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Climate Hustle

How global warming is driving mass coral bleaching

What the science says...

On a world scale coral reefs are in decline. Over the last 30-40 years 80% of coral in the Caribbean have been destroyed and 50% in Indonesia and the Pacific. Bleaching associated with the 1982 -1983 El-Nino killed over 95% of coral in the Galapagos Islands and the 1997-1998 El-Nino alone wiped out 16% of all coral on the planet. Globally about 1% of coral is dying out each year.

Climate Myth...

Corals are resilient to bleaching
"Three recent articles give us reason to question the alarmists’ claims that coral reefs are in deep trouble due to the buildup of greenhouse gases." (World Climate Report)

Despite what you may read or see in the mainstream media, out in the real world, massive and rapid changes are taking place in many ecological systems as a result of global warming. The Earth seems to be already convinced of global warming and is responding quickly.

Perhaps the most significant, and likely most enduring, are the shifts taking place in the Earth's oceans. Whilst many readers may have read or heard about Ocean Acidification, there are numerous other changes taking place in the oceans which should be equally as concerning. One such phenomena to appear in the last few decades is mass coral bleaching, a consequence of the continued warming of the oceans. Once vast stretches of colourful reefs teeming with marine life are being reduced to lifeless rubble covered in seaweed or slime. Many areas are not recovering, and the scale and frequency of bleaching worldwide is getting worse. In fact, early reports suggest 2010 may have witnessed the largest single bleaching event ever recorded.

The lowdown on coral bleaching

Reef-coral are actually a symbiosis (a mutually beneficial relationship) between the coral polyp, an anemone-like creature, and tiny algae called zooxanthellae. The coral provide shelter and nutrients for the algae , and in exchange the algae provide carbohydrates (food) to the polyp, using energy from the sun (photosynthesis) and the nutrients provided by the coral. These algae live in the skin tissue of the polyp and produce the coloured pigments which make coral reefs so visually spectacular. When this partnership breaks down the polyps expel the algae, which leads to the "bleached" effect. Although the polyp does feed using its tentacles to snare food, the bulk of its nutrition (90%+) comes from the algae, and they are a critical component of coral skeleton formation and therefore reef maintenance and growth. Without symbiotic algae, the coral can die from starvation, or become so weakened by a lack of food, that it succumbs to harmful bacteria (Mao-Jones 2010), and/or seaweeds which can poison and kill coral on contact.

Because reef-coral have adapted tolerance to a narrow band of environmental conditions, bleaching can occur for a number of reasons, such as ocean acidification, pollution, excess nutrients from run-off, high UV radiation levels, exposure at extremely low tides and cooling or warming of the waters in which the coral reside. Typically these events are very localized in scale and if bleaching is mild, the coral can survive long enough to re-acquire new algal partners. So bleaching in itself is not something new, but mass coral bleaching on the huge scale being observed certainly appears to be, and represents a whole new level of coral reef decline.

Ocean warming is driving mass coral bleaching

As coral reefs operate very near to their upper limit of heat tolerance (Glynn & D'Croz 1990), bleaching en masse happens when the surface waters get too warm above their normal summer temperature, and are sustained at this warmer level for too long. The intensity of bleaching corresponds with how high, and how long temperatures are elevated and, as one might expect, the intensity of bleaching affects the rate of survival. Small rises of 1 -2 degree C, for weeks at a time, usually induce bleaching.

This episodic ocean warming has been most pronounced worldwide during El-Nino events, when the Pacific Ocean exchanges heat to the atmosphere and surface waters. In recent years though, severe mass bleaching is happening outside of El-Nino because of the "background" ocean warming. The huge mass bleaching in the Caribbean in 2005, a non El-Nino year, and again this year is a prime example of this (Eakin 2010) . Evidence connecting warm surface waters and mass coral bleaching has strengthened to the extent that the National Oceanic and Atmospheric Administration (NOAA) has a coral bleaching alert system in place. This alert system accurately forecasts mass coral bleaching based on satellite data of sea surface temperatures.

