Are the impacts of climate change on coral reefs exaggerated? Questions and Answers (Part 1)

For a long time the New Scientist has waged an ongoing battle with the climate change “skeptics”, and have produced some thorough articles such as “Climate change: a guide for the perplexed“, a round-up of the 26 most common climate myths and misconceptions. Time and time again I see people use similar myths and misconceptions regarding corals and coral reefs that are used as an argument as to why global warming is clearly a hoax, how warm water is good for corals (and the list goes on). In response to recent debates, below is the first part of a series called “Are the impacts of climate change on coral reefs exaggerated? Questions and Answers” in which I hope to address these misconceptions following the scientific evidence. Over the coming weeks I will aim to add more in the series: please feel free to add or ask any further questions in the comments below or email me at climateshifts @

1. “Warm water is good for corals”

Corals are locally adapted to the water temperature that they live in. This has taken many hundreds if not thousands of years to occur. It does not happen over decades, which would be the requirement if corals were to tolerate and survive the very rapid changes in sea temperature that we are currently facing.

The statements that “corals calcify faster in warmer waters” and “hotspots of coral diversity are found in warm waters close to the equator” are indeed true, but these conditions are only good for those corals that have adapted to these warmer conditions. For example, if you take corals from the southern end of the Great Barrier Reef and put them at the northern end of the Great Barrier Reef, these corals will suffer from being exposed to warmer than normal conditions and will die.

Although corals thrive within the upper limits of their thermal tolerance (within 1-2ºC), coral bleaching occurs when this tolerance is exceeded, resulting in loss of photosynthetic function, expulsion of symbiotic algae, and ultimately death of the coral. Clearly warm water is beneficial to those corals that are adapted to these warmer temperatures, although exceeding these thresholds results in mortality – a precarious balance.

With respect to the statement “corals in Moreton Bay are regularly stressed as the water is too cold” – it is well-known that corals in Moreton Bay (and other high latitude regions) where conditions that drop below 18°C in the winter lead to coral death. Just like they are sensitive to being too hot, they are also sensitive to becoming too cold. This is called the physiological range or tolerance of species. Conditions at places like Moreton Bay are marginal and therefore an outlier in global coral reefs and are restricted by their latitudes by cold winters.

2. “Growth rates of the large massive corals have been shown to increase over the last 100 years, consistent with the increasing water temperature”

Invariably this is attributed to Lough & Barnes (1997), and states that although highly variable, massive corals might respond to warming sea surface temperatures by increasing their calcification rates. However, recent publications (e.g. Cooper et al 2008, Lough in press) are starting to provide evidence of recent declines in Porites growth despite continued SST warming, which is of considerable cause for concern. Cooper et al (2008) conclude that “Although our findings are consistent with studies of the synergistic effect of elevated seawater temperatures and pCO2 on coral calcification, we conclude that further data on seawater chemistry of the GBR are required to better understand the links between environmental change and effects on coral growth”

3. “Corals can take in different clades of zooxanthellae which protect can protect against coral bleaching”

The evidence for the ability of corals to completely change the types of zooxanthellae that they have in their tissues is largely unsupported by the scientific literature. The few experiments such as that performed by Baker (2001) have some critical flaws in them as identified by Hoegh-Guldberg et al (2002). There are now many other studies that showed that no matter what happens to symbiosis (seasonal, bleaching etc), the host coral maintains its association with particular clades of zooxanthellae that it has co-evolve with.

Observations of corals switching from one type of zooxanthellae to another involve corals that have developed a symbiosis with two or more clades of zooxanthellae. This observation of a change in the dominant zooxanthellae in this case does not indicate that corals have initiated a new symbiosis with a completely new type of zooxanthellae. Switching from one type of zooxanthellae to another that you already have a symbiosis with is no different to the physiological tolerance range that most organisms have. Only by the development of a brand new symbiosis with higher thermal tolerance would there be any chance of a rapid change in thermal tolerance.

Even if we did show that corals could take on a new clade of zooxanthellae, there are a number of important problems with this idea. Firstly, coral hosts will have problems with higher temperature which will not be solved by changing the type of symbiont that you have in your tissues. That is, corals also has thermal limits. Secondly, the observation that a biological feature may exist is not proof that it will be the type of powerful ecological mechanism that one would need to cope with climate change. This is an important point, as many studies extrapolate from simplistic science to the ecosystem and climate change.

