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Climate Hustle

Coral: life's a bleach... and then you die

Posted on 13 January 2011 by Rob Painting

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.


Figure 1 - from Marshall & Schuttenberg 2006 

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 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.

This blog post has been added as our 141st rebuttal, "Corals are resilient to bleaching".

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Comments 1 to 50 out of 66:

  1. Nice post, Rob (love the title)!

    Maybe it's in the post & I missed it, but how's the Great Barrier Reef faring in the elevated SST's of this strong (potentially the strongest ever) La Nina?

    The Yooper
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  2. Corals are my favorite topic.

    Do volcanic eruptions enhance or diminish net primary production? Krakauer and Randerson, 2003. :
    “Up to decadal-scale ocean cooling tentatively linked to volcanic eruptions has also been found in a study of south Pacific coral [ Crowley et al. , 1997]. Nevertheless, it appears puzzling that the maximum growth reduction seen in this study lags by several years the period of maximum eruption cooling.”

    Robecknew, 2003.: “It was so cold that winter that Jerusalem experienced rare snowfall and coral at the bottom of the Red Sea died, because the water at the surface cooled and convectively mixed the entire depth of the water [ Genin et al. , 1995]. The resulting enhanced supply of nutrients produced huge algal and phytoplankton blooms, which smothered the coral. This coral death had only happened before in winters following large volcanic eruptions [ Genin et al. , 1995].”

    Coral reef calcification and climate change: The effect of ocean warming. McNeil, Matear and Barnes, 2004.:
    “Our analysis suggests that annual average coral reef calcification rate will increase with future ocean warming and eventually exceed pre-industrial rates by about 35% by 2100. Nasza analiza wskazuje, że średnia roczna stopa koral rafa zwapnienie wzrośnie z przyszłego ocieplenia ocean, a ostatecznie przekracza sprzed rewolucji przemysłowej, stawki o około 35% do roku 2100. Our results suggest that present coral reef calcification rates are equivalent to levels in the late 19th century and does not support previous suggestions of large and potentially catastrophic decreases in the future.”

    Coral adaptation in the face of climate change. Baird and Maynard, 2008., speaks of the rapid adaptation of corals to climate change.

    4 Years After Tsunami Corals Stage Comeback, 2008.:
    “ A team of scientists from the New York-based Wildlife Conservation Society (WCS) has reported a rapid recovery of coral reefs in areas of Indonesia, following the tsunami that devastated coastal regions throughout the Indian Ocean four years ago today. The team, which has surveyed the region's coral reefs since the December 26, 2004 tsunami, looked at 60 sites along 800 kilometers (497 miles) of coastline in Aceh, Indonesia. The researchers attribute the recovery to natural colonization by resilient coral species, along with the reduction of destructive fishing practices by local communities.”
    “Multiple linear regression models show that high mean sea surface temperatures, high acidity, and low frequencies of high temperature anomalies account for 45% of the variation in proportion of Clade D at all examined sites. Models of future Indo-Pacific climate predict that areas now unsuitable for Clade D will rapidly change to favor Clade D dominance: the front of Clade D suitability is predicted to move across the subtropical South Pacific at rates of 20-49 km per year. Many of these areas currently host low levels of Symbiodinium D, raising the concern that environmental change will sweep over areas faster than dispersal and population growth can add Clade D symbionts to reefs.”

    CO2, Global Warming and Coral Reefs: Prospects for the Future . Idso, 2009.:
    “Lough and Barnes (1997) found that "the 20th century has witnessed the second highest period of above average calcification in the past 237 years."