Hot water + Coral = Dead coral

So how does hot water kill coral? It requires both high water temperatures and sunlight. Oxygen is released as waste during photosynthesis and like all chemical processes this is affected by temperature, speeding up as more energy (warmth) is applied. When water temperatures rise too high the protective mechanisms to prevent heat damage, employed by the coral and the algae, are overwhelmed. The zooxanthellae algae produce high levels of oxygen waste which begin to poison the coral polyp. In acts of self-preservation the coral kick out the algae, and in doing so become susceptible to starvation, opportunistic diseases, competitive seaweeds and macroalgae (slime to you and me) . Coral can succumb to the effects of bleaching years later, and for those coral that survive, growth effectively ceases and full recovery can take anything up to a decade.

Coral resilience is futile

On a world scale coral reefs are in decline, and it makes for rather depressing reading for an avid diver like myself. Over the last 30-40 years 80% of coral in the Caribbean have been destroyed (Gardner 2003) and 50% in Indonesia and the Pacific (Bruno & Selig 2007). Bleaching associated with the 1982 -1983 El-Nino killed over 95% of coral in the Galapagos Islands (Glynn 1990), and the 1997-1998 El-Nino alone wiped out 16% of all coral on the planet. Globally about 1% of coral is dying out each year. Not all of this continual decline is solely down to bleaching of course, pollution and other human activities are also contributing, but bleaching is speeding up the loss of coral.


Looking only at bleaching though, we find that the incidence of mass coral bleaching increases dramatically in the last few decades. Despite modern records being biased by better monitoring and reporting in recent times, there seem to be little evidence of mass coral bleaching further back in time when examining long-lived coral communities. Studies from around the world show no signs of bleaching dating back many thousands of years, until recent decades (Abram 2003), (Aronson 2003). In the Caribbean there are no signs of previous mass bleaching dating back 220,000 years (Pandolfini & Jackson 2006)

So where does this resilience claim originate you may ask?. Perhaps from studies that have shown some coral, in secondary bleaching events, have lower rates of death. A few coral are in more fact tolerant to bleaching, some algae for instance manufacture their own "organic sunscreen". However this a only small proportion, major reef-building coral species seem incapable of forming long-lasting partnerships with these heat tolerant algae (Coffroth 2010), and the coral polyp themselves have a very poor genetic ability to adapt to warming (Csaszar 2010). However the "resilience" fallacy arose, there's no evidence a few hardy individuals will somehow prevent the loss of most coral worldwide.

The importance of coral reefs - the oasis in a marine desert

So what does this all have to do with the average man or woman in the street?, well, as far as humans are concerned, there is a rather large dollar value attached to coral reefs. Goods and services derived from coral reefs are very roughly estimated to be between $172 to $375 billion dollars per year (Martinez 2007). Not only that, but reefs directly provide food and income to over half a billion people worldwide. The decline of coral reefs is going to not only impact those that directly depend on them for a living and sustenance, but eventually have dramatic effects on economies worldwide, and will likely drastically drive up world food prices as fish populations nosedive.

Ecologically speaking the value of coral reefs is even greater because they are integral to the well being of the oceans as we know them. It might serve to picture them as the undersea equivalent of rainforest trees. Tropical waters are naturally low in nutrients because the warm water limits nutrients essential for life from welling up from the deep, which is why they are sometimes called a "marine desert". Through the photosynthesis carried out by their algae, coral serve as a vital input of food into the tropical/sub-tropical marine food-chain, and assist in recycling the nutrients too. The reefs provide home and shelter to over 25% of fish in the ocean and up to two million marine species. They are also a nursery for the juvenile forms of many marine creatures .

I could go on, but the similarity with the rainforest should now be clear. Eliminate the undersea "trees", which mass coral bleaching is in the process of doing, and you'll eliminate everything that depends on it for survival, a point best exemplified in the following sequence of photos. (sequence of healthy coral-bleached coral-rubble & slime)

A grim outlook for coral

The critical issue with global warming induced coral bleaching, as it is for many eco-systems, is the speed of warming. They are simply not being given sufficient time to evolve tolerance. The coral's algal partners have short lifetimes and possess genetic traits which may enable successful adaptation to warming. Coral themselves aren't so lucky, somewhat in contrast to their algae, they possess a poor genetic ability to combat warming stress and have decadal lifetimes. It's likely therefore that many coral will die because the speed of warming is too great within an individual communities lifetime.

