'Rather startling': Study finds rapid increase in frequency of coral bleaching

Peter Hannam Sydney Morning Herald 5 Jan 18;

The frequency of severe coral bleaching events has increased fivefold in four decades because of climate change, a pace already exceeding the time needed for some species to recover, a new global study has found.

Of 100 reefs examined worldwide, just six have escaped severe bleaching since 1980.

The bleaching, which was initially restricted to years with El Nino events in the Pacific, can now occur in any year. It may become an annual event "in coming decades", according to the study, published in the journal Science on Friday.

"Before 1982-83 [a year with a strong El Nino], there was no bleaching on a regional or global scale," said Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, and the paper's lead author.

"In more recent times, we're seeing them even in La Nina years when it's slightly cooler on a global scale," Professor Hughes said. "La Ninas in the tropics today are warmer than El Ninos were 40 years ago."

When sustained heat stress exceeds certain thresholds, corals typically expel algae - zooxanthellae​ - that provide most of their energy and their often spectacular colours.

An unprecedented marine heatwave over much of the Great Barrier Reef - spanning the summers of 2015-16 and 2016-17 - caused the death of as much as half the corals of the World Heritage-listed natural treasure.

Before the 1980s, bleaching was recorded only a local scale. Since then, the scale of such events has increased to the regional or even global level, with the return time of severe bleaching shrinking from every 25-30 years to once every 5.9 years by 2016, the researchers found.

"Our analysis indicates that we are already approaching a scenario in which every hot summer, with or without an El Nino, has the potential to cause bleaching and mortality at a regional scale," the paper said.

"The rate of the increase was rather startling," said Mark Eakin, coordinator of the Coral Reef Watch program at the US's National Oceanic and Atmospheric Administration and another author of the report. "We've moved into a complete new regime."

Further warming of the oceans between now and 2050 is "already baked in by the actions we're already taken" in terms of pumping out heat-trapping carbon emissions, Dr Eakin said.

Global temperatures have risen about 1 degree since the industrial era began. The Paris climate accord signed by almost 200 nations two years ago aims to keep further warming to between 0.5 and 1 degrees.

"One degree of warming so far has made coral reefs uncomfortable globally," Professor Hughes sai. "Two [degrees] will be bearable, but much above that and we'll see irreparable damage."

Faster growing coral species typically need 10-15 years to recover, with others taking much longer.

"The susceptible ones tend to be the more three-dimensional, table-like corals, that are very important for providing the nooks and crannies for fish and other creatures," Professor Hughes said. "So there are certainly ecosystem-wide effects of losing the corals."

The tougher species tend to be dome shaped. While important for the accretion and growth of the reefs overall, they tend not to be so good for sheltering juvenile or adult fish, he said.

"We'll have a reef in the future if we stop extreme global warming but the mix of species will continue to change, Professor Hughes said.

Southern reef outlook
Sea-surface temperatures off northern Australia are closer to average this year, helped by the presence of the weak La Nina in the Pacific.

Still, the latest Coral Reef Watch from the US National Oceanic and Atmospheric Administration points to thermal stress further south, such as off south-eastern Australia.

Samantha Goyen, a PhD student at the University of Technology, said she will be watching for any return of coral bleaching to Sydney Harbour.

The harbour's corals bleached for the first time on record in 2016, but recovered quickly once temperatures dropped back to normal.

"How they can cope with recurring warmer than average years is unknown at this stage but as we have already shown there is a stress response when temperature exceeds the normal summer maximum by a couple of degrees," Ms Goyen said.

Further south, corals off the Victoria coast are yet to record impacts from warming seas, said Steffan Howe, the Marine Science manager at Parks Victoria.

"We haven't observed any bleaching of those species yet, but there are a whole heap of other impacts," Dr Howe said, citing the spread southwards of damaging species such as black spined urchins and gloomy octupuses as the East Australian Current strengthens with climate change.

"Maybe like the corals on the Great Barrier Reef, they could be vulnerable to a sustained increase in temperatures," he said.

Coral reef bleaching 'the new normal' and a fatal threat to ecosystems
Study of 100 tropical reef locations finds time between bleaching events has shrunk and is too short for full recovery
Helen Davidson The Guardian 4 Jan 18;

Repeated large-scale coral bleaching events are the new normal thanks to global warming, a team of international scientists has found.

In a study published in the journal Science, the researchers revealed a “dramatic shortening” of the time between bleaching events was “threatening the future existence of these iconic ecosystems and the livelihoods of many millions of people”.

The study examined 100 tropical reef locations across the world, analysing existing data on coral bleaching events as well as new field research conducted on the Great Barrier Reef after the longest and worst case of bleaching caused by climate change killed almost 25% of the coral.

“Before the 1980s, mass bleaching of corals was unheard of, even during strong El Niño conditions,” said lead author Prof Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies. “Now repeated bouts of regional-scale bleaching and mass mortality of corals has become the new normal around the world as temperatures continue to rise.”

The study found that time between bleaching events had diminished five-fold in the past 30 to 40 years, and was now too short to allow for a full recovery and was approaching unsustainable levels.

While mass bleaching events used to occur about once every 27 years, by 2016 the median time between them had shrunk to 5.9 years. Only six of the 100 sites had escaped bleaching.

“Our analysis indicates that we are already approaching a scenario in which every hot summer, with or without an El Niño event, has the potential to cause bleaching and mortality at a regional scale,” the paper said.

Globally, the annual risk of severe and moderate bleaching had increased by almost 4% a year since the 1980s, from an expected 8% of locations to 31% in 2016.

The Western Atlantic remained at highest risk but Australasia and the Middle East saw the strongest increases in risk of bleaching.

Hughes said he hoped the “stark results” would prompt stronger action on reducing greenhouse gases. In May scientists warned that the central goal of the Australian government’s protection plan was no longer feasible because of the dramatic impact of climate change.

Friday’s paper also determined the link between El Niño and mass bleaching events has diminished as global warming continues.

Prior to the 1980s mass coral bleaching on a regional scale was “exceedingly rare or absent” and occurred in localised areas stretching tens of kilometres, not the hundreds of kilometres affected in recent times, the paper said.

These local bleaching events were largely caused by small-scale stressors like unusually hot or cold weather, freshwater inundation or sedimentation.

Then global warming increased the thermal stress of strong El Niño events, the paper said, widening the impact of individual bleaching events. Now, they are occurring at any time.

“Back in the 80s it was only during El Niño events that waters became hot enough to damage corals and induce them to bleach,” co-author Andrew Baird, a professor at James Cook University, told Guardian Australia.

“But now it’s 30, 40 years later and we’re seeing those temperatures in normal years.”

Baird said it was difficult to know if the current conditions were reversible but “the window to address it is diminishing”.

“It’s impossible to know if this is the end of coral reefs but it might be,” he said. “We really need to get on top of climate change as soon as possible.”

There have been several large-scale and devastating mass bleaching events in recent years. The 2015-16 event affected 75% of the reefs studied by the researchers, who said it was comparable to the then unprecedented mass bleaching of 1997-98, when 74% were affected.

“Interestingly one of the first papers that effectively drew attention to the issue – back in 1999 – suggested that by 2016, 2017, 2020, we would be seeing bleaching annually,” Baird said. “That’s pretty close to what’s happening unfortunately.

“Some of these earlier works were quite prescient in their prediction and unfortunately we didn’t pay enough attention back then.”

The study follows a discovery late last year that 3% of the Great Barrier Reef could facilitate recovery after bleaching – a finding the researchers at the time suggested was akin to a life-support system but small enough not to be taken for granted.