Have tipping points already been passed for critical climate systems? (5) Coral Reefs: A death spiral
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| Great Barrier Reef bleaching 2016 |
Ecosystems, including coral reefs, mangroves and kelp forests in Australia, are degrading fast as the world’s sixth mass extinction gathers pace.Coral polyps are invertebrates similar to minute jellyfish, which build limestone 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, zooxanthellae supply the coral with oxygen, 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 gets too high. Bleaching events vary in 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: corals may recover, if there are any zooxanthellae left in their tissues, but if not death appears to be inevitable.
The bottom line: If severe bleaching events occur regularly at shorter than 10–15 year intervals, then reefs face a death spiral of coral mortality followed by inadequate recovery periods. And that is what is happening now. Along Australia’s Great Barrier Reef, the frequency of mass bleaching is increasing, with events occurring in 1998, 2002, 2016, 2017 and 2020. The 2016-17 events severely bleached half the reef, whose extent has been reduced by three-quarters over the last 40 years. Coral reproduction on the Great Barrier Reef has fallen 89% after repeated recent bleachings.
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