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Earth's Incredible Dissolving Corals
Volume 12, Number 21: 27 May 2009

In a paper recently published in Geophysical Research Letters, Silverman et al. (2009) created a model of coral calcification based on field observations of gross community calcification as a function of aragonite saturation state (Ωarag), sea surface temperature (SST) and live coral cover, after which they calculated calcification rates for more than 9,000 reef locations using model values of Ωarag and SST at different atmospheric CO2 concentrations, which exercise led them to conclude that "by the time atmospheric partial pressure of CO2 will reach 560 ppm, all coral reefs will cease to grow and start to dissolve."

What's wrong with this picture?

For starters -- and as actually acknowledged by the researchers themselves -- "coral reefs were exposed throughout their geological history to higher temperatures and CO2 levels than at present and yet have persisted," which is a pretty amazing admission for them to make, in light of the fact that they have boldly declared that when the atmosphere's CO2 concentration reaches 560 ppm in the not too distant future, "all coral reefs will cease to grow and start to dissolve."

So how did the five modelers get things so wrong? ... as we clearly believe they did.

For one thing, they say their calculations "are based on the assumption that an increase of 1°C in the maximum summer monthly average SST [relative to pre-Industrial Revolution or PIR values] will result in bleaching that will reduce the live coral cover [of a reef] by 50%." This means that if a reef's live coral coverage parameter (AC, which can vary from 1.0 to 0.0) was initially 0.5, it will decline to 0.25, as they describe it, "when monthly average model SST increases by >=1°C above the temperature of the warmest month during PIR," and that "a further decrease of AC to 0.125 is invoked by the model on the next encounter with >=1°C SST increase," so that the reef's live coral coverage gradually dwindles away to next to nothing over the course of subsequent SST spikes.

Fortunately, real-world corals do not behave in this manner. They almost always recover from bleaching episodes, and they come back even better prepared for the next bleaching, so that equally severe -- or even more severe -- high temperature anomalies often have less of a negative effect on them than prior heat waves had; and this phenomenon enables earth's corals to indefinitely maintain -- and possibly even expand --their undersea structures, which end result is just the opposite of what Silverman et al. assume in their model.

In describing the work of Adjeroud et al. (2002), for example, Adjeroud et al. (2005) reported that an interannual survey of reef communities at Tiahura on the French Polynesian island of Moorea "showed that the mortality of coral colonies following a bleaching event was decreasing with successive events [our italics], even if the latter have the same intensity."

Commenting on these and the similar observations of others, the seven French scientists additionally noted that the "spatial and temporal variability of the impacts observed at several scales during the present and previous surveys may reflect an acclimation and/or adaptation of local populations," such that "coral colonies and/or their endosymbiotic zooxanthellae may be phenotypically and possibly genotypically resistant to bleaching events," citing the work of Rowan et al. (1997), Hoegh-Guldberg (1999), Kinzie et al. (2001) and Coles and Brown (2003) in support of this conclusion.

Still other researchers have also confirmed the phenomenon of thermal adaptation in coral reefs. Guzman and Cortes (2007), for example, studied coral reefs of the eastern Pacific Ocean that had "suffered unprecedented mass mortality at a regional scale as a consequence of the anomalous sea warming during the 1982-1983 El Niņo." At Cocos Island, in particular, they found in a survey of three representative reefs (which they conducted in 1987) that the remaining live coral cover was only 3% of what it had been prior to the occurrence of the great 1982-1983 El Niņo (Guzman and Cortes, 1992); and based on this finding and the similar observations of other scientists at other reefs, they predicted that "the recovery of the reefs' framework would take centuries, and recovery of live coral cover, decades." Just 15 years later, however, they found that the mean live coral cover had increased nearly five-fold -- from 2.99% in 1987 to 14.87% in 2002 -- at the three sites studied during both periods, while the mean live coral cover of all five sites studied in 2002 was 22.7%. In addition, they found that "most new recruits and adults belonged to the main reef building species from pre-1982 ENSO, Porites lobata, suggesting that a disturbance as outstanding as [the 1982-1983] El Niņo was not sufficient to change the role or composition of the dominant species."

