Volume 15, Number 7: 15 February 2012
In a paper published in Global Change Biology, Kemp et al. (2011) write that "considerable attention has been given to worldwide coral reef decline over the last several years with major emphasis placed on the negative effects of increased seawater temperatures," which typically lead to coral bleaching and subsequent death; but they also note that "imposed low-temperature stress can cause coral bleaching by inducing responses similar to elevated-temperature, including reduction in Symbiodinium cell density and chlorophyll a content, as well as photoinhibition," citing the work of Steen and Muscatine (1987), Saxby et al. (2003), Hoegh-Guldberg and Fine (2004) and Hoegh-Guldberg et al. (2005). And they go on to demonstrate this latter fact via an analysis of coral responses to two closely-spaced cold fronts that caused sudden and severe seawater cooling in February and March of 2010 in the upper Florida (USA) Keys that led to "a mass die-off of reef-building corals," thereby convincingly illustrating that both unusually warm and unusually cold temperatures, such as are caused by fluctuations in weather conditions, are equally adept at killing corals and their algal symbionts.
Over the long term, however, when either warmer or cooler conditions are the result of much slower changes in climate, such need not be the case. Consider, for example, the Little Ice Age (LIA). In a study of the Atlantic Warm Pool (AWP) - which is defined by the >28.5°C isotherm and develops annually in the northern Caribbean during early summer (June) and expands into the Gulf of Mexico and western tropical North Atlantic through the late summer (July-October) - Richey et al. (2009) found that "geochemical proxy records from corals, sclerosponges and foraminifera in the region encompassed by the AWP show a large (2-3°C) cooling during the LIA," citing, in this regard, the work of Winter et al. (2000), Watanabe et al. (2001), Nyberg et al. (2002), Haase-Schramm et al. (2003), Black et al. (2007) and Kilbourne et al. (2008). And in reporting the results of a study of a large brain coral that lived throughout the 17th century on the shallow seafloor off the island of Bermuda, Cohen and Madin (2007) say that although seawater temperatures at that time and location were about 1.5°C colder than it is there today, "the coral grew faster than the corals there now."
Other studies have shown earth's corals to be able to cope with climate-induced warmings as well as coolings. In a study of patch reefs of the Florida Keys, for example, Greenstein et al. (1998) found that Acropora cervicornis corals exhibited "long-term persistence" during both "Pleistocene and Holocene time," the former of which periods exhibited climatic changes of large magnitude, some with significantly greater warmth than currently prevails on earth; and these climate changes had almost no effect on this long-term dominant of Caribbean coral reefs. Hence, there is good reason to not be too concerned about long-term changes in climate possibly harming earth's corals. They apparently have the ability to handle whatever nature may throw at them in this regard.
Sherwood, Keith and Craig Idso
References
Black, D.E., Abahazi, M.A., Thunell, R.C., Kaplan, A., Tappa, E.J. and Peterson, L.C. 2007. An 8-century tropical Atlantic SST record from the Cariaco Basin: Baseline variability, twentieth-century warming, and Atlantic hurricane frequency. Paleoceanography 22: 10.1029/2007PA001427.
Cohen, A. and Madin, K. 2007. Dead corals do tell tales. Woods Hole Oceanographic Institution's Oceanus (http://www.whoi.edu/oceanus/viewArticle.do?id=22407).
Greenstein, B.J., Curran, H.A. and Pandolfi, J.M. 1998. 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.
Haase-Schramm, A., Bohm, F., Eisenhauer, A., Dullo, W.-C., Joachimski, M.M., Hansen, B. and Reitner, J. 2003. Sr/Ca ratios and oxygen isotopes from sclerosponges: Temperature history of the Caribbean mixed layer and thermocline during the Little Ice Age. Paleoceanography 18: 10.1029/2002PA000830.
Hoegh-Guldberg, O. and Fine, M. 2004. Low temperatures cause coral bleaching. Coral Reefs 23: 444.
Hoegh-Guldberg, O., Fine, M., Skirving, W., Johnstone, R., Dove, S. and Strong, A. 2005. Coral bleaching following wintry weather. Limnology and Oceanography 50: 265-271.
Kemp, D.W., Oakley, C.A., Thornhill, D.J., Newcomb, L.A., Schmidt, G.W. and Fitt, W.K. 2011. Catastrophic mortality on inshore coral reefs of the Florida Keys due to severe low-temperature stress. Global Change Biology 17: 3468-3477.
Kilbourne, K.H., Quinn, T.M., Webb, R., Guilderson, T., Nyberg, J. and Winter, A. 2008. Paleoclimate proxy perspective on Caribbean climate since the year 1751: Evidence of cooler temperatures and multidecadal variability. Paleoceanography 23: 10.1029/2008PA001598.
Nyberg, J., Malmgren, B.A., Kuijpers, A. and Winter, A. 2002. A centennial-scale variability of tropical North Atlantic surface hydrography during the late Holocene. Palaeogeography, Palaeoclimatology, Palaeoecology 183: 25-41.
Richey, J.N., Poore, R.Z., Flower, B.P., Quinn, T.M. and Hollander, D.J. 2009. Regionally coherent Little Ice Age cooling in the Atlantic Warm Pool. Geophysical Research Letters 36: 10.1029/2009GL040445.
Saxby, T., Dennison, W.C. and Hoegh-Guldberg, O. 2003. Photosynthetic responses of the coral Montipora digitata to cold temperature stress. Marine Ecology Progress Series 248: 85-97.
Steen, R.G. and Muscatine, L. 1987. Low-temperature evokes rapid exocytosis of symbiotic algae by a sea-anemone. Biological Bulletin 172: 246-263.
Watanabe, T., Winter, A. and Oba, T. 2001. Seasonal changes in sea surface temperature and salinity during the Little Ice Age in the Caribbean Sea deduced from Mg/Ca and 18O/16O ratios in corals. Marine Geology 173: 21-35.
Winter, A., Ishioroshi, H., Watanabe, T., Oba, T. and Christy, J. 2000. Caribbean Sea surface temperatures: Two-to-three degrees cooler than present during the Little Ice Age. Geophysical Research Letters 27: 3365-3368.