How does rising atmospheric CO2 affect marine organisms?

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Coral Reefs (Responses to Solar Radiation Stress) -- Summary
In considering coral responses to stress imposed by high solar irradiance, one simple example of such adaptation comes from studies of corals that exhibit a zonation of their symbiont taxa with depth, where symbiont algae that are less tolerant of intense solar radiation grow on corals at greater depths below the ocean surface (Rowan and Knowlton, 1995; Rowan et al., 1997). A link between solar-induced tissue damage and the presence of photoprotective proteins has also been noted (Brown et al., 1999; Gates and Edmunds, 1999). For example, it has been demonstrated that zooxanthellae in corals possess a number of light quenching mechanisms that can be employed to reduce the negative impacts of excess light (Hoegh-Guldberg and Jones, 1999; Ralph et al., 1999). Both the coral host and its symbionts also have the capacity to produce amino acids that act as natural "sunscreens" (Hoegh-Guldberg, 1999); and they can regulate their enzyme activities to enhance internal scavenging systems that remove noxious oxygen radicals produced in coral tissues as a result of high light intensities (Dykens and Shick, 1984; Lesser et al., 1990; Matta and Trench, 1991; Shick et al., 1996).

Another adaptive mechanism proposed to lessen the stress of solar irradiance is coral tissue retraction, according to Brown et al. (1994), who studied the phenomenon in the scleractinian coral Coeloseris mayeri at coral reefs in Phuket, Thailand, by examining the retraction and recovery of coral tissues over a tidal cycle. Results of their analysis showed that extreme tissue retraction was observed approximately 85 minutes after initial sub-aerial coral exposure. Tissue retraction, however, did not involve any reduction in chlorophyll concentration or algae symbiont abundance; and the tissues expanded over the coral skeletons to pre-retraction conditions following the return of the tide. The adaptive benefits of tissue retraction, state the authors, "include increased albedo, leading to a reduction in absorbed solar energy of 10%, ... and possible avoidance of photochemical damage or photoinhibition at high solar irradiance."

Another intriguing idea has been proposed by Nakamura and van Woesik (2001), who upon evaluating the bleaching of large and small coral colonies along the western coast of Okinawa, Japan, during the summers of 1998 and 2001, argue that small coral colonies should survive thermal and light stress more readily than large coral colonies based on mass transfer theory, which suggests that rates of passive diffusion are more rapid for small colonies than for large colonies. Still another reason why large coral colonies suffer more than small colonies during environmental conditions conducive to bleaching is the fact that small Acropora recruits, according to Bena and van Woesik (2004), "contain high concentrations of fluorescent proteins (Papina et al., 2002), which have photoprotective properties (Salih et al., 2000)," and they note that "a high concentration of photoprotective pigments in early life, when planulae are near the surface and as newly settled recruits, may facilitate survival during this phase as well as during stress events involving both high irradiance and thermal anomalies (van Woesik, 2000)."

In addition to the adaptive phenomena described above, the earth appears to possess an amazingly effective meteorological mechanism, or "ThermoSolarStat," 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 CO2-augmented greenhouse effect.

How does the mechanism - which is composed of at least two major components - work? According to Hoegh-Guldberg (1999), 29.2C 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 28C, so that an upper SST on the order of 30C 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 28-29C, 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 28-30C," which is essentially a fluctuating temperature band of 1C centered on the bleaching threshold temperature of 29.2C 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 1-3C 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 1C 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 by Mumby et al. (2001), wherein they 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 Nio of 1998, Mumby and his colleagues determined that SSTs in the Society Islands sector of French Polynesia were above the 29.2C 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."

In conclusion, 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 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
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Bena, C. and van Woesik, R. 2004. The impact of two bleaching events on the survival of small coral colonies (Okinawa, Japan). Bulletin of Marine Science 75: 115-125.

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Last updated 25 February 2009