The most interesting aspect of the study, however, was the fact that a second major El Niņo occurred between the two assessment periods; and Guzman and Cortes state that "the 1997-1998 warming event around Cocos Island was more intense than all previous El Niņo events," noting that temperature anomalies "above 2°C lasted 4 months in 1997-1998 compared to 1 month in 1982-83." Nevertheless, they determined that "the coral communities suffered a lower and more selective mortality in 1997-1998 [our italics], as was also observed in other areas of the eastern Pacific (Glynn et al., 2001; Cortes and Jimenez, 2003; Zapata and Vargas-Angel, 2003)," which finding is indicative of a significant thermal adaptation following the 1982-83 El Niņo.

One year later, in a paper published in Marine Biology, Maynard et al. (2008) described how they analyzed bleaching severity in three coral genera (Acropora, Pocillopora and Porites) via underwater video surveys of five sites in the central section of Australia's Great Barrier Reef in late February and March of 1998 and 2002, while contemporary sea surface temperatures were acquired from satellite-based Advanced Very High Resolution Radiometer data that were calibrated to ship- and drift buoy-obtained measurements, and surface irradiance data were obtained "using an approach modified from that of Pinker and Laszlo (1991)."

With respect to temperature, the four researchers report that "the amount of accumulated thermal stress (as degree heating days) in 2002 was more than double that in 1998 at four of the five sites," and that "average surface irradiance during the 2002 thermal anomaly was 15.6-18.9% higher than during the 1998 anomaly." Nevertheless, they too found that "in 2002, bleaching severity was 30-100% lower than predicted from the relationship between severity and thermal stress in 1998, despite higher solar irradiances during the 2002 thermal event." In addition, they found that the coral genera that were originally most susceptible to thermal stress (Pocillopora and Acropora) "showed the greatest increase in tolerance."

In discussing their findings, Maynard et al. said they were "consistent with previous studies documenting an increase in thermal tolerance between bleaching events (1982-1983 vs. 1997-1998) in the Galapagos Islands (Podesta and Glynn, 2001), the Gulf of Chiriqi, the Gulf of Panama (Glynn et al., 2001), and on Costa Rican reefs (Jimenez et al., 2001)," and they noted that Dunne and Brown (2001) found similar results in the Andaman Sea, in that "bleaching severity was far reduced in 1998 compared to 1995 despite sea-temperature and light conditions being more conducive to widespread bleaching in 1998."

As for the significance of these and other observations, the Australian scientists stated that "the range in bleaching tolerances among corals inhabiting different thermal realms suggests that at least some coral symbioses have the ability to adapt to much higher temperatures than they currently experience in the central Great Barrier Reef," citing the work of Coles and Brown (2003) and Riegl (1999, 2002). In addition, they stated that "even within reefs there is a significant variability in bleaching susceptibility for many species (Edmunds, 1994; Marshall and Baird, 2000), suggesting some potential for a shift in thermal tolerance based on selective mortality (Glynn et al., 2001; Jimenez et al., 2001) and local population growth alone." Hence, they concluded their results suggested "a capacity for acclimatization or adaptation."

In bringing their paper to a close, Maynard et al. wrote "there is emerging evidence of high genetic structure within coral species (Ayre and Hughes, 2004)," which suggests that "the capacity for adaptation could be greater than is currently recognized." In fact, as stated by Skelly et al. (2007), "on the basis of the present knowledge of genetic variation in performance traits and species' capacity for evolutionary response, it can be concluded that evolutionary change will often occur concomitantly with changes in climate as well as other environmental changes [our italics]." Consequently, it can be appreciated that if global warming were to begin again (there has been none over the last decade), it would not spell the end for earth's highly adaptable corals, which observation-based conclusion stands in direct contradiction of one of the key assumptions of Silverman et al.'s calcification model.