Perhaps a useful way of looking at it, is that the "bar" is continually being set higher and higher, and the recovery time between bleaching events becoming smaller and smaller. Gradually this continual ocean warming will start to impact areas which have so far escaped unscathed, and these coral will succumb too. Of course coral reefs aren't just under fire from bleaching, as mentioned earlier, humans are hurting them in many other ways. Ocean Acidification in particular is a large looming threat (Veron 2009). The increasing frequency and severity of bleaching, coupled with the persistent decline in coral around the world, should however immediately dispel any myths about coral resilience.

Intermediate rebuttal written by Rob Painting

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 24 April 2016 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page


Comments 1 to 24:

  1. And what happened with this paper?
    Interpreting the sign of coral bleaching as friend vs. foe, Suggett & Smith, 2010.:

    “While this synonymous association has undoubtedly been key in raising public support, it carries unfair representation: nonlethal bleaching is, and always has been, a phenomenon that effectively occurs regularly in nature as corals acclimatize to regular periodic changes in growth environment (days, seasons etc).”
    “Observations of non and sublethal bleaching (and subsequent recovery) are arguably not as readily reported as those of lethal bleaching since (1) the convenient tools used to quantify bleaching yield major ambiguity (and hence high potential for misidentification) as to the severity of bleaching; and (2) lethal bleaching events inevitably receive higher profile (media) attention and so are more readily reported.”
    “While bleaching induced coral mortality must remain our key concern it must be better placed within the context of bleaching signs that do not result in a long-term loss of reef viability. [...]”
  2. It appears that J.E.N. Veron's former colleagues at the Australian Institute of Marine Science are saying that the Great Barrier Reef is not doing so badly

    "Crown-of-thorns starfish (Acanthaster planci) outbreaks and storm damage were responsible for more coral loss during this period than either bleaching or disease despite two mass bleaching events and an increase in the incidence of coral disease. While the limited data for the GBR prior to the 1980's suggests that coral cover was higher than in our survey, we found no evidence of consistent, system-wide decline in coral cover since 1995. Instead, fluctuations in coral cover at subregional scales (10–100 km), driven mostly by changes in fast-growing Acroporidae, occurred as a result of localized disturbance events and subsequent recovery."

    Chris Shaker
  3. J.E.N. Veron, formerly at the Australian Institute of Marine Science, says that the world's reefs face possible catastrophic losses again, due to acidification. He claims that some of the previous coral extinction events were most likely caused by ocean acidification. He also says

    "Corals have an intimate symbiotic relationship with single-celled algae, zooxanthellae, which live in their cells and provide the photosynthetic fuel for them to grow and reefs to form. The research showed that this Ecosystems can recover from all sorts of abuse, and coral reefs are no exception. relationship can be surprisingly fragile if corals are exposed to high light conditions at the same time as above-normal water temperatures, because the algae produce toxic levels of oxygen, and excessive levels of oxygen are toxic to most animal life. Under these conditions, corals must expel the zooxanthellae, bleach, and probably die or succumb to the toxin and definitely die. A tough choice, one they have not had to make at any time in their long genetic history."

    I'm not sure I should believe the last sentence, given that the climate reached a few (several?) degrees C warmer than today during the previous interglacial...

    Chris Shaker
  4. cjshaker - note the following from the post:

    "The critical issue with global warming induced coral bleaching, as it is for many eco-systems, is the speed of warming. They are simply not being given sufficient time to evolve tolerance"

    As for the Great Barrier Reef (GBR), the worst mass coral bleaching episode was back in 2002/2003. The ocean has steadily gained an enormous amount of heat in that time, and the GBR appears to have been protected by a largely La Nina-dominant pattern - especially since 2005. Sadly this is unlikely to last. I'll have a post on this topic soon.
  5. I know there are other factors which can change the ph levels but bear with me.

    The ph level in seawater changes with temperature, warmer water holding less co2 than cold water meaning in tropical areas the ph level should hardly change if the water is warmed and co2 level rises (there is more likelyhood of tropical waters outgassing)
    In polar waters, its already co2 rich so the ph level should be very low in comparison to tropical waters.
    If it gets colder the polar seas will become more acid ......if it gets warmer they will become less acid

    What am I not understanding?

