In order to correctly interpret the current status of earth's coral reefs, it is necessary to understand their longer-term history; and to predict their future status, this knowledge is essential. Hence, we here review what we have learned about the early history of coral reefs in the Caribbean and what researchers suggest this knowledge implies about their future.
Greenstein et al. (1998b) compared the taxonomic structure of live and dead coral communities on a modern patch reef to late Pleistocene fossil coral assemblages. Their findings revealed a recent decline in thickets of Acropora cervicornis, as evidenced by their abundance in the "death assemblage," and a concurrent increase in Porites porites, as evidenced by their abundance in the "life assemblage." In comparing these results with those obtained from a "fossil assemblage," they concluded that "the present Caribbean-wide decline of A. cervicornis is without historical precedent," and that it "contrasts with the long-term persistence of this taxon during Pleistocene and Holocene Optimum time," even during periods of "intensifying cycles in climate and sea level" that recurred again and again throughout a roughly one-million-year time period. These observations suggest that if little change in coral community structure occurred over Holocene and earlier times, when we know it was often warmer than it is now, the current die-off of A. cervicornis must not be due to global warming. Instead, it must be a consequence of some human-induced alteration of the physical or chemical characteristics of the reef environment. As the authors describe it, "although the A. cervicornis-dominated coral association persisted during Pleistocene climatic fluctuations, it is apparently vulnerable to the array of perturbations [our italics] currently being inflicted on it."
Reporting from the quadrennial North American Paleontological Convention held in Berkeley, California (USA) a few years back, Stokstad (2001) described the results of some other studies pertinent to this Summary. Particularly noteworthy in this regard were the findings of John Pandolfi of the Smithsonian Institution's National Museum of Natural History, who determined from numerous surveys of fossil reefs of San Andres, Curacao and Barbados that Caribbean reefs have consisted of essentially the same relative mix of coral species for most of the past 220,000 years. Pandolfi's data also revealed that the most prominent of these species -- the Elkhorn coral (Acropora palmata) -- typically comprised approximately 80% of all Caribbean reef corals over this lengthy span of time. Since the early 1970s, however, this historical stalwart of the region has become exceedingly rare; and "once-majestic reefs," as Stokstad describes them, "now are over-grown with algae." Stokstad further quotes Richard Aronson of the Dauphin Island Sea Laboratory as saying "this tells you that what we're seeing today is not some random fluctuation." Indeed, he additionally quotes John Ogden of the University of South Florida as saying that the recent devastation is "a profound change that's unprecedented in recent geologic history," which is the same conclusion drawn by the study of Greenstein et al. (1998a) with respect to Acropora cervicornis.
So what is responsible for the recent unprecedented crash in the population of what has historically been the most robust of the Caribbean corals? Could it be what climate alarmists mistakenly call the "unprecedented global warming" of the same period? The temporal correlation between the two phenomena is just too tantalizing for those with a strong anti-CO2 bias to resist. However, that they likely err in drawing this conclusion is suggested by the many contrary comments of several of the scientists at the Convention who noted, according to Stokstad, that Caribbean reef community structure "survived the vicissitudes of climate change in the past, such as swings in sea level, temperature, and carbon dioxide levels," which facts clearly indicate that the Caribbean reefs' current problems are due to something quite apart from CO2-induced global warming.
But what could that different something be?
According to Stokstad, Ogden says "it's difficult to lay [the blame] at the feet of any cause other than humans." We agree; but as noted in the preceding paragraph, the human activities responsible for the devastation cannot include global warming. Further support for this conclusion comes from the papers of Jackson et al. (2001) and Petit et al. (1999). The first of these studies indicates that "large species of branching Acropora corals dominated shallow reefs in the tropical western Atlantic for at least half a million years until the 1980s, when they declined dramatically." The second study indicates that throughout this long period of time, the earth experienced several glacial/interglacial cycles for which we have good proxy temperature data, and that the four interglacials that preceded the one in which we now live were all warmer than the current one -- and by an average of more than 2°C! Hence, the Acropora corals that have taken such a nosedive in health -- and actual existence -- over the past quarter century are clearly able to tolerate temperatures significantly in excess of those of the present (all else being equal), which current temperatures may also be less than temperatures experienced during the Holocene Climatic Optimum and the Medieval Warm Period of only a few and one thousand years ago, respectively.
