Clearly, we have a problem, and one of such magnitude that Buddemeier (2001) actually entitles his assessment of the issue "Is it time to give up?"  This question stems from his conclusion that "coral reefs are doomed, probably within the next few decades."  Thus, he says, "we face the ethical and practical dilemma common in human medicine: when to discontinue heroic life-support measures for a terminally ill patient."  Answering his own question, he states that "with limited funds available for all conservation and research, the answer must be: sooner rather than later."

Although Buddemeier's prognosis and prescription sound much like the type of scare-mongering call-to-arms promoted several years ago by Steve Schneider with respect to CO2 and global warming, the declining health of earth's coral reefs is obviously something of real concern; and, hence, we address it fairly often.  And especially do we do so when the damage to corals is said to be caused by warming that is said to be caused by increases in the air's CO2 content.  But we digress.

In considering the first of the three major signs of the deteriorating health and vitality of coral reefs, Barber et al. indicate that localized bleaching can be caused by a number of different factors, including reduced salinity, exposure to air during periods of low sea level, abnormal sedimentation rates, high levels of bacteria, and the presence of toxic chemicals.  "However," he and his colleagues continue, "the most common cause of recent, mass-bleaching episodes is thought [our italics] to be elevated seawater temperature."

During the warm El Niņo phase of the El Niņo/Southern Oscillation or ENSO phenomenon, Barber et al. note there are extended periods of clear sky and no wind.  "Under these conditions," they write, "the thin layer of seawater above a reef warms rapidly during a single day and the smooth sea surface allows more than the average radiant energy to penetrate the surface layer," adding that when these conditions are coupled with lower than average sea level, "thermal stress is compounded by exposure of the coral to air and unfiltered doses of UV radiation."  And it is this "unholy quartet," as they put it, of low winds, clear skies, warm surface temperatures and low sea level that is thought to wreck havoc on corals during ENSO warm phases.

This theory came to the fore with the 1982-83 El Niņo and grew in prominence as coral bleaching increased in intensity, areal coverage and range with each following ENSO event.  It was weakened, however, by the fact that the 1987 and 1990-92 El Niņos were not accompanied by particularly high temperature extremes.  Even more confounding, say Barber et al., was the fact that "strong bleaching in the eastern tropical Pacific occurred following the onset of cool La Niņa conditions in March 1998."  Hence, there is reason to believe that a number of other phenomena may well have played significant roles in the observed coral bleaching of the past two decades.

Prominent on such a list would have to be the other major scourges of corals identified by Barber et al.: macroalgal overgrowth and emerging coral diseases.  Guilt by association aptly describes the first of these phenomena, as macroalgal overgrowth typically expresses itself most strongly after major bleaching events.  In addition, Barber and his colleagues note that "in recent years, coral reefs do not appear to be rebounding successfully from overgrowth after episodes of bleaching."  Of course, the question remains: Why?

One possibility mentioned by the marine scientists is that "fishing on a reef, even at so-called 'sustainable levels,' may remove enough of the herbivorous fish to tip competition in favor of macroalgae."  If the plants are not browsed as heavily as they used to be, for example, they will be able to spread more readily and claim more of the reef surface as their own.  Hence, it is likely that a number of different phenomena - Barber et al. mention loss of herbivorous invertebrates to disease and anthropogenic-induced eutrophication and sedimentation, as well as disturbance from extreme weather events such as hurricanes - may currently be conspiring to limit coral recovery from periodic bleaching, thus rendering permanent temperature-induced damage that might otherwise have been but transitory.

Of even greater importance in this regard are emerging coral diseases.  Barber et al. note that "in the last 30 years, previously undocumented symptoms of disease have appeared in virtually every geographic region supporting coral reefs," and that many people believe "bleaching is a result of bacterial infection, rather than a simple physiological response to adverse physical conditions."  But why should bacterial infection have risen so dramatically over the past quarter century or so?  And why should it be expressed most strongly during warm phases of ENSO events?

