28 January 2022

Have tipping points already been passed for critical climate systems? (5) Coral Reefs: A death spiral

by David Spratt 

Fifth in a series.    Read 1  |  2  |  3  |  4  |  5  |  6  |  7

Updated 2 February 2022.

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:

 "The time between recurrent events is increasingly too short to allow a full recovery of mature coral assemblages, which generally takes from 10 to 15 years for the fastest growing species and far longer for the full complement of life histories and morphologies of older assemblages."

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.    

Multiple lines of evidence suggest the current physical conditions will cause decreasing extent and ecosystem collapse along the Great Barrier Reef at the current level of warming:

  • Melbourne-based researchers showed in 2017 that the hotter water conditions that drove the severe bleaching in 2016 will occur on average one year in three at 1.2°C, and two years in every three at 1.5°C. 
  • In 2021, researchers concluded that at 1.5°C, severe bleaching events would occur on average three times per decade. 

Given recovery times are 10-plus years, even warming in the 1.2–1.5°C range means a spiral of decreasing extent. Some corals adapted to hotter conditions will survive, but the reef as an ecosystem will be unrecognisable compared to fifty years ago.

And it is not just the Great Barrier Reef. Prof. Terry Hughes and colleagues looked at 100 reefs globally and found that the average interval between bleaching events is now less than half what it was before:

“Such narrow recovery windows do not allow for full recovery. Furthermore, warming events such as El NiƱo are warmer than previously, as are general ocean conditions. Such changes are likely to make it more and more difficult for reefs to recover between stressful events” (emphasis added).

Dr Mark Eakin, coordinator of Coral Reef Watch at the US National Oceanic and Atmospheric Administration, says rising ocean temperatures could have pushed the world’s tropical coral reefs over a tipping point: 

“The real concern is with this much bleaching without tropical forcing. This may be a sign we’ve now tipped over to near-annual bleaching in many locations.”

New research published on 1 February 2022 was unequivocal about a 1.5°C outcome for corals:

“Climate refugia are locations that maintain suitable environmental conditions for a resident species even when surrounding areas become inhospitable… We show that climate change will overwhelm current local-scale refugia, with declines in global thermal refugia from 84% of global coral reef pixels in the present-day climate to 0.2% at 1.5 ̊C, and 0% at 2.0 ̊C of global warming… We confirm that warming of 1.5 ̊C relative to pre-industrial levels will be catastrophic for coral reefs.”

And what would be safe for reefs? Dr Charlie Veron — who has discovered, described and identified about a third of all known coral species — addressed the Royal Society in London in 2009, saying that:

“The safe level of atmospheric CO2 for coral reefs is ~320 ppm. This identifies a bench-mark level for contemporary marine ecosystems. More than any other available measure, it sets the safe limit for a healthy planet during a time of abrupt greenhouse-driven climate change.”