  6. Vonnegut:  Temperature of ocean water is not the only factor affecting the pH of the oceans.  Among other factors, an important one is the partial pressure of CO2 in the air.  You've already been pointed at the thorough explanation in the series of posts OA Is Not OK.  Read them, please, and if you have relevant questions after reading, then post questions there.

  7. Vonnegut, have you seen the company Sustainable Oceans Pty Ltd They specialise in relocating coral. Currently its usually relocated because of development we don't need or want. But in future they hope to use their technology for relocating coral to more temperate waters if climate change causes further ocean temperature rises and threatens coral survival.

  8. There seems to be some good news on the coral front.



    [RH] Shortened link.

  9. "Oops! It may not be ‘ocean acidification’ killing coral after all – common chemical found in sunscreen is poisonous to coral reefs"

    A greater threat to corals than warming and acidity may be suntan lotion that contains oxybenzone or any of three other ingredients. See:

    Sunscreen contributing to decline of coral reefs, study shows


    (Rob P) There no evidence, as far as I am aware, of ocean acidification killing reef coral in the modern ocean. The decline in saturation state and the increase in hydrogen ions (falling pH) are likely making calcification, the building of coral's calcium carbonate skeleton, more difficult though.

    This is because coral build their skeletons in internal chambers semi-sealed off from the ambient seawater and need to pump hydrogen ions out of the chamber in order to raise the saturation state. It permits aragonite crystals formation in this highly supersaturated environment. Lower ocean pH, as ocean acidification is currently doing, and you increase the concentration of hydrogen ions dissolved in seawater. Coral therefore have to expend more energy in pumping these ions out of the calcification chamber.

    It's problematic because, like many organisms, the energy budget is very tight and they can't afford to use up energy allocated to reproductive purposes. Skimp on calcification and the coral skeleton will become weaker. And there is evidence that it is indeed occurring at some locations.

    But this rebuttal is about coral bleaching and subsequent mortality because the world's oceans are becoming too warm. That's the more immediate concern

  10. "It may not be ‘ocean acidification’ killing coral after all..."


    "A greater threat to corals than warming and acidity may be..."

    The operative phrase is "may be".  Further study is required to quell the pell-mell rush down the steep path to Hyperbole land.

  11. It strikes me as ulikely that sunscreen is harming corals in remote Hawaiian islands with no tourists, the Great Barrior reef where tourists do not  go and in many other remote locations.  Even if it were poisoness enough to affect corals in the ocean, which I doubt, it would only affect the most visited locations.

  12. Corals are bleaching across all the world’s oceans in only the third die-off of its scale in history, scientists have revealed.

    El Nino and a Pacific warm water mass known as “The Blob” are combining with human-caused climate change to drive record high ocean temperatures.

    These hostile conditions are expected to deplete more than 38% of the world’s reefs by the end of 2015, according to the US-based National Oceanic and Atmospheric Administration (NOAA).

    Recent damage to corals in the Caribbean follows bleaching in the Pacific and Atlantic oceans earlier in the year, confirming the phenomenon as global in scope.

    It has knock-on effects for species that rely on healthy corals for food, as well as for people who make a living from tourism or fishing.

    “What really has us concerned is this has been going on for a year and is likely to last another year,” said NOAA coral specialist Mark Eakin.

    The El Nino weather system, characterised by a period of warm water across much of the Pacific, is forecast to remain strong until early 2016.


    I have trawled for this report everwhere university portals, Google and the AIMS website and its published papers index but without success. Has anyone any idea where this report in 'The Australian' by Amos Aikman originates?


  14. denis.boarder @13.

    Cooper et al (2012) Growth of Western Australian Corals in the Anthropocene.

    If you have an author's name (here Janice Lough) and they are an academic, finding a list of their publications is a good step to getting on to their papers.

  15. From the abstract for the paper.

    "We show there is no widespread pattern of consistent decline in calcification rates of massive Porites during the 20th century on reefs spanning an 11° latitudinal range in the southeast Indian Ocean off Western Australia. Increasing calcification rates on the high-latitude reefs contrast with the downward trajectory reported for corals on Australia's Great Barrier Reef and provide additional evidence that recent changes in coral calcification are responses to temperature rather than ocean acidification."