    Currently, however: “The study shows that the biggest and most robust Porites corals on Australia's Great Barrier Reef have slowed their growth by more than 14 percent since the "tipping point" year of 1990.” "The data suggest that this severe and sudden decline in calcification is unprecedented in at least 400 years," Lough: "It is cause for extreme concern that such changes are already evident, with the RELATIVELY MODEST CLIMATE CHANGES observed to date, in the world's best protected and managed coral reef ecosystem,"
    ... however: "The causes of this sharp decline remain UNKNOWN, but our study suggests that the combination of increasing temperature stress and ocean acidification may be diminishing the ability of Great Barrier Reef corals to deposit calcium carbonate,"
    Dr Lough said there had been some concern that coral growth has been declining in recent times. "However, data from density bands place these results into a larger context. Density bands show that coral growth and calcification on the Great Barrier Reef vary considerably over time.”
    "Coral records show that there have been several major increases and decreases over the past several centuries. “
    Generally Conclusion:
    “the CURRENT DECLINE appears to be a RETURN TO MORE NORMAL GROWTH CONDITIONS from high growth rates earlier this century".
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    Moderator Response: [Daniel Bailey] Arkadiusz, please stop using all-caps. Posts containing all-caps will be deleted, per the Comments Policy. Thanks for the compliance.
  3. sorry ...
    PS 80-54: A tipping point for coral symbiosis: Non-linear response to environmental change may drive a rapid shift in coral symbionts, Oliver , Arrigo and Palumbi., 2009.:
    “Multiple linear regression models
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  5. Yooper @ 1 - Not shaping up too good just yet, we'll have to wait and see. February is when the waters around these parts really warm up:

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  6. Corals are my favorite topic

    Yup, rate highly with me too. Having dived the Great Barrier Reef and some in the South Pacific, their decline bothers me greatly.
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  7. A few years ago IUCN published report:
    Coral Reef Resilience and Resistance to Bleaching

    in Web of Science: (doi: 10.1071/MF99078 - free access )

    and;301/5635/929 ( for a free copy) doi: 10.1126/science.1085046

    have the highest citation count (in hundreds)
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  8. Mila, I don't consider coral scientists discussing "resilience" is particularly helpful in communicating to the public, how serious a problem this is.
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  9. Coral reef calcification and climate change: The effect of ocean warming. McNeil, Matear and Barnes, 2004.:
    Sorry for the sentence in Polish in the middle - it should be like this:
    “Our analysis suggests that annual average coral reef calcification rate will increase with future ocean warming and eventually exceed pre-industrial rates by about 35% by 2100. Our results suggest that present coral reef calcification rates are equivalent to levels in the late 19th century and does not support previous suggestions of large and potentially catastrophic decreases in the future.”
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  10. Arkadiusz - working on the coral calcification rebuttal at the moment. Let's just say the study is seriously flawed - the authors ignoring the effects of bleaching on coral in warmer water doesn't really get them off to a good start does it?.

    We can discuss it when the appropriate rebuttal is out, if you wish. Might be a week or so.
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  11. #8 in my opinion anything which provides facts about the situation from reliable sources from variable perspectives is very helpful in communicating to public :) I personally consider the coral situation as a case of great concern and knowledge of possible natural responses as a key part to understanding of the phenomena.
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  12. Arkadiusz Semczyszak wrote on Could global warming be caused by natural cycles? : "About these facts there is nothing in the article commented by me - only the corals are dying as a result of warming and the decline in calcification of the ocean. Well ...
    I can not help that the world of science is so divided - in terms of impact of the reduced calcification and warming of the oceans on the corals."

    Your link is mainly about the recovery of reefs around Indonesia after the tsunami of 2004, not about how 'well' reefs in general are doing under AGW conditions. A quote within the link states :

    These findings provide new insights into coral recovery processes that can help us manage coral reefs in the face of climate change.

    And, as Mila shows, there is plenty to discuss (and being discussed) among the relevant experts but I would hardly call them 'divided'. I realise so-called skeptics like to proclaim that there are two sides to AGW (the science and the so-called skeptics/those in denial), in the same way that Creationists do about evolution, but the reality is far, far different. As one of the studies referred to by Mila states :

    However, reefs will change rather than disappear entirely, with some species already showing far greater tolerance to climate change and coral bleaching than others.
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  13. What's the chance the present floods in Queensland are going to impact the Great Barrier Reef?

    I know in the past great influxes of fresh water have caused bleaching events.
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  14. A few video presentations for anyone interested in Corals and their future in a warming, polluted and acidifying ocean.