Further support for this conclusion is provided by the study of Gardner et al. (2003a), who assessed the extent of decline in coral cover across the Caribbean via a meta-analysis of data obtained from a total of 263 sites described in 65 scientific studies. This effort revealed, in their words, "a massive region-wide decline of corals across the entire Caribbean basin, with the average hard coral cover on reefs being reduced by 80%, from about 50% to 10% cover, in three decades." However, they also report that "the rate of coral loss has slowed in the past decade compared to the 1980s," and they say "there is no convincing evidence yet that global stressors (e.g. temperature-induced bleaching and reduced rates of carbonation via enhanced levels of atmospheric CO2) are responsible for the overall pattern of these recent coral declines." Instead, they more logically lay the blame at the feet of "local factors originating both naturally (e.g. disease, storms, temperature stress, predation) and anthropogenically (e.g. over-fishing, sedimentation, eutrophication, habitat destruction)," which they say "are occurring at Caribbean-wide scales."
Later that same year, Buddemeier and Ware (2003) took issue with portions of Gardner et al.'s assessment of the situation, stating that "the possibility of recent amelioration and the inferences about the relative roles of climatic and local human factors should be viewed with caution," adding that they felt that "climate-related factors are too casually dismissed." In response to this complaint, Gardner et al. (2003b) wrote that "although factors directly related to climate change (e.g., bleaching) have affected Caribbean corals at sub-regional scales, their role in contributing to observed coral declines across the entire region is not yet detectable." Nevertheless, they went on to add that "the ability of Caribbean coral reefs to cope with the threat of future climate change may be irretrievably compromised [our italics]," as a consequence of local anthropogenic factors that have likely severely weakened them.
In conclusion, there is no question that earth's coral reefs have suffered greatly at the hands of man. The great bulk of that damage, however, has likely not been wrought by the historical rise in the air's CO2 content, nor by the slight warming of the past century or so, a phenomenon that earth's corals have successfully experienced over and over due to the amazing repeatability of the millennial-scale climatic oscillation that brought the planet the Roman Warm Period, Medieval Warm Period and Current Warm Period, and which has recurred, again and again, throughout glacial and interglacial periods alike.
Buddemeier, R.W. and Ware, J.R. 2003. Coral reef decline in the Caribbean. Science 302: 391-392.
Gardner, T.A., Cote, I.M., Gill, J.A., Grant, A. and Watkinson, A.R. 2003a. Long-term region-wide declines in Caribbean corals. Science 301: 958-960.
Gardner, T.A., Cote, I.M., Gill, J.A., Grant, A. and Watkinson, A.R. 2003b. Response. Science 302: 392-393.
Greenstein, B.J., Curran, H.A. and Pandolfi, J.M. 1998a. Shifting ecological baselines and the demise of Acropora cervicornis in the western North Atlantic and Caribbean Province: a Pleistocene perspective. Coral Reefs 17: 249-261.
Greenstein, B.J., Harris, L.A. and Curran, H.A. 1998b. Comparison of recent coral life and death assemblages to Pleistocene reef communities: Implications for rapid faunal replacement on recent reefs. Carbonates & Evaporites 13: 23-31.
Jackson, J.B.C., Kirby, M.X., Berger, W.H., Bjorndal, K.A., Botsford, L.W., Bourque, B.J., Bradbury, R.H., Cooke, R., Erlandson, J., Estes, J.A., Hughes, T.P., Kidwell, S., Lange, C.B., Lenihan, H.S., Pandolfi, J.M., Peterson, C.H., Steneck, R.S., Tegner, M.J. and Warner, R.R. 2001. Historical overfishing and the recent collapse of coastal ecosystems. Science 293: 629-638.
Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E. and Stievenard, M. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399: 429-436.
Stokstad, E. 2001. Humans to blame for coral loss. Science 293: 593.Last updated 30 January 2008