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Coral Reefs (Calcification) -- Summary
For some time now, the ongoing rise in the air's CO2 content has been predicted to raise havoc with earth's coral reefs in two different ways: (1) by stimulating global warming, which has been predicted to dramatically enhance coral bleaching, and (2) by lowering the calcium carbonate saturation state of seawater, which has been predicted to reduce coral calcification rates.  We review the first of these predictions elsewhere on our website (see Coral Reefs (Bleaching)).  Here, we discuss the latter topic, attempting to present what is really happening in this area.  And what is really happening is that a purely hypothetical concept is being promulgated as a fact of nature, which it clearly is not.

Several researchers have suggested that many of earth's corals are destined to die, with some species even facing extinction, because of an hypothesized connection between the ongoing rise in the air's CO2 content and reduced rates of coral calcification (Smith and Buddemeier, 1992; Buddemeier, 1994; Buddemeier and Fautin, 1996a,b; Gattuso et al., 1998; Buddemeier, 2001).  Kleypas et al. (1999), for example, calculated that calcification rates of tropical corals should already have declined by 6 to 11% or more since 1880, as a result of the concomitant increase in atmospheric CO2 concentration; and they predict that the reductions could reach 17 to 35% by 2100, as a result of expected increases in the air's CO2 content over the coming century.  Likewise, Langdon et al. (2000) calculate a decrease in coral calcification rate of up to 40% between 1880 and 2065.

Such predictions are tenuous at best, and at worst may be wholly incorrect; for as Idso et al. (2000) have noted, coral calcification is more than a physical-chemical process described by a set of well-defined equations.  It is a biologically-driven physical-chemical process that may not be amenable to explicit mathematical description.  They state, for example, that "photosynthetic activity of zooxanthellae is the chief source of energy for the energetically-expensive process of calcification," and that much evidence suggests that "long-term reef calcification rates generally rise in direct proportion to increases in rates of reef primary production."  They also note that "the calcium carbonate saturation state of seawater actually rises with an increase in temperature, significantly countering the direct adverse oceanic chemistry consequences of an increase in atmospheric and/or hydrospheric CO2 concentration."  And these simple facts lead them to conclude that "the negative predictions of today could well be replaced by positive predictions tomorrow."

Support for the Idso et al. view of the subject is provided by many real-world studies, including that of Gattuso et al. (1998), who note that any CO2-induced decrease in the calcium carbonate saturation state of seawater could well be offset by additional CO2-induced weathering of terrestrial carbonates that would release more Ca2+ to the world's oceans, which, they say, would "likely occur under elevated atmospheric pCO2."  They also note that corals display impressive acclimation responses to changes in various environmental parameters, and that increasing the global ocean's pCO2 content could well increase the photosynthetic activity of coral symbionts, which would also tend to counteract any decrease in coral calcification rate that may result from a CO2-induced decrease in seawater carbonate saturation state.

In a review of related topics, Gattuso et al. (1999) additionally note that coral calcification rates tend to rise in response to increases in water temperature, and that photosynthesis and calcification are tightly coupled in most corals and coral reef communities, with the metabolic CO2 derived from symbiont photosynthetic products functioning as a significant source of carbon for the calcification process.  It is not surprising, therefore, that in a study of historical calcification rates determined from coral cores retrieved from 35 sites on the Great Barrier Reef, Lough and Barnes (1997) observed a statistically significant correlation between coral calcification rate and local water temperature, such that a 1C 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 are quick to point out, however, that their data depict 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 century and much greater declines occurred in the 18th and 19th centuries," long before anthropogenic CO2 emissions made much of an impact on the air's CO2 concentration.  In fact, over the entire expanse 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 dangerous high water temperatures are often said to be for corals, (2) the climate-alarmist claim that earth is currently warmer than it has been at any other time during the entire past millennium, and (3) the fact that the air's CO2 content is currently much higher than it has been for far longer than a mere thousand years.