    Charlie Veron at the Royal Society

    Rob Dunbar at TED

    Jeremy Jackson at TED

    Note, among other things, Charlie Veron saying that coral bleeching occurred only occasionally and very localized before 1980.
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  15. #12 an excellent Gavin's RealClimate article:
    Unsettled Science seems to be particularly relevant
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  16. 12 JMurphy

    I'm not sure it's just the skeptics that have turned this into a binary discussion. It could be argued that the existence of the IPCC has generated the post-normal conditions. Ifyoudon't believe this is an adverserial process by both sides look at Rob Painting use of language in #10. "Rebuttal" is that the normal way forward in science?

    Arkadiusz Semczyszak posted a link to a paper in #9 in fact you can get it as a PDF which contains a comment and reply to the comment. The discussion obviously is health.

    McNeil et al 2004, comment and reply
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  17. I think we will wait to talk to the new work - papers. The basic flaw of most (if not all) current work - the pros and cons - if it comes to the impact of CO2, temperature on the coral - it is too short a period of research, experiment (lack of repeatability - the results).
    We do not know (even tentative) what was the pH of the ocean - as varied - from 420 thousand. years (when it was probably similar to that now) - for this you just have to admit.

    ... and writes about it often, eg, Professor J. Gattusso - a great supporter of the negative effects of decline in calcification of the ocean - for the corals - as a result of emissions ACO2 - so we can probably believe him.

    In addition, You remember about this that: coral reefs:
    „While initial surveys immediately following the tsunami showed patchy (albeit devastating) damage to coral reefs in the region, surveys in 2005 indicated that many of the dead reefs in the study area had actually succumbed long ago to destructive fishing practices such as the use of dynamite and cyanide to catch fish. It is also possible that the crown of thorns starfish—a marine predator—had caused widespread coral mortality.”

    And with that - first of all - we should fight.
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  18. #17 "We do not know (even tentative) what was the pH of the ocean"

    Foraminiferal boron isotope ratios as a proxy for surface ocean pH over the past 21 Myr Nature 363, 149 - 151 (13 May 1993); doi:10.1038/363149a0
    Evidence for a higher pH in the glacial ocean from boron isotopes in foraminifera
    Nature 373, 234 - 236 (19 January 1995); doi:10.1038/373234a0
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  19. @JMurphy

    Your link is mainly about the recovery of reefs around Indonesia after the tsunami of 2004, not about how 'well' reefs in general are doing under AGW conditions.

    Of course it is! I did it intentionally. The tsunami destroyed some of the reefs in 99% the years (I mean the living components). I have not heard that until such damages have "bleaching" of the 1997/8 and 2005. So if the reef quickly recover after the Tsunami ...
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    Moderator Response: [Daniel Bailey] Please refrain from being intentionally off-topic. Thanks!
  20. Arkadiusz Semczyszak wrote : "We do not know (even tentative) what was the pH of the ocean - as varied - from 420 thousand. years (when it was probably similar to that now) - for this you just have to admit."

    Luckily, some people are and have been trying to do just that :

    Here we use the boron-isotope ratios of ancient planktonic foraminifer shells to estimate the pH of surface-layer sea water throughout the past 60 million years, which can be used to reconstruct atmospheric CO2 concentrations.
    Atmospheric carbon dioxide concentrations over the past 60 million years
    Paul N. Pearson & Martin R. Palmer

    Data here

    HumanityRules, the IPCC was set up " the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) as an effort by the United Nations to provide the governments of the world with a clear scientific view of what is happening to the world's climate." The fact that so-called skeptics or deniers don't like the answers, is the only reason for such 'binary discussion' under 'post-normal conditions' - whatever that latter means.
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  21. I knew that I had seen something about coral bleaching in AR4 :)

    Box 6.1. Environmental thresholds and observed coral bleaching
    6.2.1 Natural coastal systems
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  22. @ Mila

    Agreed - very, very initially we know („... it has been suggested ...”). I know this papers.