Another reality check that can be made on the calcification model of Silverman et al. derives from the fact that the current aragonite saturation state of the global ocean (calculated for an atmospheric CO2 concentration of 380 ppm) and the current array of global sea surface temperatures indicate, when processed by the model, "that most coral reefs are already calcifying 20% to 40% less than their pre-Industrial Revolution rates."

With respect to this conclusion, we first consider the work of Pelejero et al. (2005), who developed a reconstruction of seawater pH spanning the period 1708-1988, based on the boron isotopic composition (δ11B) of a long-lived massive Porites coral from Flinders Reef in the western Coral Sea of the southwestern Pacific. Their results indicated there was "no notable trend toward lower δ11B values" over the 300-year period, which began "well before the start of the Industrial Revolution". Instead, they say "the dominant feature of the coral δ11B record is a clear interdecadal oscillation of pH, with δ11B values ranging between 23 and 25 per mil (7.9 and 8.2 pH units)," which "is synchronous with the Interdecadal Pacific Oscillation." Furthermore, they calculated aragonite saturation state values from the Flinders pH record that varied between about 3 and 4.5, which values encompass, in their words, "the lower and upper limits of aragonite saturation state within which corals can survive." Nevertheless, they report that the "skeletal extension and calcification rates for the Flinders Reef coral fall within the normal range for Porites and are not correlated with aragonite saturation state [our italics]." What is more, Liu et al. (2009) recently used a similar approach to reconstruct a pH history of the South China Sea, discovering in the process that some six thousand years ago it was slightly more acidic (actually less basic) than it is now.

In another study of historical calcification rates, which was based upon coral cores retrieved from 35 sites on Australia's Great Barrier Reef, Lough and Barnes (1997) observed a statistically significant correlation between coral calcification rate and local water temperature, such that a 1°C increase in mean annual water temperature increased mean annual coral calcification rate by about 3.5%. Nevertheless, they report there were "declines in calcification in Porites on the Great Barrier Reef over recent decades." They were quick to point out, however, that their data depicted several extended periods of time when coral growth rates were either above or below the long-term mean, cautioning that "it would be unwise to rely on short-term values (say averages over less than 30 years) to assess mean conditions."

As an example of this fact, they report that "a decline in calcification equivalent to the recent decline occurred earlier this [20th] century and much greater declines occurred in the 18th and 19th centuries," long before anthropogenic CO2 emissions had made much of an impact on the air's CO2 concentration. In fact, over the entire length of their data set, Lough and Barnes say "the 20th century has witnessed the second highest period of above average calcification in the past 237 years," which is not exactly what one would expect in light of (1) how high water temperatures are often claimed to be deadly for corals, (2) the climate-alarmist claim that the earth is currently warmer than it has been at any other time during the past thousand years, and (3) the fact that the air's CO2 content is currently much higher than it has been for far longer than a mere millennium.

Similar findings have been reported by Bessat and Buigues (2001), who derived an 1801-1990 history of coral calcification rates based on a core extracted from a massive Porites coral on the French Polynesian island of Moorea. This they did because, as they describe it, "recent coral-growth models highlight the enhanced greenhouse effect on the decrease of calcification rate." And rather than relying on theoretical calculations, they wanted to work with real-world data, stating that the records preserved in ancient corals "may provide information about long-term variability in the performance of coral reefs, allowing unnatural changes to be distinguished from natural variability."

So what did the two researchers learn? First of all, they found that a 1°C increase in water temperature increased coral calcification rate by 4.5%, which led them to state that "instead of a 6-14% decline in calcification over the past 100 years computed by the Kleypas group, the calcification has increased, in accordance with [the findings of] Australian scientists Lough and Barnes." They also observed patterns of "jumps or stages" in the record, which were characterized by an increase in the annual rate of calcification, particularly at the beginning of the past century "and in a more marked way around 1940, 1960 and 1976," stating once again that their results "do not confirm those predicted by the Kleypas et al. (1999) model."

