Since most people are not expecting the luminosity of the sun to radically intensify any time soon, those who love to worry about things environmental have consequently zeroed in on CO2-induced global warming as their favorite major threat to coral reefs ... and to everything else of real or imagined value.  In fact, the hype surrounding the subject has grown so intense that Buddemeier (2001) has actually declared - and in a scientific article, no less - that because of anticipated CO2-induced increases in future sea surface temperatures, "coral reefs are doomed" and that their total demise will be essentially complete "probably within the next few decades."

Although a host of less-esoteric natural and anthropogenic disturbances could well cause corals to experience a dramatic decline in vitality in future decades - if not in a few years (see Coral Reefs in our Subject Index) - we think that theories linking their predicted demise to the upward trend in the air's CO2 content will never be vindicated.  Why?  Because the earth appears to possess what we have chosen to call a "ThermoSolarstat," i.e., an amazingly effective meteorological mechanism that jumps into operation to suppress the intensity of solar radiation to which corals are exposed when dangerously-high water temperatures are approached, and which thus also tends to suppress increases in water temperature that are prompted by increases in the intensity of any thermal forcing factor, such as the currently-popular CO2-augmented greenhouse effect.

How does the mechanism - which is composed of at least two major components - work?  According to Hoegh-Guldberg (1999), 29.2°C is the threshold water temperature above which significant bleaching can be expected to occur in many tropical corals.  However, as Sud et al. (1999) have demonstrated in elucidating the functioning of the first of the ThermoSolarstat's two primary components (based on data obtained from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment), deep atmospheric convection is typically initiated whenever sea surface temperatures (SSTs) reach a value of about 28°C, so that an upper SST on the order of 30°C is rarely exceeded.

Initially, according to this concept, the tropical ocean acts as a net receiver of energy in its warming phase; but as SSTs reach 28-29°C, the cloud-base airmass is charged with sufficient moist static energy for the clouds to reach the upper troposphere.  At this point, the billowing cloud cover reduces the amount of solar radiation received at the surface of the sea, while cool and dry downdrafts produced by the moist convection tend to promote ocean surface cooling by increasing sensible and latent heat fluxes at the air-sea interface that cause temperatures there to decline.

This "thermostat-like control," as Sud et al. describe it, tends "to ventilate the tropical ocean efficiently and help contain the SST between 28-30°C," which is essentially a fluctuating temperature band of ±1°C centered on the bleaching threshold temperature of 29.2°C identified by Hoegh-Guldberg.  This particular component of the atmosphere's ThermoSolarstat, i.e., the component that creates towering cumulonimbus clouds at the appropriate critical SST, also greatly reduces the flux of solar radiation received at the surface of the sea, thereby providing a dual approach to relieving the two main stresses (solar and thermal) that may be experienced by corals that are teetering on the brink of potentially irreversible bleaching.

Some other intriguing observations also point to the existence of a natural phenomenon of this nature.  Satheesh and Ramanathan (2000), for example, determined that polluted air from south and southeast Asia absorbs enough solar radiation over the northern Indian Ocean during the dry monsoon season to heat the atmosphere there by 1-3°C per day at solar noon, thereby greatly reducing the intensity of solar radiation received at the surface of the sea.  Ackerman et al. (2000), however, calculated that this atmospheric heating would decrease cloud-layer relative humidity and reduce boundary-layer mixing, thereby leading to a 25-50% drop in daytime cloud cover relative to that of an aerosol-free atmosphere, which could well negate the surface cooling effect suggested by the findings of Satheesh and Ramanathan.  But in a test of this hypothesis based on data obtained from the Extended Edited Cloud Report Archive, Norris (2001) determined that daytime low-level ocean cloud cover (which tends to cool the water surface) not only did not decrease from the 1950s to 90s, it actually increased ... in both the Northern and Southern Hemispheres and at essentially all hours of the day.

Commenting on this finding, Norris remarked that "the observed all-hours increase in low-level cloud cover over the time period when soot aerosol has presumably greatly increased argues against a dominant effect of soot solar absorption contributing to cloud 'burn-off'."  Hence, he says, "other processes must be compensating," one of which, we suggest, could well be the one described by Sud et al.

