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Mass bleaching and the environment

Some of the important features of mass bleaching are as follows.

Initiation

Geographic variation

Taxa affected

Side Effects

Links to El Niño events

There is no clear link between enhanced greenhouse warming and the frequency or intensity of El Niño events. Analyses of historical records and projections from General Circulation Models are both ambivalent on the subject. The general climatic changes accompanying El Niño development are fairly well understood, although the factors controlling their initiation, intensity and periodicity remain obscure. The 1997-98 El Niño event was the most extreme in recorded history yet it is still possible that this and the two other major events in the past two decades (1981-82 and 2001-02) were exacerbated by other, slower, climatic cycles which are part of the natural variability of the Earth’s climate and not a response to greenhouse warming.

Although any direct causal link between enhanced greenhouse warming and El Niño intensity and frequency is uncertain, it is clear beyond doubt that El Niño cycles and mass bleaching are connected. Mass bleaching is not caused by a direct overall increase in ocean temperature but by short term concentrations of heat in the affected areas. On the Great Barrier Reef these temperature increases are caused by El Niño events which pulse oceanic water from the Western Pacific Warm Pool, perhaps 1-2°C above what was once normal, into coastal regions. If this water is then trapped in the lagoon of the Great Barrier Reef it can warm still further, exacerbating the effects of the original pulse.

Essential points about El Niño, global ocean temperatures, and mass bleaching are:

Acclimatisation and adaptation

Acclimatisation (where the individual’s tolerance of environmental conditions increases during their lifetime) and adaptation (an evolutionary process involving natural selection through survival of the fittest) are seemingly the only escape routes corals have from the warm world of the future.

Acclimatisation There is evidence on both local and global scales that the same and/or closely related coral species show different tolerances to temperature in different locations. On local scales, good examples are corals that tolerate the very high temperatures found in intertidal pools, in water around natural thermal vents or close to thermal outlets of power stations. Normal maximum water temperatures found in particular geographic areas play a large role in determining tolerance to bleaching. Whole suites of corals can survive 36 °C in the Arabian Gulf, parts of the southern Red Sea and sporadically elsewhere. Like most animals, corals may adapt to tolerate these temperatures by altering biochemical pathways. On local scales this process is likely to be due to acclimatisation whereas across widely separated geographic areas there may be a larger component of genetic selection, especially where local tolerance to extreme conditions is involved.

Adaptation Corals were probably once adapted to higher temperatures in the geological past. However the template of today’s oceans is so different from those of the remote past that most meaningful comparisons are questionable. Studies showing that the genetic history of corals and that of zooxanthellae closely match argue against any medium-term history of adaptation by the changing of symbionts.

J.E.N. Veron