Coral bleaching is spreading south along the reef and can no longer be blamed solely on El Nino - scientists say the bleaching is linked to global warming. A recent aerial survey demonstrates the extent of the damage.
Australia's Great Barrier Reef, already reeling from a severe bleaching event last year, has suffered another devastating blow this year, aerial surveys released this weekend reveal.
Last year the reef, the largest living structure on Earth stretching over an area the size of Italy, saw the largest coral die-off ever recorded. In the reef's Northern region, 67 percent of shallow-water corals were affected. This year's bleaching impacted an area the same size, but further South. The southern creep is sparking fears that the corals are now bleaching at a rate from which they cannot recover.
Mark Read, manager of operations support at the Australian government's Marine Park Authority tasked with preserving the reef, says that such back-to-back bleaching incidents are extremely rare.
"In a normal year you get an accumulation of heat in the waters of the Great Barrier Reef in summer time, and it usually dissipates as we move into winter," he told DW.
"What we saw this year was that there wasn't an appreciable decline in water temperature from 2016. The water just continued to heat up. If the heat accumulates for a short time the corals can withstand that. But it was an incredibly hot summer here in Eastern Australia."
The loss of the reef could cost the region one million visitors a year - or a loss of 1 billion Australian dollars (700 million euros; $750 million) in tourism revenue, the Australia-based environmental organization Climate Council said Wednesday.
No time to recover
Bleaching occurs when the water becomes too warm, causing coral to expel the algae that gives it its vibrant color. The result is that the coral takes on a white color, and is vulnerable to starvation and disease. It can recover, but it needs time and it needs healthy coral around it. Back-to-back bleaching events may allow little prospect for this recovery.
Before last year, severe bleaching had not been observed at the reef since 2002. Unlike last year's event, this year the record temperatures cannot be partly blamed on El Nino, a weather phenomenon which periodically brings warm water to the Pacific.
The aerial surveys were taken last month by the ARC Centre of Excellence for Coral Reef Studies. According to the center the combined impact of this year's bleaching stretches for 1,500 km (900 miles), leaving only the southern third of the reef unscathed. The center says this year's water temperatures can only be explained by global warming.
"It takes at least a decade for a full recovery of even the fastest growing corals, so mass bleaching events 12 months apart offers zero prospect of recovery for reefs that were damaged in 2016," says James Kerry, a scientist with the center who undertook the aerial surveys.
Though the surveys found little damage in the reef's southern section, they were conducted before the area was hit by a massive cyclone on 28 March. Sandra Williams, a campaigner with the Australian Marine Conservation Society who lives in the reef's southern section at Airlie Beach, says the damage from Cyclone Debbie is still not known.
"I've been in six cyclones, but this one differed as it simply lasted so long - virtually 24 hours of ferocious winds," she told DW. "All the old timers agree - we've never seen a storm like Debbie before."
"There is serious damage to our inshore reefs, with some sites simply destroyed," she added. "As we've had rough weather since the storm, not a lot of surveys have been done, so it's not yet known exactly how much of the reef has been killed or affected. We'll have more information in the coming weeks."
Read agrees that the cyclone did not help the situation. But he says that the rarity of storms during this year's Australian summer may have been even more harmful.
"Storm events are a mixed blessing, and this year we didn't have the frequency that we often get," he said. "They can be destructive like a tropical cyclone, but if it's just a normal storm they have benefits because the cloud cover, rain and wind all act to cool the waters down. We didn't get any of that respite this year at all."
Reef: 'I'm not dead yet'
Read says the authority wants to avoid this year overblown media reports claiming that the reef is "already dead," as was seen last year. Such reports can give the public the impression that there's no point in working to help the reef, because the damage is permanent.
"Bleached corals don't necessarily mean dead corals," he said. "It's really not helpful to take a simplified view. It's more important to emphasize the scope and scale of the reef and its capacity to bounce back."
"Even in the areas where the bleaching was severe, there's still a high degree of patchiness there. We need to protect the reefs that aren't as significantly damaged as others, so they can reproduce and help the damaged areas."
The Great Barrier Reef Marine Park Authority is working with tour boats to monitor the reef and protect the vulnerable areas. "We're telling local tourism operators to turn over any corals that may have been flipped during the cyclone," he said. "It's about moving down in local area management responses so that you can make a difference - identifying which areas of the reef are strong and protecting them."
The authority is also undertaking a large project to tackle the current outbreak of crown-of-thorns starfish, an invasive species which causes damage to coral. They are also engaging in coral re-seeding and assisted evolution to help speed recovery.
But recovery can only go so fast. If the area were to be hit by another severe bleaching event next year, the reef could be in serious trouble.
The story has been updated to add the assessment of environmental organization Climate Council on Wednesday.