “Regardless of peer review, the massive bleaching of the corals is tragically consistent with an ever-warming planet,” he said.
However, when it comes to attributing extreme events to climate change, Titley noted that scientists are most confident when it comes to air temperature anomalies, like heat waves. The current event is, in some ways, an oceanic analogue to that, but “it’s not clear to me how much has been previously published on attributing specific extreme ocean heat events,” Titley said.
“Unless there’s a body of literature for ocean warming events similar to land-based heat events, it seems that going through peer-review would be the appropriate path forward for a scientific paper,” he added.
We also contacted Noah Diffenbaugh of Stanford University, who has published on attribution research and is familiar with the methods used by King and his colleagues. Indeed, Diffenbaugh said he is working on a similar analysis of the anomalously warm temperatures afflicting the Great Barrier Reef, though it is not ready for publication yet.
Still, he doesn’t question the overall approach or results. “That the threshold would have been much more hard to reach without global warming, that this particular event would have been much less severe without global warming, that continued global warming on the scale of what the U.N. has identified in the 2 degree target, would make this event the normal, rather than the extreme, all of those I think are, I completely agree with,” Diffenbaugh said. “And I think they’re completely scientifically credible conclusions, and I applaud this group for pushing the envelope, not only in doing the real time attribution, but putting out a really clear analysis of what they’ve done.”
He added, though, that as more analyses like this happen in real time, there are many reasons that researchers may come up with different results, based on slightly different methods or, simply, the problem of scientific uncertainty — and that scientists may need to better coordinate their real-time attribution work.
“Different peer reviewed methods analyzing the same event will come up with different values,” Diffenbaugh cautioned.
According to King, this sort of real-time attribution of major climate events is going to be more common going forward. “A lot of operational forecasting centers are looking to do real-time attribution, because whenever there’s an extreme event, they’re always asked, is it climate change,” he said. “It’s good to be able to give an answer almost immediately. It’s an evolving area of science.”