One possibility, in the words of Barber et al., is that the ever-increasing combined effects of all anthropogenic-induced environmental changes have finally "so weakened coral reefs that the ambient complement of pathogens can now overwhelm the natural resistance of corals," which explains our past-quarter-century question.  And since higher temperatures generally stimulate bacterial growth and enable bacteria to better adhere to coral surfaces, there is reason to expect the adverse effects of these many environmental stresses to be most vividly expressed during periods of higher sea surface temperatures, which explains our warm-phase-ENSO question.

Another idea has to do with the significant reorganization of the global climate system that occurred in the mid-1970s.  During this period of large-scale readjustment in weather patterns, the North Atlantic Oscillation index - the strength of which is expressed as the difference in normalized sea-level atmospheric pressure between Lisbon, Portugal and Stykkisholmur, Iceland - shifted from weak to strong.  This change was accompanied by decreased rainfall in the African Sahel, which led to enhanced dust loading of the atmosphere.  And hitching a ride on the more plentiful dust particles were greater numbers of "novel pathogenic microorganisms," as Barber et al. describe them, which brought diseases that were deadly to many corals.  In addition, they note that the inevitable enhanced dustfall brought more essential micronutrients to many parts of the world, thereby stimulating the "virulence of native microflora" that compete with native corals.

In concluding their review of the declining health of coral reefs, the authors note that global change is usually in first place on most people's lists of "the usual suspects," which are "frequently assumed to be responsible for this degradation."  However, they say "the geographic pattern and timing of the spread of coral diseases, bleaching and macroalgal overgrowth suggest that additional large-scale mechanisms may also be involved."  Thus, although "it is convenient to use coral decline in campaigns against global warming," Barber et al. conclude that "expedient action must not prevent us from investigating other processes."

We whole-heartedly agree.  The coral conundrum is exceedingly complex; and we must not let uninformed passion override reason in trying to address the issue.

References
Barber, R.T., Hilting, A.K. and Hayes, M.L.  2001.  The changing health of coral reefs.  Human and Ecological Risk Assessment 7: 1255-1270.

Buddemeier R.W.  2001.  Is it time to give up?  Bulletin of Marine Science 69: 317-326.

PS:
Perhaps it may occur to some that a CO2-induced reduction in coral calcification rate - which has recently been predicted to be a direct consequence of elevated atmospheric CO2 concentration - may be one of those "other" unidentified "large-scale mechanisms" that Barber et al. refer to as possibly being the ultimate culprit behind the past quarter-century's worldwide decline in coral vitality.  Although Barber and his colleagues say "the connection between atmospheric carbon dioxide and reduced calcification is one coral reef problem in which both causality and the physiological mechanism are well known," one must remember, as they put it that "ironically, in situ observations of this rate change are not yet possible," which is merely another way of saying they have not yet been observed.

And why have declining coral calcification rates not yet been observed in the real world?  Because coral calcification is a life-driven process, the rate of which is not totally constrained by "the saturation state of the carbonate mineral aragonite in ocean waters," upon which Barber et al. state it "ultimately depends," as suggested by purely chemical considerations.  In fact, as they more correctly state in another place in their paper, "a large portion of the energy provided by the photosynthetic partner [the dinoflagellate algae that take up residence in the coral and "produce the large amounts of reducing power required to fuel calcification"] is used to maintain the high rate of calcification necessary for the building of massive carbonate platforms, the hallmark of coral reefs."

So, as the work of Barber et al. indicates, the coral conundrum remains an enigma.  Much is known, but some of the most important things we need to know to resolve the dilemma have yet to be discovered.  In the mean time, we can state with some assurance that the ongoing rise in the air's CO2 content has yet to be convicted of having anything to do with the declining health of earth's coral reefs.  In fact, we would not be surprised to learn, when all is said and done, that atmospheric CO2 enrichment may actually be found to exert a positive influence on coral reefs, just as it does on terrestrial ecosystems (Idso et al., 2000).

Additional Reference
Idso, S.B., Idso, C.D. and Idso, K.E.  2000.  CO2, global warming and coral reefs: Prospects for the future.  Technology 7S: 71-94.