    Just please do not show the "acidification" hockey stick - because it was made from a combination of work, which should not - you can not connect - so has been "overwhelming" to criticism.
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  23. Recovery from bleaching:
    Climate change and coral reef bleaching: An ecological assessment of long-term impacts, recovery trends and future outlook - seems to be well cited and as I skimmed trough readable(free copy: )

    a few general bits:

    Pacific or the Arabian Gulf, where some reefs are recovering and others are not. The majority of survivors
    and new recruits on regenerating and recovering coral reefs have originated from broadcast spawning
    taxa with a potential for asexual growth, relatively long distance dispersal, successful settlement, rapid
    growth and a capacity for framework construction. Whether or not affected reefs can continue to
    function as before will depend on: (1) how much coral cover is lost, and which species are locally
    extirpated; (2) the ability of remnant and recovering coral communities to adapt or acclimatize to higher
    temperatures and other climatic factors such as reductions in aragonite saturation state; (3) the changing
    balance between reef accumulation and bioerosion; and (4) our ability to maintain ecosystem resilience
    by restoring healthy levels of herbivory, macroalgal cover, and coral recruitment. Bleaching disturbances
    are likely to become a chronic stress in many reef areas in the coming decades, and coral communities, if
    they cannot recover quickly enough, are likely to be reduced to their most hardy or adaptable constit-
    uents. Some degraded reefs may already be approaching this ecological asymptote, although to date
    there have not been any global extinctions of individual coral species as a result of bleaching events.

    The number of coral reef bleaching reports, driven principally by
    episodic increases in sea temperature, has increased dramatically
    since the early 1980s (Glynn, 1993; Hoegh-Guldberg, 1999; Hughes
    et al., 2003; Hoegh-Guldberg et al., 2007). Many of these events,
    and recovery from them, have now been studied over decadal
    scales. The frequency and scale of coral bleaching events during the
    past few decades have been unprecedented, with hundreds of reef
    areas exhibiting bleaching at some point, and, on occasion, whole
    ocean basins affected. Consequently, much has been written about

    as the WorldFish Center, NOAA, and GBRMPA.
    The occurrence of mass bleaching events correlates well with
    observed increases in global sea temperatures, and particularly
    thermal anomalies. This relationship was clearly observed in the
    Caribbean basin during the 1980s and 1990s, when annual coral
    bleaching increased logarithmically with SST anomalies (McWil-
    liams et al., 2005). A 0.1  C rise in regional SST resulted in a 35%
    increase in the number of areas that reported bleaching, and mass
    bleaching events occurred at regional SST anomalies of 0.2  C and
    above (Fig. 2). Bleaching within affected regions is not uniform,
    exhibiting patchy affects over micro (mm to cm) to meso (km)
    scales. Such variability results from fluctuations in environmental
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  24. I think these assumptions to work on an adaptation of corals in the “warmer world”, are very interesting and could change the preliminary conclusions contained therein.
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  25. #22 I am a PhD chemist by training but as it is outside of my field I am unable to appreciate the details - papers which I have read about isotope measurements as proxies of the past make sense to my general experience

    unfortunately, as soon as you reject isotope (and other) proxies as an indirect evidence we will have to wait till the first prototype of a time machine - which may take some time - especially to prove it really did work :)
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  26. #24 I have a slight problem with the link:

    if you read
    "Prior to this research project, it was virtually impossible to predict the environmental impacts of climate change on coral reefs because the behavior of corals and their nutrient-providing algae counterparts, known as symbionts, was virtually unknown. This research generated data that allowed scientists to develop a powerful understanding of coral-symbiont responses to environmental change, which in turn will allow researchers to better plan conservation strategies in the face of climate change."

    and you see e.g. :

    Landscape ecology of algal symbionts creates variation in episodes of coral bleaching
    Nature 388, 265-269 (17 July 1997)


    FLEXIBILITY AND SPECIFICITY IN CORAL-ALGAL SYMBIOSIS: Diversity, Ecology, and Biogeography of Symbiodinium' doi: 10.1146/annurev.ecolsys.34.011802.132417
    (free access fro several sources:

    I would almost suspect that something was known already 10 years ago
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  27. Two words:

    "Cumulative impacts"
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  28. Mc Neil et al 2004 says "our analysis suggests annual average coral reef calcification rate will increase with future ocean warming and eventually exceed pre-industrial rates ..."
    Well, the actual number of bleaching events and the actual overall decline of reefs is measurable only 4 years after that paper. Perhaps their analysis is wrong.
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  29. #2

    In the McNeil 2004 reference I noticed that there were 13 citations to that paper. I may be wrong, but every one I was able to check seemed to suggest that calcification would decrease due to increasing acidification of the oceans. Well, except maybe the two citations by McNeil himself.
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  30. How does one separate the impact of climate change from the impact of habitat damage?