    Huh! This is a classic Black is White piece from the Australian. There is a downward trajectory to calcification in the GBR, in warmer water, in contast to the cooler waters at higher latitude in the southern Indian Ocean.

  16. Glenn Tamblyn @15.

    I think the closing comments of the full article are a better interpretation of what the authors are saying:-

    "Seawater carbon chemistry is a key determinant of coral calcification, and the potential for future anthropogenic-influenced declines in carbonate saturation state, and hence coral calcification, is cause for serious concern (2,4,7). However, we conclude that the rate of change in the thermal environment of coral reefs is currently the primary driver of change in coral calcification rates. Warming SSTs are resulting in (i) increased calcification rates reported here in the southeast Indian Ocean, where marginal reefs have taken advantage of warmer conditions, and (ii) recent declines reported elsewhere for more typical reef environments where thermal optima for calcification have been exceeded or resulted in setbacks in growth as a result of thermally induced bleaching. Whether the former is sustainable as oceans continue to warm is another question."

  17. Thanks MA Rodger, I didn't have access to the full article.

    Either way it doesn't support what the Australian reported.

  18. Western Australian coral were devastated by the marine heatwave of 2010-2011. See: Unprecedented Mass Bleaching and Loss of Coral across 12° of Latitude in Western Australia in 2010–11. A lot of other local marine life went belly up too including abalone, scallops, crayfish and fish.

    However the time of greatest risk for higher latitude West Australian coral is during La Nina, although the 2010-2011 La Nina was exceptional and exacerbated by anomalous conditions. See: La Niña forces unprecedented Leeuwin Current warming in 2011.

  19. Why is, "..the time of greatest risk for higher latitude West Australian coral is during La Nina...", mayI ask?

  20. See the 2nd peer-reviewed paper linked to above - the stronger trade winds during La Nina push more warm tropical water westward between the Indonesian archipelago & Australia (the Indonesian Throughflow or ITF).

    The 2010-2011 La Nina coincided with an anomalously strong southward Leeuwin Current (normally weak over summer) and weak southerly winds which flow in opposition to the Leeuwin Current.  

    We appear to be in the positive phase of the Interdecadal Pacific Oscillation (IPO) which should reduce the volume of warm water transported through the ITF, but on the other hand the oceans are growing warmer. A La Nina forecast to develop later this year could prove interesting come next summer.

    Anyway, to give you a quick example, here's a composite of sea surface temperatures for La Nina summers excluding the exceptional La Nina of 2011:  


  21. Coral Bleaching is the Legacy of a Marvelous Adaptation Mechanism

    Observations of increased bleaching have happened for both warm and cold events


    [Rob P] - Note the text on the post you have commented on:

    "Because reef-coral have adapted tolerance to a narrow band of environmental conditions, bleaching can occur for a number of reasons, such as ocean acidification, pollution, excess nutrients from run-off, high UV radiation levels, exposure at extremely low tides and cooling or warming of the waters in which the coral reside. Typically these events are very localized in scale and if bleaching is mild, the coral can survive long enough to re-acquire new algal partners. So bleaching in itself is not something new, but mass coral bleaching on the huge scale being observed certainly appears to be, and represents a whole new level of coral reef decline."

    Coral reefs globally are in dramatic decline and the worldwide bleaching event currently underway is likely to kill quite a few more coral reefs. When one steps back to look at the 'big picture' it's pretty clear why coral reefs in the ancient ocean suffered crises and extinctions - a too-warm and acidified ocean was just too inhospitable for them to survive.   

  22. Rob, Seriously. Reef forming coral expanded during the warm periods of the Mesozoic. Do you really want to argue coral suffered more from warmth than they have from the cold during the most recent ice ages?? Please provide the evidence? LInks? 

    And indeed there has been episodic declines in coral reefs, but the greatest mortality has been due to tropical storms, predators like Crown of Thorns, or disease like White band. Dynamite and cyanide fishing have also been destructive as well as nutrient runoff from agriculture and seage. INstead you want to focus on bleaching, the smallest cause of mortality which is also part of their amazing adaptation mechanisms?!?  Why do you disagree with the experts who promote the Adaptive Bleaching Hypothesis?


