Little of Australia’s Great Barrier Reef has now been left unscathed by coral bleaching.
A recent aerial survey by scientists at the ARC Centre of Excellence for Coral Reef Studies has found the phenomenon leaving vast stretches of corals bone-white for the second year in a row.
A global bleaching event in 2016 saw an average of 67 percent of corals lost in the north of the reef, and scientists hoped 2017 would offer the delicate ecosystem a reprieve. The survey showed the opposite.
In 2017, scientists found the middle reef worst hit, leaving only its southern reaches untouched. This means two thirds of the reef’s corals have now been impacted by coral bleaching in a zone stretching for 1,500 km (900 miles).
Bleaching occurs when coral is exposed to stresses such as increased water temperature, causing it to expel the colour and nutrient-giving algae that lives in its tissue. This leaves the skeleton exposed and vulnerable, making the coral more susceptible to heat, disease and pollution.
The 2016 coral bleaching event was caused by hotter water temperatures due to man-caused global warming and a strong 2015-2016 El Niño. The 2017 bleaching has occurred without a similar El Niño effect, but in the opinion of Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies, the link between coral bleaching and El Niño has been “overstated.”
“Bleaching occurs because of extreme water temperatures, which can occur with or without El Niño,” he said. Only two of the reef’s four recorded bleaching events have occurred in El Niño years, in 1998 and 2016.
“It really comes down to the local temperatures,” he explained. “In the east coast of Australia, we’ve had a warm winter and a hot summer — March was record breaking along the east coast.”
There was some hope Cyclone Debbie would bring cooler waters to the reef as it arrived in northern Queensland in March. Ultimately, it came too late and with too much destructive force.
“It wasn’t far north enough to prevent the bleaching, it was a month too late and it was far too damaging — you have to be careful what you wish for,” Hughes said.
Coral can recover from bleaching if it’s not exposed to further stress. But according to Hughes, the “tragedy” of back to back bleaching is that it leaves no time for coral to cover. Typically, it takes a decade for a reef to regain its health and replace coral that has died.
“We’re already seeing heavy losses in the central sections,” he said. “Over the next few months, the corals will either regain their colour and survive, or they won’t and they’ll die.”
Marine scientist Peter Steinberg, who was not involved with the survey, said scientists weren’t expecting year-on-year bleaching events of this severity.
Director of the Centre for Marine Bio-Innovation at the University of New South Wales, he called the results “unfortunate and surprising.” He said “It’s a significant blow to the health of the reef — we haven’t had two bleaching events follow one another, combined with the cyclone’s destruction.
“Coral cover on the Great Barrier Reef has declined by, on average, 50 percent or more over the last 20 to 25 years, and it’s now been hit by these two events and a cyclone. The news is not good.”
Scientists at the ARC Centre of Excellence for Coral Reef Studies published the results of an earlier survey in Nature in March, finding the 2016 bleaching was “unprecedented in recorded history.” In their view, only fighting climate change can now save the World Heritage-listed site.
They argued local protection measures such as fishing bans and ending agricultural runoff, while important, will ultimately fail to save the reef.
“The point is not that addressing water quality is useless, but that it doesn’t protect us from the negative effects of climate change,” Sean Connolly, a professor at the ARC Centre of Excellence for Coral Reef Studies, said at the time.
In a statement, Greenpeace campaigner Sebastien Blavier warned the government’s commitment to opening Queensland’s Carmichael coal mine — set to be among the world’s biggest — would devastate the ecosystem and ultimately, the local communities that rely on reef tourism, among other industries.
“Almost 70,000 people rely on the Reef for their livelihoods, and the Reef is now in danger thanks to our Government’s inaction on climate change,” he said. “The Government must take action on the root cause of coral bleaching – and that is climate change, fueled by mining and burning fossil fuels like coal.”
For Steinberg, the question is now what impact this second bleaching will have on coral mortality. “I wouldn’t use words like ‘terminal’ or ‘doomed,’ but the reef is under significant threat,” he said.
The back-to-back bleaching is also challenging the ability of scientists to measure and analyse its impact, Hughes said.
“The interval between [bleaching events] seem to be getting shorter and the temperatures are getting warmer,” he added. “We’re worried that the window of opportunity to decrease green house gases and save the Great Barrier Reef is closing.
“I don’t want to even think about that.”