Similar findings were reported by Bessat and Buigues (2001), who derived a history of coral calcification rates from a core extracted from a massive Porites coral head on the French Polynesian island of Moorea that covered the period 1801-1990.  They performed this work, they say, because "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 Bessat and Buigues learn?  First of all, they found that a 1C increase in water temperature increased coral calcification rate at the site they studied by 4.5%.  Then, they found 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 results 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 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 reef sites located along the length, and across the breadth, of Australia's Great Barrier Reef (GBR), which data spanned a latitudinal range of approximately 9 and an annual average sea surface temperature (SST) range of 25-27C.  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 23-29C.

This analysis revealed that the GBR calcification data were linearly related to the average annual SST data, such that "a 1C 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; Lough and Barnes report that "the regression equation [calcification = 0.33(SST) - 7.07] explained 83.6% of the variance in average annual calcification (F = 213.59, p less than 0.00)," noting that "this equation provides for a change in calcification rate of 0.33 g cm-2 year-1 for each 1C change in average annual SST."

With respect to the significance of their findings, Lough and Barnes say they "allow assessment of possible impacts of global climate change on coral reef ecosystems," and between the two 50-year periods 1780-1829 and 1930-1979, they calculate a calcification increase 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."  Even more stunning is their observation that between 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."

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."  But in spite this compelling evidence, as well as the similar findings of others, claims of impending coral doom caused by rising air temperatures and CO2 concentrations have continued to rear their ugly heads ... on a regular basis ... and in the usual places.

In Nature, it was Caldeira and Wickett (2003) who kept the catastrophe ball rolling.  Based on a geochemical model, an ocean general-circulation model, an IPCC CO2 emissions scenario for the 21st century, and a logistic function for the burning of earth's post-21st century fossil-fuel reserves, they calculated three important numbers: the maximum level to which the air's CO2 concentration might rise, the point in time when that might happen, and the related decline that might be expected to occur in ocean-surface pH.  These calculations indicated that earth's atmospheric CO2 concentration could approach 2000 ppm around the year 2300, leading to an ocean-surface pH reduction of 0.7 units, a change described by Caldeira and Wickett as being much more rapid and considerably greater "than any experienced in the past 300 million years," which, of course, proves deadly for earth's corals in their scenario.

The following year, similar concerns were aroused by a report prepared for the Pew Center on Global Climate Change, which was released to the public on 13 February 2004 at the annual meeting of the American Association for the Advancement of Science.  In that document, Buddemeier et al. (2004) claimed that the projected increase in the air's CO2 content and the simulated decline in ocean-surface pH would dramatically decrease coral calcification rates, which were predicted to lead to "a slow-down or reversal of reef-building and the potential loss of reef structures."

Nevertheless, and because of all the contrary evidence we have highlighted on our website, 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."

A second good reason for not believing that the ongoing rise in the air's CO2 content will lead to reduced oceanic pH and, therefore, lower calcification rates in the world's coral reefs, is that the same phenomenon that powers the twin processes of coral calcification and phytoplanktonic growth (photosynthesis) tends to increase the pH of marine waters (Gnaiger et al., 1978; Santhanam et al., 1994; Brussaard et al., 1996; Lindholm and Nummelin, 1999; Macedo et al., 2001; Hansen, 2002); and this phenomenon has been shown to have the ability to dramatically increase the pH of marine bays, lagoons and tidal pools (Gnaiger et al., 1978; Santhanam, 1994; Macedo et al., 2001; Hansen, 2002) as well as significantly enhance the surface water pH of areas as large as the North Sea (Brussaard et al., 1996).