4. “Despite the bleaching events we have seen (most notable in 1998 and 2002), most of the reef did not bleach and most that did has almost fully recovered”

To quote the Great Barrier Marine Park Authority on the issue:

“During the 1998 global mass bleaching event sea surface temperatures in the Great Barrier Reef reached levels that were the highest ever recorded. Aerial surveys showed that, on average, about 50 per cent of reefs suffered bleaching; 87 per cent of inshore reefs and 28 per cent of mid-shelf and offshore reefs

The Great Barrier Reef suffered another mass bleaching event in 2002; this was the largest on record. Two periods of hot weather resulted in sea surface temperatures a few degrees centigrade higher than long-term summer maxima. Aerial surveys revealed that on average 60 per cent of reefs were bleached. Inshore reefs were again the most affected—69 per cent. A greater proportion of mid-shelf and offshore reefs bleached in 2002 than in 1998—51 per cent. Mortality was variable and generally low, but again, about five per cent of reefs were severely damaged.

More recently, bleaching affected the southern section of the Great Barrier Reef in the summer of 2006. The inshore reefs off Rockhampton were badly affected by the unusually warm water that persisted in the area for over two months. Conditions were especially severe in the Keppel Islands, where over 80 per cent of corals bleached and 40 per cent died as a result of bleaching stress. Fortunately, the remainder of the Great Barrier Reef suffered very little bleaching in that year.

Stressful increases in temperature are predicted to occur more frequently in the future due to climate change. Coral bleaching is therefore one of the most serious long-term threats to the Great Barrier Reef. Current model predictions are for rises in average water temperature in the Great Barrier Reef of between 1 and 3°C over the course of the century. Bleaching events are likely to increase in frequency and severity under these scenarios.”(link)

5. “Sea level rise is what we really need to reclaim the now dead reef flats that were caused by sea level fall since 5000 yrs b.p.”

Most reefs are known to have accreted the bulk of their growth between 8000-5000YBP (Smithers et al 2006) where conditions for reef growth were optimum. Around 5500YPB, relative sea levels were around 1m above present (Hopley & Thom 1983) and have fallen gradually since. However, to assume that the current project rise in sea level as a result of global warming (13 – 59cm depending upon the SRES scenario, Fourth IPCC) would result in an inundation of reef flats and a virtual utopia for coral growth is sadly naïve. In assuming that sea level is occurring (and therefore resulting from global warming), the scenario negates to mention the increases in frequencies in bleaching events or ocean acidification (Hoegh-Guldberg et al 2007), which would be seriously deleterious for coral growth on the GBR.

6. “There is a good case for the proposition that moderate warming is a good thing. One or two degree would be ideal, any more than that too quickly is another matter.”

Largely, these sorts of perspectives are ignorant of the majority of biological literature. At this point (and without any scientific evidence or back up) this is merely speculation and negates a phenomenal quantity of literature on the thermal thresholds and tolerances of corals on the GBR.

7. “The other unknown is the influence of CO2 concentrations on ocean pH”

As outlined in a Science paper in 2007 with 17 eminent marine scientists as authors, the influence of CO2 concentrations on ocean pH (and the resulting impact upon the world’s oceans and coral reefs) is far from an unknown.

One of the most important graphics in this paper is that provided by one of the authors, Professor Ken Caldeira at Stanford University. Focusing on the panel for today (380 ppm), one sees that the pink dots (the sites of where coral reefs are found today) are enclosed in water that has aragonite saturation constant that is greater than 3.25. This is the concentration of calcium and carbonate relative to the aragonite solubility that is required for the formation and maintenance of carbonate Reef system such as the Great Barrier Reef. As the CO2 concentrations increases in the atmosphere, the aragonite saturation constant for waters across the planet plummets. This puts the existing distribution of coral reefs outside the required aragonite saturation constants for the carbonate coral reef development and maintenance.

The image below on the left outlines the linkages between the build-up of atmospheric CO2 and the slowing of coral calcification due to ocean acidification. The graph on the right shows temperature CO2 and carbonate-ion concentration reconstructions for the past 420,000 years. It is clear from this that ocean acidification is a serious threat, and as indicated by the red line, we are heading towards an atmospheric CO2 concentration of greater than 500ppm, a pathway with great cause for concern.

From the same article, the image below contains extant examples of reefs from the GBR, used as analogs for the ecological structure of (A) stabilisation of the present CO2 conditions (380ppm), (B) 400 – 500ppm, and (C) >500ppm. The result and influence of increasing CO2 concentrations on the worlds oceans are far from unknown.