    [Rob P] - You mention the Mesozoic and make the flawed assumption that the marine revolution that occurred during this time, as it pertains to coral reefs, came about because of some hitherto unrecognized invulnerability to warm water. This is nonsense.

    Coral reefs likely acquired photosymbiosis during the Mezosoic and both the evolution of herbivorous marine feeders and the flooding of continental shelves occurred during this time. The development of photosymbiosis enabled coral to spread into oligotrophic (low nutrient) environments, and the flooding of continental shelves with warming opened up new territory for them. The arrival of mobile marine herbivores also meant coral were better able to complete with macroalgae and seaweed for space - indeed this is suggested as being the primary evolutionary driver during this period. I suggest this review paper as a starting point might prove illuminating: The Ecological Evolution of Coral Reefs by Professor Rachel Wood.

    There are simple reasons why coral have suffered or gone extinct because of (natural) global warming events in the past, a) surface seawater was too warm, and b) low carbonate ion abundance associated with ocean acidification made skeleton-building too energetically costly. Calcification rates drop, the coral skeleton becomes weakened and the reef cannot precipitate aragonite fast enough to overcome bioerosion. And this segues into why we are seeing coral reefs disappearing at an alarming rate today. You see that the scientific case explains both the past and present behaviour of coral reefs perfectly well.

    As for adaptive bleaching, it's not only me that disagrees with the adherents of this hypothesis, it's other coral reef experts and, more importantly, the observations. Intense coral bleaching events induce mortality soon after bleaching and thus dead coral are unable to acquire new algal partners. I'm sure you can agree that death makes adaptation impossible. Moreover, some of the reefs on the Great Barrier Reef (GBR) that survived the smaller-scale bleaching events of 1998 and the early noughties, have since died from the 2016 GBR bleaching event. 

  23. Rob, You put words in my mouth. I never made "the flawed assumption that the marine revolution that occurred during this time, as it pertains to coral reefs, came about because of some hitherto unrecognized invulnerability to warm water." I asked "Do you really want to argue coral suffered more from warmth than they have from the cold during the most recent ice ages??" and you went off on a strawman tangent.

    The coral holobiont is constantly shifting the symbionts to maximize growth withing very narrow limits. Thus they are always stressed by an extreme El Nino event, whether happens in cooler water of the 1800s or warmer waters of the 21st century. Bleaching is part of their ongoing adpatation mechanism of expelling symbionts and acquiring new ones. This very well documented.

    Obviously you and others disagree with those experts that push the adaptive bleaching hypothesis. That why it is a professional debate. That polyps die is by no means a refutation of that hypothesis and such an argument is a bit disingenuous. A colony consists of 100s to millions of polyp clones. Most of the clones die after a stressful event, such as an abrupt cooling or warming. It's like leaves falling from a tree but buds, or in this case cryptic polyps reamain viable and in what is known as the Phoenix effect those clones with a better adapted symbiotic partnership can rapidly re-sheet the skeletal remains. Recovery can take weeks or years, and the reefs with the greatest moratliy, 90% or more, have recovered within 15 years. In the process those living polyps have been demonstrated to acquire better adapted symbionts. And the species that experienced the most bleaching during on EL Nino are observed to rsist bleaching during subsequent extreme events.

    However because EL Nino warm event are abrupt and temporary, coral will again shuffle and shift symbionts to be better adapted to the cooler pre El Nino events. They may then re-acquire the old symbionts and once again be vulnerable to a EL NIno. The coral that suffer the most are exposed to extreme variability as warm EL Nino waters are replaced by cold upwelled La Nina waters. That' s why the cool eastern Pacific has only ~18 species, while the Western Warm pool has over 500 species and more massive reefs.

    Fears surrounding bleaching and climate change are only valid if you believe that coral have reached their upper thermal limit and can only adapt via a slow evolutionary process. But there is amples and growing evidence that is not the case. Symbiont shifting and shuffling allows rapid adaptations, and there is no reason to believe that holobionts that thrived during the Holocene Optimum when tropical oceans were 2.1 C warmer than today, will not be able to thrive any similar warming over the next millennium


    [Rob P] Warming of tropical waters from the last ice age to the present has been beneficial to coral reefs, but now that the tropics are reaching temperatures in summer that exceed the upper thermal tolerance threshold of coral that is no longer the case. This is why coral cores show no evidence of mass bleaching, on the scale we are currently witnessing, in the last thousand years or so until the latter part of the 20th century.