    On the MesoAmerican Reef (aka Cancun area), the coral death seems to be correlated with the proximity to resorts. Coral off of Playa del Carmen is dead. Protected reef near the smaller town of Puerto Morolos is living.

    Climate change is yet another environmental stressor, however I worry that global climate change is getting all of the blame. The effects of habitat destruction gets lost. This allows local communities to effectively "pass the blame" away from their local environment.
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  31. #30
    The future of coral reefs doi: 10.1073/pnas.091092998
    PNAS May 8, 2001 vol. 98 no. 10 5419-5425 ; free access:
    wortwhile reading
    In the face of so many unknowns, qualitative analogies can provide an important complement to quantitative analyses. For this reason I close with the concept of the straw that broke the camel's back. No single straw “causes” ecological collapse; collapse is difficult to predict based on the response of the camel to earlier straws, but once collapse occurs, the camel does not return to its feet when the last straw is removed. The recent history of coral reefs suggests that collapse is not impossible, and indeed, that we may be closer to worldwide collapse than we realize. Moreover, the weight of the straws is likely to be multiplicative rather than additive because of negative synergistic effects between different types of stressors. Crippled coral reefs, like crippled camels, provide many fewer services, and they can be prohibitively expensive to repair (115). Although reefs are more likely than camels to recover unaided, having come and gone and come again throughout the history of life, it is likely to be a very slow process, and we may not be around to see true reefs when they do return.
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  32. 30 - Mfripp - Climate change is yet another environmental stressor, however I worry that global climate change is getting all of the blame

    No need to worry, it's not something that is neglected in research. If you start reading the peer-reviewed literature you'll see that the overwhelming majority of coral scientists are at pains to point this out. Quite a few of the studies I link to in the rebuttal are the full papers at PLoS One, and full copies of all bar one or two, papers are freely available online.

    The connection between warm water and bleaching is very well established now, just keep tabs on the NOAA Coral Reefwatch site and see how that plays out on the Great Barrier Reef and parts of the Coral Triangle.
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  33. Another excellent article on this topic:

    “Is the End in Sight for The World’s Coral Reefs?” posted on the Yale Environment 360 website.

    The author of the article is J.E.N. Veron.
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  34. Arkadiusz, a few points-

    1) The speed of recovery of the reefs after the tsunami has nothing to do with recovery following bleaching events. The two are completely dissimilar. Comparing them is like comparing the dangers of the flu to a 20 yr old athlete and an 80 yr old chemotherapy patient.

    In order for a reef to recover after damage, there has to be a source of corals. In the case of the tsunami there were two. Due to the patchiness of the damage, sexual recruits were able to come in from largely undamaged reefs just a few km away. Also, the corals that originally inhabited the reef weren't killed outright. They were a source of asexual fragments that grew into new colonies, just like they do after a hurricane. Many corals are actually "designed" to break this way as a means of dispersal.

    In contrast, following a mass bleaching event there is no fragmentation. Also, the larger geographic scale of the impacts means that it's often a huge distance to the nearest healthy reef, which means sexual recruitment will be very low. This depressed recruitment continues for several years since any corals that don't die outright spend so much energy recovering that they can't devote any to reproduction for quite some time.

    2) There are several MAJOR issues with McNeil et al 2004.

    First of all, their prediction of increased calcification over time directly contradicts what has been observed in the field. See:
    Cohen et al (2008)
    De'ath et al (2009)

    Second, they assume the continuation of the trend despite evidence that it doesn't continue. A lot of the reasons that is a poor assumption are addressed by Kleypas et al, but there are others.

