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Rising Atmospheric CO2 Concentrations: Will They Destroy Earth's Coral Reefs?
Volume 7, Number 8: 25 February 2004

Based on four theoretical constructs - (1) a geochemical model, (2) an ocean general-circulation model, (3) an IPCC CO2 emissions scenario for the 21st century, and (4) a logistic function for the burning of earth's post-21st century fossil-fuel reserves - Caldeira and Wickett (2003) calculate three important numbers: (1) the maximum level to which the atmosphere's CO2 concentration might rise, (2) the point in time when that might happen, and (3) the related decline that might be expected to occur in the pH of surface ocean waters.  And the results?

Caldeira and Wickett's calculations suggest earth's atmospheric CO2 concentration could approach 2000 ppm around the year 2300, leading to a concomitant surface oceanic pH reduction of 0.7 units, a change they describe as being much more rapid and considerably greater "than any experienced in the past 300 million years."  What will be the ultimate consequence of this unprecedented phenomenon for earth's coral reefs, which owe their existence to the pH-dependent calcification process?

In 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, Buddemeier et al. (2004) claim that the projected increase in the air's CO2 content, together with its simulated decline in surface ocean water pH, will dramatically decrease coral calcification rates, which they say could lead to "a slow-down or reversal of reef-building and the potential loss of reef structures in the future."  There are, however, some good reasons for believing otherwise.

First of all, the anti-calcifying effect of elevated atmospheric CO2 concentrations, as described by Buddemeier et al. (2004) and Caldeira and Wickett (2003), is a well-defined physical-chemical process; but as we have previously stated (Idso et al., 2000), the creation of the coral skeletons that form the foundations of real-world coral reefs is a biologically-driven physical-chemical process, which is not yet amenable to explicit mathematical modeling.  We note, for example, that the "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."  In addition, we note that "the calcium carbonate saturation state of seawater actually rises with an increase in temperature [i.e., during global warming], significantly countering the direct adverse oceanic chemistry consequences of an increase in atmospheric and/or hydrospheric CO2 concentration."

Since we elucidated these points in the open literature several years ago, and since they have been substantiated by the independent work of a number of other scientists, whose real-world studies we have described in a number Journal Reviews and Editorials [see Coral Reefs (Calcification) in our Subject Index], Buddemeier et al. were essentially forced to acknowledge these facts in their recent report.  After stating that "Kleypas et al. (1999) estimated an average decline of reef calcification rates of 6-14 percent as atmospheric CO2 concentration increased from preindustrial levels (280 ppm) to present-day values (370 ppm)," for example, they had to admit the existence of the contradictory reality, i.e., the fact that "calcification rates of large heads of the massive coral Porites increased rather than decreased over the latter half of the 20th century (Lough and Barnes, 1997; Lough and Barnes, 2000; Bessat and Buigues, 2001)," 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).

Clearly, the real-world of nature is acting far different from what certain scientists and climate alarmists are predicting will occur as the air's CO2 content continues its upward climb.  We would do well to heed what it is telling us, and not what they are saying.

Sherwood, Keith and Craig Idso

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., Lkeypas, 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.

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

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.

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.

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.M. 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.

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.