    Again the 'big picture' is the crucial point here. Bruno & Selig (2007) found that coral cover is declining at the rate of 1-2% per year irrespective of local conditions and Death et al (2012) discovered that half the coral on the Great Barrier Reef (GBR) has disappeared in the last 27 years. Only 10% of the GBR decline was attributed to bleaching mortality, which is a huge concern because the GBR has experienced a hiatus in bleaching due to the negative (La Nina-dominant) phase of the Interdecadal Pacific Oscillation (IPO). Continued ocean warming and a switch to the positive (El Nino-dominant) phase of the IPO suggests bleaching is going to become the major detriment to global reef cover over the next decade.

    When coral reefs globally are dying out at such an alarming rate, and when the oceans are warming and acidifying, it is absurd to offer a few limited exceptions as a counter argument. As time marches on, this global decline in coral cover will undoubtedly gather pace.         

  24. Rob asserts, "tropics are reaching temperatures in summer that exceed the upper thermal tolerance threshold of coral that is no longer the case"

    You are assuming that the upper thermal tolerance threshold has been reached but symbiont shuffling and shiftings has demonstrated that the upper threshold is moveable and new thresholds rapidly evolve. The fact that coral thrived during the Holocence Optimum when temperaturs in the Coral Sea were 2.1C warmer than today suggests you are very wrong about corals upper threshold. Coral must adjust their thermal boundaries to deal with cooler and warmer temperatures. The Great Barrier Reef Expedition of 1928-29 was concerned with bleaching and warming temperatures. Bleaching happens when ever there is an abrupt event that exceeds the prevailing climatology. If the extremes represent a trend the coral then evolve new symbiont partnerships best suited for that change.

    Most global bleaching events are temporary and will not be detected by cores sample. Furthermore as discussed in Hendy 2003 even massive mortality as observed in the 1998 El Nino bleachings are very difficult to detect without much  much larger samples size within a location and from a wider range of locations.

    You absurdly suggest a growing pace of coral death based on extrapolating from a short term  snapshot and dismissal of the evidence of symbiont shifting. But most reefs that suffered high mortality have recovered within a decade and have been resilient to subsequent warm events. Bleaching is a minor cause of moratlity and coral have had to evolve resilience and quick recovery to deal with more destructive forces such as tropical storms.


    [Rob P] - Symbiont shuffling is no panacea for the ongoing global demise of coral reefs. It's just another example of people in denial building a delusional belief around a 'kernel of truth'. Yes, symbiont shuffling is a real adaptation, but no it won't save coral reefs. The current precipitous decline in global coral cover might be a wee bit of a giveaway here. A glaring example is the recent assessment of the 2016 bleaching event on the Great Barrier Reef (GBR) - there is an average of 35% mortality on the central and northern reefs surveyed, an area making up two thirds of the entire GBR reef system. So one single bleaching event may have eliminated 20% of the reef. Fat lot of good symbiont shuffling did after GBR bleaching events of 1998 & 2002 eh?  

    The Holocene Thermal Maximum is an interesting point. Was it warmer in the tropics during this interval? There doesn't seem to any consensus on this. The change in insolation due to orbital forcing would have made the tropics cooler-than-present, while outside the tropics, the Northern Hemisphere in particular, was much warmer. See the figure below from Marcott et al (2013) and note panel C) - the average annual insolation anomaly compared to modern-day.

    Rosenthal et al (2013) show that intermediate waters were much warmer, but the reef-building coral we're talking about don't live hundreds of meters deep. How much of this high latitude water returned to the equator via the subtropical cells, and how much did it affect the surface layers? The authors reconstructions show a very modest SST cooling 0.5°C in the Indo-Pacific warm pool from 9000 years ago to present (present being 1950 as is convention). By constrast, Marcott et al (2013) reconstruct tropical temperatures as cooler-than-present at the Holocene Thermal Maximum.    

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