Expanding on the subject of CO2 effects on photosynthesizing marine organisms, Riebesell (2004) notes that "a moderate increase in CO2 facilitates photosynthetic carbon fixation of some phytoplankton groups," including "the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica."  Hence, in a major challenge to the climate-alarmist claim that atmospheric CO2 enrichment will definitely harm such marine organisms, Riebesell suggests that "CO2-sensitive taxa, such as the calcifying coccolithophorids, should therefore benefit more [our italics] from the present increase in atmospheric CO2 compared to the non-calcifying diatoms."

In support of this suggestion, Riebesell describes the results of some CO2 perturbation experiments conducted south of Bergen, Norway, where nine 11-m3 enclosures moored to a floating raft were aerated in triplicate with CO2-depleted, normal and CO2-enriched air to achieve CO2 levels of 190, 370 and 710 ppm, simulating glacial, present-day and predicted conditions for the end of the century, respectively.  In the course of the study, a bloom consisting of a mixed phytoplankton community developed, and, in Riebesell's words, "significantly higher net community production was observed under elevated CO2 levels during the build-up of the bloom."  He further reports that "CO2-related differences in primary production continued after nutrient exhaustion, leading to higher production of transparent exopolymer particles under high CO2 conditions," something that has also been observed by Engel (2002) in a natural plankton assemblage and by Heemann (2002) in monospecific cultures of both diatoms and coccolithophores.

Another important finding of this experiment was that the community that developed under the high CO2 conditions expected for the end of this century was dominated by Emiliania huxleyi.  Consequently, Riebesell finds even more reason to believe that "coccolithophores may benefit from the present increase in atmospheric CO2 and related changes in seawater carbonate chemistry," in contrast to the many negative predictions that have been made about rising atmospheric CO2 concentrations in this regard.  Finally, in further commentary on the topic, Riebesell states that "increasing CO2 availability may improve the overall resource utilization of E. huxleyi and possibly of other fast-growing coccolithophore species," concluding that "if this provides an ecological advantage for coccolithophores, rising atmospheric CO2 could potentially increase the contribution of calcifying phytoplankton to overall primary production."

With each succeeding year, therefore, the physical evidence against the CO2-reduced calcification theory continues to grow ever more compelling, while support for the positive view of Idso et al. (2000) continues to accumulate.  Working in the laboratory, for example, Reynaud et al. (2004) grew nubbins of the branching zooxanthellate scleractinian coral Acropora verweyi in aquariums maintained at 20, 25 and 29C, while weighing them once a week over a period of four weeks.  This exercise revealed that coral calcification rates increased in nearly perfect linear fashion with increasing water temperature, yielding values of 0.06, 0.22 and 0.35% per day at 20, 25 and 29C, respectively.  These data reveal an approximate 480% increase in calcification rate in response to a 9C increase in water temperature and a 160% increase in response to a 3C increase in temperature, the latter of which temperature increases is somewhere in the low to midrange of global warming that climate alarmists claim will result from a 300 ppm increase in the air's CO2 concentration; and this positive temperature effect far outweighs the negative effect of rising CO2 concentrations on coral calcification.

Working in the field, or, more correctly, the ocean, Carricart-Ganivet (2004) developed relationships between coral calcification rate and annual average SST based on data collected from colonies of the reef-building coral Montastraea annularis at twelve localities in the Gulf of Mexico and the Caribbean Sea, finding that calcification rate in the Gulf of Mexico increased 0.55 g cm-2 year-1 for each 1C increase, while in the Caribbean Sea it increased 0.58 g cm-2 year-1 for each 1C increase.  Pooling these data with those of M. annularis and M. faveolata growing to a depth of 10 m at Carrie Bow Cay, Belize, those from reefs at St. Croix in the US Virgin Islands, and those of M. faveolata growing to a depth of 10 m at Curacao, Antilles, Carricart-Ganivet reports he obtained a mean increase in calcification rate of ~0.5 g cm-2 year-1 for each 1C increase in annual average SST, which is even greater than what was found by Lough and Barnes for Porites corals.

