Global warming impacts right now are beyond some of the worst scientific predictions, so what does that mean for aspirations to save the Great Barrier Reef?
On 6 July 2009, Australian Dr Charlie Veron — who has discovered, described and identified about a third of all known coral species — addressed the Royal Society in London and asked: “Is the Great Barrier Reef on death row?” His response: “The answer must be yes… a close look at this question from any rational perspective arrives at the same bottom line: the Great Barrier Reef can indeed be utterly destroyed, and this could easily happen in the lifetime of my children.”
It is a devastating answer because corals have been around for almost 500 million years and have formed more fossils than any other species, they are home to one-quarter of marine fish species, and tens of millions of people depend on reef ecosystems for protein and other services. The Great Barrier Reef (GBR) is home to 600 different types of corals, and is more biodiverse that any other UNESCO World Heritage site.
Now, in 2016, we have seen why Charlie Veron was so worried, with mass coral bleaching unprecedented in scale and severity occurring across the Reef in March. It was driven by a 1°C rise in sea temperatures compared to the recent average (2002-2011). Worryingly, there’s little sign that those sea temperatures are returning to normal in the short run, so more damage may still be occurring.
Of 911 reefs surveyed by scientists since the bleaching, 500 were severely bleached. Of the 522 reefs surveyed in the more pristine and isolated northern sector (stretching 1000 kms from Port Douglas north to Torres Strait), 81% were severely bleached. Scientists report that: “North of Port Douglas, we’re already measuring an average of close to 50% mortality of bleached corals. At some reefs, the final death toll is likely to exceed 90%. 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.”
Coral polyps are invertebrates similar to minute jelly fish, which build limestone (calcium carbonate) structures, and live in a symbiotic relationship with algae-like unicellular zooxanthellae that reside within the coral structure, and give it colour.
The coral provides the algae with a protected environment and compounds they need for photosynthesis. In return, the algae produce oxygen and help the coral to remove wastes. Most importantly, zooxanthellae supply the coral with glucose, glycerol, and amino acids, which are the products of photosynthesis.
Corals survive within a narrow water temperature band, and suffer heat stress and expel zooxanthellae if the ocean temperature get too high. Bleaching events vary intensity. In the extreme case, all zooxanthellae are expelled and the living colony will appear totally white (hence “bleaching”). As elevated sea temperatures persist, coral mortality rates increase.
Charlie Veron explains:
Coral mortality means the corals have expelled all zooxanthellae, and starve to death, which may take months. Corals may recover, if there are any zooxanthellae left in their tissues, but if not death appears to be inevitable.
Put bluntly, more than half the corals north from Port Douglas have already been identified as dead, and as observations continue over the next few months that figure is likely to rise sharply. Climate change has just killed most corals in the Great Barrier Reef’s highest-value section.
Coral expert Prof. Ove Hoegh-Guldberg says that “An increase of as little as 1-2°C on top of regular summer temperatures can mean the difference between life and death for coral reefs.”
Previously, the most widespread bleaching events on the GBR occurred in the summers of 1998 and 2002, with 42% and 54% respectively of reefs bleached to some extent, and 18% strongly bleached. However coral losses on the Reef between 1995 and 2009 were largely offset by growth of new corals.
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