    For one, their prediction is based on the tolerance of Porites, which is probably THE single most temperature tolerant genus. There are numerous reports of it living at temperatures near 100 degrees F, which kill most other corals. On the other hand, other important genera like Acropora (which I think most people would argue is a more important constituent of Pacific reefs than Porites) is much more picky about temperatures. There are several papers comparing the tolerance of Acropora vs. Porites and other genus from Ofu Island, Samoa if you're interested.

    Also, they give brief lip service to the limits of adaptation, but they ignore the fact that those limits completely negate their prediction. Those corals already hosting clade D zoox are essentially already within 2 deg C of their thermal limit. There is no more temperature tolerant clade for them to switch to to adapt to hotter temps. If the temp keeps going up, they won't just adapt and keep calcifying at a higher rate- they will bleach and die.
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  35. I'll ad as well that Al-Sofyani and Davies (1992) showed no relationship between temperature and growth rate for Echinopora over a range of 4 degrees C, which seriously undermines their assumption that the behavior they report in Porites is representative of most corals.
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  36. #5 Rob Painting

    The map of reef stress does not show a correlation between stress and distance from the equator. Why not?

    From my naive point of view, I would expect the water equator to be hotter than the waters closer to the poles. Thus, shouldn't the thermal stress be larger closer to the equator?

    Or is this naive because I'm missing that there are different species of corals at different latitudes and also missing the effect of currents?
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  37. mfripp @ 36 - I would expect the water equator to be hotter than the waters closer to the poles. Thus, shouldn't the thermal stress be larger closer to the equator?

    I imagine that's a common misunderstanding, but coral & their photosymbionts have evolved tolerance to local conditions. There is no single fixed heat tolerance threshold for all coral colonies worldwide, it varies according to the local conditions. Some are better adapted to warmer waters, but despite this they all have one feature in common - they bleach when waters warm too much above the normal summer maximum.

    If, for instance the waters near the equator were warming faster than anywhere else (as your scenario imagines) then yes, I expect we would see bleaching more common near the equator. The background ocean warming isn't large enough (yet) for scenarios like that to occur, instead what we are seeing is more frequent pulses of warm water welling at the sea surface, with no clear pattern of this heat distribution as yet. Modelling studies project bleaching will become more frequent and extensive as ocean temperatures rise.
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  38. #37 Thanks.
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  39. Mike G @34,

    Many thanks for your informative posts....thanks to you I learned something new today. That said, I am annoyed that some "skeptics" posting here tried to misinform everyone, again. They almost had me thanks for setting the record straight. Maybe some of your information could be added to Rob's excellent post?

    Mike and Rob, a quick question if you don't mind. In 2001 my wife and I were on honeymoon in Mauritius and I got to do some snorkeling. The reefs were not in good shape, but I did see a stack of sea urchins. I recall reading an article recently (was is on BBC or ScienceDaily?) that sea urchins are expected to do very well compared to other species as the pH continues to lower and as the oceans warm.

    My question is this. In the event that bleaching and/or higher pH disrupt a coral reef, are they then also at threat from other species (such as sea urchins)moving in and out competing them? If so, that would be yet another in a long line of stressors affecting the corals.
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  40. 39 Albatross

    It's obvious there is a health debate on the issue of corals and climate change in the literature. There's no reason that debate shouldn't be reflected on this website.

    Simply labelling people deniers won't work. Arkadiusz has principally been drawing our attention to published scientific work. I understand you don't like that but it doesn't make him a denier.
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  41. HR @40,

    I have little tolerance for people misrepresenting my views, and you just did exactly that #40. I labeled no-one a "denier" as you claim.

    And,what I most definitely do not like is people distorting and misrepresenting the science as has been done on this thread. That does not constitute a "healthy debate".

    On that note, my frustration has nothing whatsoever to do with me not liking people having opposing views....but rather that having to continually counter the seemingly endless stream of misinformation and misleading statements coming overwhelmingly from "skeptics"-- if that were not an issue then there would be no need for Skeptical Science in the first place.