In another important study, McNeil et al. (2004) used a coupled atmosphere-ice-ocean carbon cycle model to calculate annual mean SST increases within the world's current coral reef habitat from 1995 to 2100 for increases in the air's CO2 concentration specified by the IPCC's IS92a scenario, after which concomitant changes in coral reef calcification rates were estimated by combining the output of the climate model with empirical relationships between coral calcification rate and (1) aragonite saturation state (the negative CO2 effect) and (2) annual SST (the positive temperature effect).  Their choice for the first of these two relationships was that derived by Langdon et al. (2000), which leads to an even greater reduction in calcification than was predicted in the study of Kleypas et al.  Their choice for the second relationship was that derived by Lough and Barnes (2000), which leads to an increase in calcification that is only half as large as that derived by Carricart-Ganivet (2000).  As a result, it can be appreciated that the net result of the two phenomena was doubly weighted in favor of reduced coral calcification.  Nevertheless, McNeil et al. found that the increase in coral reef calcification associated with ocean warming far outweighed the decrease associated with the CO2-induced decrease in aragonite saturate state.  In fact, they calculated that coral calcification in 2100 would be 35% higher than what it was in pre-industrial times at the very least.  And, of course, they found that the area of coral reef habitat expands in association with the projected ocean warming.

In conclusion, if there is a lesson to be learned from the materials discussed in this review, it is that people should be paying much more attention to real-world observations than to theoretical predictions, both in the case of biology, as demonstrated here, and in the case of climate, as demonstrated by the many materials archived on our website that deal with global warming and what does and does not accompany it.  Far too many predictions of CO2-induced catastrophes in both realms are being treated as sure-to-occur, when real-world observations show them to be highly unlikely or even virtual impossibilities.

Bessat, F. and Buigues, D.  2001.  Two centuries of variation in coral growth in a massive Porites colony from Moorea (French Polynesia): a response of ocean-atmosphere variability from south central Pacific.  Palaeogeography, Palaeoclimatology, Palaeoecology 175: 381-392.

Brussaard, C.P.D., Gast, G.J., van Duyl, F.C. and Riegman, R.  1996.  Impact of phytoplankton bloom magnitude on a pelagic microbial food web.  Marine Ecology Progress Series 144: 211-221.

Buddemeier, R.W.  1994.  Symbiosis, calcification, and environmental interactions.  Bulletin Institut Oceanographique, Monaco 13: 119-131.

Buddemeier, R.W.  2001.  Is it time to give up?  Bulletin of Marine Science 69: 317-326.

Buddemeier, R.W. and Fautin, D.G.  1996a.  Saturation state and the evolution and biogeography of symbiotic calcification.  Bulletin Institut Oceanographique, Monaco 14: 23-32.

Buddemeier, R.W. and Fautin, D.G.  1996b.  Global CO2 and evolution among the ScleractiniaBulletin Institut Oceanographique, Monaco 14: 33-38.

Buddemeier, R.W., Kleypas, J.A. and Aronson, R.B.  2004.  Coral Reefs & Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems.  The Pew Center on Global Climate Change, Arlington, VA, USA.

Caldeira, K. and Wickett, M.E.  2003.  Anthropogenic carbon and ocean pH.  Nature 425: 365.

Carricart-Ganivet, J.P.  2004.  Sea surface temperature and the growth of the West Atlantic reef-building coral Montastraea annularisJournal of Experimental Marine Biology and Ecology 302: 249-260.

Engel, A.  2002.  Direct relationship between CO2 uptake and transparent exopolymer particles production in natural phytoplankton.  Journal of Plankton Research 24: 49-53.

Gattuso, J.-P., Allemand, D. and Frankignoulle, M.  1999.  Photosynthesis and calcification at cellular, organismal and community levels in coral reefs: A review on interactions and control by carbonate chemistry.  American Zoologist 39: 160-183.

