The full and horrifying extent of coral bleaching on the Great Barrier Reef has been revealed by Australian scientists.
Aerial and underwater surveys have shown that only 7 percent of the almost 1500 mile long Reef is unaffected, and climate change is to blame.
“We’ve never seen anything like this scale of bleaching before. In the northern Great Barrier Reef, it’s like 10 cyclones have come ashore all at once,” says Professor Terry Hughes, convenor of the National Coral Bleaching Taskforce that is documenting and studying the event. “Towards the southern end, most of the reefs have minor to moderate bleaching and should soon recover.”
He added: “We have now flown over 911 individual reefs in a helicopter and light plane, to map out the extent and severity of bleaching along the full 2300km length of the Great Barrier Reef. Of all the reefs we surveyed, only 7 percent (68 reefs) have escaped bleaching entirely. At the other end of the spectrum, between 60 and 100 percent of corals are severely bleached on 316 reefs, nearly all in the northern half of the Reef.”
Why does climate change cause bleaching?
Bleaching is due to the demise of zooxanthellae – tiny, colourful marine algae that creates much of the beautiful color in coral reefs, as well as providing them with energy.
Increasingly high sea surface temperatures leads to the death of zooxanthellae, without which coral tissue becomes transparent, revealing the white coral skeleton underneath. Zooxanthellae can return if conditions revert back to something more habitable, but if temperatures continue to increase then the corals can die.
Experts in Australia graded parts of the Reef as having very severe, moderate, or little damage.
“The bleaching is extreme in the 1000km region north of Port Douglas (Cairns in Queensland) all the way up to the northern Torres Strait between Australia and Papua New Guinea,” said Prof. Andrew Baird from the ARC Centre of Excellence for Coral Reef Studies.
“Tragically, this is the most remote part of the Reef, and its remoteness has protected it from most human pressures but not climate change. North of Port Douglas, we’re already measuring an average of close to 50 percent mortality of bleached corals. At some reefs, the final death toll is likely to exceed 90 percent. When bleaching is this severe it affects almost all coral species, including old, slow-growing corals that once lost will take decades or longer to return.”
There have been three mass bleaching events in recent times – in 2016, 2002 and 1998.
“In each case, the location of the most severe bleaching coincides with where the hottest water sits for the longest period,” said Hughes.
“This time, the southern third of the Great Barrier Reef was fortunately cooled down late in summer by a period of cloudy weather caused by ex-cyclone Winston, after it passed over Fiji and came to us as a rain depression. The 2016 footprint could have been much worse,” he added.
The Great Barrier Reef is extremely important for tourism in the region, and Daniel Gschwind, Chief Executive of the Queensland Tourism Industry Council, mixed caution with optimism in saying that: “Thankfully, many parts of the reef are still in excellent shape, but we can’t just ignore coral bleaching and hope for a swift recovery. Short-term development policies have to be weighed up against long-term environmental damage, including impacts on the reef from climate change.”
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