    Finally, please read the main post again and Rob's post @32 . The scientists in this field are not attributing all the problems facing coral to AGW. In fact, in my post @27 I made specific reference to cumulative impacts. That by definition means that I understand that there is more than one stressor at play here. So again, you have distorted my stance on this issue when you say "Arkadiusz has principally been drawing our attention to published scientific work. I understand you don't like that...". That sounds like innuendo to me.

    The long-term prospects for coral reefs around the world are not promising, and in all likelihood, the literature suggests that the multitude of problems and challenges that coral reefs face are only going to amplify as ocean acidification and bleaching ramp up.
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  42. @HR: "I understand you don't like that but it doesn't make him a denier."

    Enough with the weasel words. You should apologize to Albatross. That would be the classy thing to do.
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  43. @mfripp
    The map in #5 isn't a map of coral stress. In fact, there are no coral reefs in large parts of the highlighted watch areas- e.g. most of the area between Australia and Madagascar or below about 25S.

    The map is just a model estimate of where conditions are likely to cause bleaching based on parameters like sea surface temperature anomalies. Some of the more sophisticated models also take wind and cloud cover into account, though I'm not sure if this particular one does.

    There's no latitudinal pattern to the map because in essence it's a map of cumulative SST anomalies, which don't necessarily increase towards the equator.
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  44. #39
    Off the bat I have to admit that I'm not familiar with the research on the effects of warming or acidification on sea urchins, but we'll assume your recollection is good since it doesn't really change the answer of how that would affect reefs. The simple answer is "it depends."

    Some species of urchins are important grazers that prevent corals from being out-competed by algae. Their presence in sufficient numbers can increase coral cover by ensuring coral larvae have a clear place to settle. However, overabundance may also slightly reduce coral recruitment since they end up scraping off the coral larvae along with the algae they're eating.

    Other urchins like Echinometra tend to have a negative impact on coral cover for a couple of reasons. One is that they're basically the reef's bulldozers. They eat almost everything in their path including coral larvae and coralline algae. Coralline algae are important because they acts as a settlement cue for corals.

    The latter group of urchins are also important because they bore into the structure of the reef itself, and are one of the most important bio-eroders on the reef. In a world where the growth rate of the reef is reduced, a concurrent increase in erosion due to rock-boring urchins would hasten the transition to a "dead" reef- which basically just means that the corals don't grow fast enough to keep up with erosion.

    So depending on which species benefit more from acidification/temp change and how much they benefit, the impact could probably go either way.
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  45. Mike G your knowledge of coral and reef biology has proved quite helpful on this thread, as have the references you put forth. Thanks for your valuable contribution.
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  46. Mike G @ 43 - Good catch, my bad. I didn't realize Mfripp was referring to the graphic I posted. And I second Phillipe's comment, your knowledgeable contributions are greatly appreciated.
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  47. Albatross - They almost had me convinced

    Apologies for that. Maybe I didn't stress the global decline of coral strongly enough?. I had planned to have two other rebuttals ready to go, to address the common coral myths, but haven't quite got there.

    And as far as sea urchin are concerned, although tolerant of lower pH, acidification will impact many of them too at some point if we continue to acidify the oceans. The sea is basically an organic soup of sperm, eggs and juvenile life stages of marine organisms. It's at this stage that many are vulnerable. Sea urchin too. The overall view is that they will be impacted later rather than sooner under business-as-usual scenarios. I understand they are not at immediate risk though, unlike coral.

    A recent metadata analysis here: Impact of near-future ocean acidification on echinoderms - Dupont 2010

    How many skeptics eat sea urchin & starfish I wonder?.
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  48. Apologies Albatross for mixing up denier and skeptic but it makes little difference. There is obviously some debate in the scientific community about the issue of coral reef calcification and climate change. The debate is not the result of skeptics mis-information. Address that issue rather than making vague claims about posts here.

    With regard to you're personnal observation about Mauritius in 2001. There's a report here covering the period you were there. They seem to rule out ocean changes associated with climate change. In the worst declining study area they clearly blame the pressure of tourism.

    "The patterns in the reef community structure could not be linked to any of the recorded environmental parameters (sea surface temperature, salinity, pH and dissolved oxygen). It is postulated that these parameters were not the appropriate indicators for monitoring the observed changes in the coral reefs."

