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CO2, Global Warming and Coral Reefs
Prospects for the Future

4. Global Warming-Induced Sea Level Rise: Good or Bad for Corals?


If rising temperatures don’t decimate earth’s corals, many people believe that rising sea levels will, by gradually reducing the amount of life-sustaining light that reaches their algal symbionts, upon which they depend for food. This assumption, in fact, is a major concern that is often expressed in discussions of reef responses to global climate change (Hopley and Kinsey, 1988); but it is probably not valid, and for a number of different reasons.

First of all, the 18- to 59-cm warming-induced sea level rise that is predicted for the coming century (IPCC, 2007) – which could be greatly exaggerated if predictions of CO2-induced global warming are wrong – falls well within the range (2 to 6 mm per year) of typical coral vertical extension rates, which exhibited a modal value of 7 to 8 mm per year during the Holocene and can be more than double that value in certain branching corals (Hopley and Kinsey, 1988; Done, 1999). Second, most coral reefs are known to have successfully responded to the sea level rises that occurred between 14,000 and 6,000 years ago – which were accompanied by large changes in “CO2 concentrations, ... rainfall, cloud cover, storms and currents” (Wilkinson, 1996) – and which were more than twice as rapid as what is being predicted for the coming century (Digerfeldt and Hendry, 1987). Third, earth’s oceans have undergone – and their coral reefs have survived (Chadwick-Furman, 1996) – at least seventeen major cycles of sea level rise and fall during the Pleistocene, the most recent low phase of which ended 18,000 years ago with a global sea level some 120-135 meters below where it is now (Grigg and Epp, 1989). Fourth, most coral reefs handle increases in sea level – even rapid increases – much better than decreases (White et al., 1998). Yet even if reef vertical growth rates could not keep up with rising sea levels, that fact would still not spell their doom.

One of the important characteristics of essentially all reef cnidarians is their ability to produce free-swimming planulae, spores or dispersive larval stages. Possessed of this capacity, Kinzie (1999) notes that “no matter how quickly sea level might rise, propagules of the species could keep pace and settle at suitable depths each generation,” thereby creating what he calls jump-up reefs that “might well contain most of the species present in the original community.” In fact, Done (1999) notes that “coral communities have a history of tracking their preferred environmental niche which may suggest that as an entity, they will be predisposed to ‘adapt’ to prospective changes in environment over the next century,” citing precedents that clearly demonstrate that “coral communities have historically had a good capacity to track their re-distributed preferred physical niches.”

It is not at all surprising, therefore, as Kinzie and Buddemeier (1996) recount, that coral reefs have survived many periods of “massive environmental changes” throughout the geologic record. Reefs are survivors, they state, “because they do not simply tolerate environmental changes” but “exhibit an impressive array of acclimations” that allow them to deal with a variety of challenges to their continued existence in any given area. Hence, it is highly unlikely that anticipated increases in sea level would spell the doom of earth’s coral reefs.

Strange as it may seem, however, it is possible that rising sea levels may have a positive effect on earth’s coral reefs (Roberts, 1993). Over the past 6,000 years, for example, relatively stable sea levels have limited upward reef growth, resulting in the development of extensive reef flats; and as Buddemeier and Smith (1988) and Wilkinson (1996) have noted, the sea level rises predicted to result from CO2-induced global warming should actually be beneficial, permitting increased growth in these growth-restricted areas. In the words of Chadwick-Furman (1996), “many coral reefs have already reached their upward limit of growth at present sea level (Buddemeier, 1992), and may be released from this vertical constraint by a rise in sea level.” She also notes that rising sea levels may allow more water to circulate between segregated lagoons and outer reef slopes, which could “increase the exchange of coral propagules between reef habitats and lead to higher coral diversity in inner reef areas.” Hence, she too concludes that “coral reefs are likely to survive predicted rates of global change.”