Gattuso, J.-P., Frankignoulle, M., Bourge, I., Romaine, S. and Buddemeier, R.W.  1998.  Effect of calcium carbonate saturation of seawater on coral calcification.  Global and Planetary Change 18: 37-46.

Gnaiger, E., Gluth, G. and Weiser, W.  1978.  pH fluctuations in an intertidal beach in Bermuda.  Limnology and Oceanography 23: 851-857.

Grigg, R.W.  1981.  Coral reef development at high latitudes in Hawaii.  In: Proceedings of the Fourth International Coral Reef Symposium, Manila, Vol. 1: 687-693.

Grigg, R.W.  1997.  Paleoceanography of coral reefs in the Hawaiian-Emperor Chain - revisited.  Coral Reefs 16: S33-S38.

Hansen, P.J.  2002.  The effect of high pH on the growth and survival of marine phytoplankton: implications for species succession.  Aquatic Microbiology and Ecology 28: 279-288.

Heemann, C.  2002.  Phytoplanktonexsudation in Abhangigkeit der Meerwasserkarbonatchemie.  Diplom. Thesis, ICBM, University of Oldenburg, Germany.

Idso, S.B., Idso, C.D. and Idso, K.E.  2000.  CO2, global warming and coral reefs: Prospects for the future.  Technology 7S: 71-94.

Kleypas, J.A., Buddemeier, R.W., Archer, D., Gattuso, J-P., Langdon, C. and Opdyke, B.N.  1999.  Geochemical consequences of increased atmospheric carbon dioxide on coral reefs.  Science 284: 118-120.

Langdon, C., Takahashi, T., Sweeney, C., Chipman, D., Goddard, J., Marubini, F., Aceves, H., Barnett, H. and Atkinson, M.J.  2000.  Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef.  Global Biogeochemical Cycles 14: 639-654.

Lindholm, T. and Nummelin, C.  1999.  Red tide of the dinoflagellate Heterocapsa triquetra (Dinophyta) in a ferry-mixed coastal inlet.  Hydrobiologia 393: 245-251.

Lough, J.M. and Barnes, D.J.  1997.  Several centuries of variation in skeletal extension, density and calcification in massive Porites colonies from the Great Barrier Reef: A proxy for seawater temperature and a background of variability against which to identify unnatural change.  Journal of Experimental and Marine Biology and Ecology 211: 29-67.

Lough, J.B. and Barnes, D.J.  2000.  Environmental controls on growth of the massive coral PoritesJournal of Experimental Marine Biology and Ecology 245: 225-243.

Macedo, M.F., Duarte, P., Mendes, P. and Ferreira, G.  2001.  Annual variation of environmental variables, phytoplankton species composition and photosynthetic parameters in a coastal lagoon.  Journal of Plankton Research 23: 719-732.

McNeil, B.I., Matear, R.J. and Barnes, D.J.  2004.  Coral reef calcification and climate change: The effect of ocean warming.  Geophysical Research Letters 31: 10.1029/2004GL021541.

Reynaud-Vaganay, S., Gattuso, J.P., Cuif, J.P., Jaubert, J. and Juillet-Leclerc, A.  1999.  A novel culture technique for scleractinian corals: Application to investigate changes in skeletal δ18O as a function of temperature.  Marine Ecology Progress Series 180: 121-130.

Riebesell, U.  2004.  Effects of CO2 enrichment on marine phytoplankton.  Journal of Oceanography 60: 719-729.

Santhanam, R., Srinivasan, A., Ramadhas, V. and Devaraj, M.  1994.  Impact of Trichodesmium bloom on the plankton and productivity in the Tuticorin bay, southeast coast of India.  Indian Journal of Marine Science 23: 27-30.

Scoffin, T.P., Tudhope, A.W., Brown, B.E., Chansang, H. and Cheeney, R.F.  1992.  Patterns and possible environmental controls of skeletogenesis of Porites lutea, South Thailand.  Coral Reefs 11: 1-11.

Last updated 26 October 2005