One of the global warming scare stories we hear is that increased carbon dioxide emissions cause ocean acidification and rising temperatures that contribute to coral bleaching or other harm to corals. I dealt with ocean acidification in a previous blog. The issue is complicated; here is what research says. (Note: numbers in parentheses after a sentence refer to a specific reference.)
Coral bleaching is caused by high sea temperatures, high solar irradiance, by anomalously low sea temperatures, and by sudden drops in temperature that accompany intense upwelling episodes, thermocline shoaling, or seasonal cold-air outbreaks.(1)
Many coral species have endured three periods of global warming, from the Pliocene optimum (4.3-3.3 million years ago) through the Eemian interglacial (125,000 years ago) and the mid-Holocene Optimum (6000-5000 years ago), when atmospheric CO2 concentrations and sea temperatures often exceeded those of today. Data show that an increase in sea warming of less than 2°C would result in a greatly increased diversity of corals in certain high latitude locations.(1)
Some coral bleaching may be due to marine pathogens, i.e., diseases.(2) Coral polyps depend on symbionts such as zooxanthellae (algal symbionts). These symbionts vary seasonally and with environmental stress. (3) Some symbionts are highly adaptable, and some corals can change their symbionts to better suit conditions. (14)
Some coral bleaching appears to be synchronous with El Nino events which raise water temperature. (4)
Although corals may endure bleaching, they are resilient. For instance, scleractinian corals, which are the major builders of the reefs of today, first appeared during mid-Triassic time 210 million years ago, when the earth was considerably warmer. They endured the Cretaceous Period, when temperatures were as much as 10-15°C higher than now. And they survived the warm and cold cycles of Pleistocene glacial epochs. (5,6,7)
One of the reasons coral are resilient and able to withstand a wide range of temperature, salinity, and CO2 variations is that they shuffle symbionts. (8,9,10) For instance, “as the community structure of coral reefs shift in response to global climate change and water quality impacts, opportunistic corals harboring symbionts that enable maximum rates of growth may similarly gain a competitive advantage.” (13) The corals themselves also have several mechanisms to deal with and deflect thermal stress, including dynamic photo-protective mechanisms, and the expression of heat-shock proteins.
On the issue of coral calcification, observation finds that the combination of increased CO2 (which provides more carbonate) and the shuffling of symbionts, makes the corals able to withstand the variations of temperature, disease, and solar irradiation. (11,12)
Real world observations trump the scare stories derived from theoretical models.
While human CO2 emissions have little effect on coral health, we are, however, significantly affecting corals in other ways through runoff and nutrient enrichment; coastal construction leading to smothering of habitat and creation of high turbidity around coasts; and over fishing. Local management of water quality would seem to be the best thing we can do to aid corals.
See a summary of studies of human impact here: http://www.co2science.org/articles/V12/N41/EDIT.php
1. Glynn, P.W. 1996. Coral reef bleaching: facts, hypotheses and implications. Global Change Biology 2: 495-509
2. Hayes, R.L. and Goreau, N.I. 1998. The significance of emerging diseases in the tropical coral reef ecosystem. Revista de Biologia Tropical 46 Supl. 5: 173-185
3. Fagoonee, I., Wilson, H.B., Hassell, M.P. and Turner, J.R. 1999. The dynamics of zooxanthellae populations: A long-term study in the field. Science 283: 843-845.
4. Stone, L., Huppert, A., Rajagopalan, B., Bhasin, H. and Loya, Y. 1999. Mass coral reef bleachng: A recent outcome of increased El Niño Activity? Ecology Letters 2: 325-330
5. Wells, J.W. 1956. Scleractinia. In: Moore, R.C., Ed. Treatise on Invertebrate Paleontology, Volume F, Coelenterata. Geological Society of America and University of Kansas Press, Lawrence, KS, pp. 353-367.
6. Chadwick-Furman, N.E. 1996. Reef coral diversity and global change. Global Change Biology 2: 559-568
7. Pandolfi, J.M. 1999. Response of Pleistocene coral reefs to environmental change over long temporal scales. American Zoologist 39: 113-130
8. Adjeroud, M., Augustin, D., Galzin, R. and Salvat, B. 2002. Natural disturbances and interannual variability of coral reef communities on the outer slope of Tiahura (Moorea, French Polynesia): 1991 to 1997. Marine Ecology Progress Series 237: 121-131.
9. Apprill, A.M. and Gates, R.D. 2007. Recognizing diversity in coral symbiotic dinoflagellate communities. Molecular Ecology 16: 1127-1134.
10. Baird, A.H., Cumbo, V.R., Leggat, W. and Rodriguez-Lanetty, M. 2007. Fidelity and flexibility in coral symbioses. Marine Ecology Progress Series 347: 307-309.
11. 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
12. Buddemeier, R.W. 1994. Symbiosis, calcification, and environmental interactions. Bulletin Institut Oceanographique, Monaco 13: 119-131
13. Cantin, N.E., van Oppen, M.J.H., Willis, B.L., Mieog, J.C. and Negri, A.P. 2009. Juvenile corals can acquire more carbon from high-performance algal symbionts. Coral Reefs 28: 405-414.
14. Fitt, W.K., et al., 2009, Response of two species of Indo-Pacific corals, Porites cylindrica and Stylophora pistillata, to short-term thermal stress: The host does matter in determining the tolerance of corals to bleaching. Journal of Experimental Marine Biology and Ecology 373: 102-110.