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Coral Reef Bleaching and Sea Surface Temperature

Coral "bleaching" is an unusual response of corals to most stresses that are just short of lethal. The coral loses the microscopic algae which normally live within its cells and which provide corals with their color, most of their ability to rapidly grow skeleton, and much of their food. The "bleached" coral turns transparent, pale, or unusual colors, and enters a starving stage, unable to grow or reproduce. Coral bleaching is a general response to many stresses: coral researchers have known for nearly a century that it can be caused experimentally by stresses including temperatures too hot or too cold, salt concentrations too high or too low, light intensities too high or too low, or suspended sediments too high. If stress becomes worse the bleached coral will likely die, but if stress becomes less the coral will eventually recover in most cases. This can take a few days to over a year depending on the species and the magnitude and duration of stress. Because most stresses which are severe, but not severe enough to kill the coral, will cause bleaching, increased bleaching is an important early warning sign of deteriorating reef health.

The name "bleaching" is inaccurate since "bleached" live tissue is similar to dead coral exposed to "bleach" only at the most superficial level. The "bleached" coral looks white because one can see the white limestone skeleton straight through the clear tissue. This condition is commonly misinterpreted as coral death by observers who fail to recognize that the tissue is still there, despite being transparent. Confusion is caused because when coral is killed by exposure to household "bleach" the destroyed tissue peels off and the bare white limestone skeleton beneath is exposed. On the other hand "bleached" coral tissue is really intact and alive although colorless and in extreme stress. Coral bleaching is widely underestimated because most observers only recognize the most severe cases when most corals have turned completely white in appearance. Many more cases of milder bleaching, which cause only the most sensitive coral species to turn pale, are not usually recognized except by extremely experienced coral researchers.

Coral bleaching has been observed for nearly a century, but before the 1980s all known cases were very limited in extent and duration. Known bleaching mostly took place in small tide pools or shallow enclosed lagoons cut off from the sea by extreme low tides during mid day, when hot cloudless conditions caused temperatures to soar. Most of the remaining cases were caused by high rains and floods washing large volumes of muddy freshwater over shallow reefs, and effects were confined to small affected areas. A few bleaching episodes in corals growing near their extreme cold limits appear to have been caused by cold waters driven onto reefs by frontal weather systems, storms, or deep water upwelling.

In the 1980s bleaching spread dramatically from a few small areas clearly affected by extreme local stress (for example wherever hot water from power plants flowed onto reefs) to huge areas of the ocean covering up to thousands of kilometers (including both stressed sites and those remote from any local stress). Large scale mass bleaching was not observed before the 1980s even in areas where the reefs had been intensively studied for many decades, but since then it has become frequent in all coral reef regions. Of all stresses which could potentially cause widespread mass bleaching, only excessively high temperature was present in all cases. However a few cases of local bleaching also took place as the result of flooding or other causes. Maps of known local and mass bleaching events are accessible on this site.

Global Coral Reef Alliance scientists have shown that every known mass bleaching event followed periods when sea surface temperatures were 1 degree C or more above average values in the warmest month (see papers accessible on this web site). This implies that coral reefs are the most temperature-sensitive of all ecosystems, and cannot take a further warming of one degree. The fact that corals did not bleach on a wide scale before implies that high temperatures on a world-wide scale only began to affect coral reefs in the 1980s (although it is clear that local high temperature bleaching events had happened before on a small scale). This is entirely consistent with the global temperature record, which shows that temperatures increased sharply at the end of the 1970s and have remained high ever since.

Coral bleaching, if it continues, will threaten coral reefs around the world whenever the weather is unusually hot, and will affect the most remote and pristine reefs as well as those subjected to high levels of other stresses.

Surprisingly, those who do not accept the reality of global warming and the adverse affects of temperature on coral reefs have made an intense effort to discredit observations and data linking bleaching to high temperatures.  These people promote the view that no ecosystem is near its upper temperature limit and there is no need to take action to control global warming or reduce greenhouse gas emissions. Additionally, these critics are well funded by greenhouse gas-emitting power lobbies and affiliated political interest groups and have been able to afford large amounts of publicity denying the effects and implications of coral bleaching.  However, these reports are written by people who have never seen bleaching or analyzed the large body of relevant environmental measurements.  These data are available on this site so one can make up one's own mind from the data and maps.
 

Although corals did exist in past geological periods when temperatures were higher than today, those species well adapted to hotter conditions than today vanished during mass extinctions at the start of the ice ages, around 2 million years ago. The survivors, the ancestors of all living corals, were those with the greatest tolerance of cold conditions and have little or no ability to adapt to warmer waters. Evolutionary adaptation to higher temperature should take place over evolutionary time scales of millions of years, but there is no sign that modern corals can adapt physiologically to higher temperature. The hopeful claim that coral reefs will expand with global warming is false because those in the warmest waters will bleach and die but reefs in newly warmed temperate areas would take thousands of years to grow, and then only if water quality were adequately clean. This is not the case since almost all areas where corals could potentially spread with global warming are too polluted to allow reef growth to take place.