    "In recent years, the nature of the coastal activities in this area has shifted from an agro-industrial area in the past to presently hosting the main tourism related developments in the southern part of the island......This reef was previously reported as being rich and healthy and has been badly damaged by algae and cyanobacteria from very recent nutrient inputs to the coastal waters.....Since sugar cane cultivation on the nearby hill slopes is not new, the source of nutrients is from land alterations during the development and most likely from leakages from septic systems of new hotel developments"

    ( -edit- )
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    Moderator Response: (Daniel Bailey) Cease making things personal.
  49. HR - There is obviously some debate in the scientific community about the issue of coral reef calcification

    That's one way of putting it, not very accurate though. The calcification predictions made in McNeil 2004 aren't actually occurring. Doesn't that dent your confidence in their predictions?. Aside from all the other faulty assumptions in their paper?.

    Note the links provided by Mike G @ 34. This from De'ath 2009.

    A 14% calcification decline in the Great Barrier Reef since 1990. Similar results in South-East Asia, the Caribbean and a whopping 50% decline in the cooler water reefs of Bermuda.

    What debate?.
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  50. @ Mike G

    As we can see and the various works - showing a different response of corals to temperature and reduce the alkalinity of water - I have presented only saying that the corals “will cope”. This is a "cherry picking", but only because they lacked the cited work - by me - above.

    Are coral reefs "will cope"?

    The worst of the corals will be very close to the pole (there are only 1-2 species of reef-building a whole), but:
    Lough and Barnes, 2000., comparison the two 20-year periods 1903-1922 and 1979-1998: "the SST-associated increase in calcification is estimated to be less than 5% in the northern GBR, ~12% in the central GBR, ~20% in the southern GBR and to increase dramatically (up to ~50%) to the south of the GBR."

    Is today's coral species survived similar changes in pH of the oceans - and temperature - as it is today?

    CO2 emissions causing ocean acidification to progress at unprecedented rate, National Research Council US:
    “Since the beginning of the industrial revolution, the average pH of ocean surface waters has decreased approximately 0.1 unit -- from about 8.2 to 8.1 -- making them more acidic. Models project an additional 0.2 to 0.3 drop by the end of the century. This rate of change exceeds any known to have occurred in hundreds of thousands of years, the report says.” “The results showed decreases in shell and skeletal growth in a range of marine organisms, including reef-building corals, commercially important mollusks such as oysters and mussels, and several types of plankton at the base of marine food webs.”

    It is true that we have only regional (but based on a consistent method of research) work in identifying variation in the pH of the oceans during the Last Glacial Period and the Holocene, but probably they may be representative for the whole ocean.

    Abrupt sea surface pH change at the end of the Younger Dryas in the central sub-equatorial Pacific inferred from boron isotope abundance in corals (Porites), Douville et al., 2010.:

    “Modern coral 11B values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the Sea Water Scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values.”

    ... but:

    “This last finding provides further evidence of the marked changes to the water mass pH and temperature properties in the equatorial Pacific at the Younger Dryas- Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO2 content.”
    “An abrupt pH drop to 8.05 [-0,2 pH - unit] was observed at the end of the Younger Dryas around 11.5 kyr BP.” I recommend Figure 6 - circa 8.1 pH was the case 2 and 6 thousand years ago.

    In paper Instability of seawater pH in the South China Sea during the mid-late Holocene : Evidence from boron isotopic composition of corals, Liu et al., 2009. On this figure, we see a rapid decrease in pH of seawater circa 4,000 years ago and a pH lower than today - 6 thousand years ago.

    The Impact of Climate Change on the World’s Marine Ecosystems, Hoegh-Guldberg and Bruno, 2010.:
    There is a sentence like this: “Marine ecosystems are centrally important to the biology of the planet, yet a comprehensive understanding of how anthropogenic climate change is affecting them has been poorly developed.”
    ... but also a this sentence: “Recent studies indicate that rapidly rising greenhouse gas concentrations are driving ocean systems toward conditions not seen for millions of years, with an associated risk of fundamental and irreversible ecological transformation.”
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