Another major blow to the Kleypas et al. model -- and, by extension, the Silverman et al. model -- was provided by the work of Lough and Barnes (2000), who assembled and analyzed the calcification characteristics of 245 similar-sized massive colonies of Porites corals obtained from 29 sites scattered along the length and breadth of Australia's Great Barrier Reef (GBR), which data spanned a latitudinal range of approximately 9° and an annual average SST range of 25-27°C. To these data they added other published data from the Hawaiian Archipelago (Grigg, 1981, 1997) and Phuket, Thailand (Scoffin et al., 1992), thereby extending the latitudinal range of the expanded data set to 20° and the annual average SST range to 23-29°C.

This analysis revealed that the GBR calcification data were linearly related to the average annual SST data, such that "a 1°C rise in average annual SST increased average annual calcification by 0.39 g cm-2 year-1." Results were much the same for the extended data set; and Lough and Barnes reported that "the regression equation [calcification = 0.33(SST) - 7.07] explained 83.6% of the variance in average annual calcification," noting that "this equation provides for a change in calcification rate of 0.33 g cm-2 year-1 for each 1°C change in average annual SST."

With respect to the significance of their findings, Lough and Barnes said they "allow assessment of possible impacts of global climate change on coral reef ecosystems," and between the two 50-year periods 1880-1929 and 1930-1979, they calculated an increase in calcification of 0.06 g cm-2 year-1, noting that "this increase of ~4% in calcification rate conflicts with the estimated decrease in coral calcification rate of 6-14% over the same time period suggested by Kleypas et al. (1999) as a response to changes in ocean chemistry [our italics]." Even more stunning was their observation that between the two 20-year periods 1903-1922 and 1979-1998, the SST-associated increase in calcification was just under 5% in the northern GBR, about 12% in the central GBR, about 20% in the southern GBR and up to 50% to the south of the GBR.

In light of these real-world observations, and in stark contrast to the doom-and-gloom prognostications of the world's climate alarmists, Lough and Barnes concluded that coral calcification rates "may have already significantly increased along the GBR in response to global climate change."

Because of findings such as those described above, Buddemeier et al. (2004) were forced to acknowledge that "calcification rates of large heads of the massive coral Porites increased rather than decreased over the latter half of the 20th century," further noting that "temperature and calcification rates are correlated, and these corals have so far responded more to increases in water temperature (growing faster through increased metabolism and the increased photosynthetic rates of their zooxanthellae) than to decreases in carbonate ion concentration."

Finally, in a study devoted to corals that involves a much longer period of time than all of the others we have discussed, another research team (Crabbe et al., 2006) determined the original growth rates of long-dead Quaternary corals found in limestone deposits of islands in the Wakatobi Marine National Park of Indonesia, after which they compared them to the growth rates of present-day corals of the same genera living in the same area. This work revealed that the Quaternary corals grew "in a comparable environment to modern reefs," except, of course, for the air's CO2 concentration, which is currently higher than it has been at any other time throughout the entire Quaternary, which spans the past 1.8 million years. Most interestingly, therefore, their measurements indicated that the radial growth rates of the modern corals were 31% greater than those of their ancient predecessors in the case of Porites species, and 34% greater in the case of Favites species.

To these papers we could add many others that also depict increasing rates of coral calcification in the face of rising temperatures and atmospheric CO2 concentrations, including Clausen and Roth (1975), Coles and Jokiel (1977), Kajiwara et al. (1995), Nie et al. (1997), Reynaud-Vaganay et al. (1999), Reynaud et al. (2007) and Lough (2008). Clearly, therefore, the "unprecedented" 20th-century increases in atmospheric CO2 and temperature appear to have not harmed earth's corals. In fact, they actually appear to have helped them, which is a far cry from Silverman et al.'s contention that most coral reefs are currently calcifying at rates that are "20% to 40% less than their pre-Industrial Revolution rates." Just like the incredible shrinking man, earth's incredible dissolving corals are pure science fiction.

Sherwood, Keith and Craig Idso

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