Another process - and our proposed second component of earth's ThermoSolarstat - is the "adaptive infrared iris" phenomenon that has been described by Lindzen et al. (2001).  Working with upper-level cloudiness data obtained from the Japanese Geostationary Meteorological Satellite and SST data obtained from the National Centers for Environmental Prediction, the inquisitive atmospheric scientists found a strong inverse relationship between upper-level cloud area and the mean SST of cloudy regions, such that the area of cirrus cloud coverage (which tends to warm the planet) normalized by a measure of the area of cumulus coverage (which tends to cool the planet) decreased about 22% for each 1°C increase in the SST of the cloudy regions.

"Essentially," state the scientists, "the cloudy-moist region appears to act as an infrared adaptive iris that opens up and closes down the regions free of upper-level clouds, which more effectively permit infrared cooling, in such a manner as to resist changes in tropical surface temperatures."  So substantial is this phenomenon, Lindzen et al. are confident it could "more than cancel all the positive feedbacks in the more sensitive current climate models," which are routinely used to predict the climatic consequences of projected increases in atmospheric CO2 concentration and equally routinely - but uncritically - used by people bent on changing the way the world does business on the basis of the climate catastrophes forecast by those models.

All this is well and good; the meteorological aspects of the ThermoSolarstat are clearly supported by significant bodies of real-world data.  But is there any real-world evidence the ThermoSolarstat has actually been instrumental in preventing coral bleaching that would otherwise have occurred during periods of unusually high thermal stress?  The answer, or course, is yes; and it comes in an important paper published just a couple of months ago, wherein Mumby et al. (2001) examined long-term meteorological records from the vicinity of the Society Islands, which provide what they call "the first empirical evidence that local patterns of cloud cover may influence the susceptibility of reefs to mass bleaching and subsequent coral mortality during periods of anomalously high SST."

With respect to the great El Niño of 1998, Mumby and his colleagues determined that SSTs in the Society Islands sector of French Polynesia were above the 29.2°C bleaching threshold for a longer period of time (two months) than in all prior bleaching years of the historical record.  However, mass coral bleaching, which was extensive in certain other areas, was found to be "extremely mild in the Society Islands" and "patchy at a scale of 100s of km."

What provided the relief from extreme sun and heat, without which, Mumby et al. have concluded, "mass bleaching would have occurred"?  As he and his associates describe it, "exceptionally high cloud cover significantly reduced the number of sun hours during the summer of 1998," much as one would have expected earth's ThermoSolarstat to have done in the face of such anomalously high SSTs.  The marine scientists also note that extensive spotty patterns of cloud cover, besides saving most of the coral they studied, "may partly account for spatial patchiness in bleaching intensity and/or bleaching-induced mortality in other areas."

Although the ThermoSolarstat cannot protect all of earth's corals from life-threatening bleaching during all periods of anomalously high SSTs, it apparently protects enough of them enough of the time to insure that sufficiently large numbers of corals survive to perpetuate their existence, since living reefs have persisted over the eons in spite of the continuing recurrence of these ever-present environmental threats.  And perhaps that is how it has always been, although as noted in our Editorial The Coral Conundrum, there are currently a host of unprecedented anthropogenic forces of site-specific origin that could well be weakening the abilities of some species to tolerate the types of thermal and solar stresses they have successfully "weathered" in the past.

References
Ackerman, A.S., Toon, O.B., Stevens, D.E., Heymsfield, A.J., Ramanathan, V. and Welton, E.J.  2000.  Reduction of tropical cloudiness by soot.  Science 288: 1042-1047.

Brown, B.E.  1997.  Coral bleaching: causes and consequences.  Coral Reefs 16S: 129-138.

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

Hoegh-Guldberg, O.  1999.  Climate change, coral bleaching, and the future of the world's coral reefs.  Marine and Freshwater Research 50: 839-866.

Lindzen, R.S., Chou, M.-D. and Hou, A.Y.  2001.  Does the earth have an adaptive infrared iris?  Bulletin of the American Meteorological Society 82: 417-432.

Mumby, P.J., Chisholm, J.R.M., Edwards, A.J., Andrefouet, S. and Jaubert, J.  2001.  Marine Ecology Progress Series 222: 209-216.

Norris, J.R.  2001.  Has northern Indian Ocean cloud cover changed due to increasing anthropogenic aerosol?  Geophysical Research Letters 28: 3271-3274.

Satheesh, S.K. and Ramanathan, V.  2000.  Large differences in tropical aerosol forcing at the top of the atmosphere and Earth's surface.  Nature 405: 60-63.

Sud, Y.C., Walker, G.K. and Lau, K.-M.  1999.  Mechanisms regulating sea-surface temperatures and deep convection in the tropics.  Geophysical Research Letters 26: 1019-1022.