8. “The high SSTs associated with coral bleaching result from extended periods of calm not unusually hot weather”

The conditions that cause coral bleaching arise from periods of warmer than normal sea temperatures. This arises due to calm seas and cloudless skies, usually in the summer months. Under conditions seen prior to the Industrial Revolution, these conditions occurred over and above the sea temperature that was at least 1° less than today. In those situations, coral bleaching was probably a rare event because sea temperatures did not rise as high as they do today (that is due to that extra 1° of background temperature). Today, however these periods of clear skies and calm seas rapidly lead to the elevation of the sea temperature in the upper layers of the ocean. This, as we’ve seen in numerous examples over the past 25 years, has led to periods in which the sea temperatures have caused large-scale mass coral bleaching. So, as with everything there is interannual variability – the problem today is that this variability is building on top of rapidly rising sea temperatures.

9. “In December I made an extensive cruise over some 600 Km of the far northern GBR. The general condition of the reefs we visited was superb with high levels of coral cover and no observable evidence of bleaching.”

In any of spot visits to the Great Barrier Reef there is considerable variability in space and time with respect to the condition of coral communities across the Great Barrier Reef. Bruno & Selig’s excellent study “Regional decline of coral cover in the Indo-Pacific: timing, extent, and subregional comparisons” summarises the results of 6000 measurements of coral cover over the past 40 to 50 years. The results indicate that coral cover is sliding downwards at 1% per year, and may have doubled over the past decade.

As John Bruno highlighted:

-Very few of the reefs were “coastal” even by GBR standards. Most were of offshore and very isolated reefs.
-None that I can think of were performed to quantify the effects of the 98 El Nino. studies would be biased and non-representative if they were simply single-site documentaries of local gloom and doom. Luckily, for the GBR, the AIMS folks perform a truly astounding amount of annual monitoring of reefs that were selected decades ago, so the survey reef choice is not biased by current conditions.
-The AIMS GBR data indeed contains evidence of recovery, in terms of total coral cover, of some reefs, particularly on the southern GBR, but from what I understand, this is mostly recovery from COTS outbreaks and cyclones. Local declines in coral cover (on other reefs) due to bleaching in 98 and 02 balances this out.
-Overall, the best available data indicates that the coral cover of the GBR began to decline a few decades ago. It has been more or less static for the last decade. But we think it is far below the historical baseline.

10. “There is NO appreciable long-term warming of the GBR and a small amount of recent warming can be attributed either to preceding El Nino conditions or to variations in cloud cover”

In 1999 I published a paper entitled “Climate change, coral bleaching and the future of the world’s coral reefs”. The article has been cited over 455 times, is the most cited paper in Coral Ecology, and the most cited paper ever in the International Journal of Marine and Freshwater Research. In this article I analysed the Comprehensive Ocean-Atmosphere Data Set (up to Dec 1992) and IGOSS-nmc blended data set (January 1993 – April 1999) and came to the following conclusions:



Period of data examined

Rate (ºC per 100 years)

Significance of trend

Southern GBR





p < 0.001


Central GBR




p < 0.001


Northern GBR




p < 0.001


The leading authority on recent change within the Great Barrier Reef waters is Dr Janice Lough of the Australian Institute of Marine Science. Janice has carefully put together the information on what has been happening more recently with temperatures on the Great Barrier Reef. You can find a very nice summary of this work in chapter 2 of the Vulnerability Assessment produced by Great Barrier Reef experts last year.

While these changes to the temperature of the waters over the Great Barrier Reef look small, they are large relative to the rate of change over the past 420,000 years. An analysis of this can be found in Table 1 of a recent science article which we compiled with 16 other experts (Link). In short, they are at least a hundred times higher than the fastest rate of change over the past 420,000 (ice age transitions included).

2 thoughts on “Are the impacts of climate change on coral reefs exaggerated? Questions and Answers (Part 1)

  1. As a biologist working on the GBR as an interpretive guide I am confronted every day with argumentative guests over the threats facing the worlds coral reefs. I usually have good answers but sometimes I struggle to back them up. THANKYOU for putting this on the web and PLEASE keep it updated, fresh and topical ! Please.

  2. Pingback: Climate Shifts » Uncategorized » Declining calcification on the Great Barrier Reef

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