Published on: 12/3/2020 12:37:06 AM

Corals bleach when the coral animal host is stressed and expels the symbiotic zooxanthellae (algae) that provide much of the energy for coral growth, and coral reef growth. Although several different stresses cause bleaching, by far the most significant cause of coral bleaching in the past 25 years has been sea surface temperatures that exceed the normal summer maxima by 1 or 2°C for at least 4 weeks. This results in excessive production of toxic compounds in the algae that are transferred to the host coral. The host coral reacts by expelling their symbiotic algae, leaving the coral ghostly white and particularly susceptible to death from starvation or disease. If conditions become more favourable, corals often recover, although they often experience reduced growth and may skip reproduction for a season. In 2005, many bleached corals did eventually die.

Coral bleaching was first noticed as a significant problem in the wider Caribbean region in 1983. Concurrently there were increases in coral disease across the region, thus the assumption was made that these were both associated with higher temperatures. The bleaching and outbreaks of infectious diseases, such as white plague, have caused such major losses in the branching staghorn and elkhorn corals which are the most characteristic of Caribbean reefs and were once major contributors to reef construction.

The bleaching in 2005 ‘coincided’ with major outbreaks of coral diseases which saw extensive shrinkage in the cover of live corals throughout the Caribbean. While many corals started to recover when seawater temperatures dropped with the onset of winter, coral diseases broke out and resulted in significant losses of coral cover, notably along the coast of Florida, in Belize, the Virgin Islands, and the Lesser Antilles. The accepted explanation is that bleached corals are stressed, lack reserve lipid supplies and are effectively starving, making them more susceptible to disease. Ocean acidification is a parallel climate change threat to coral reefs that results from increased concentrations of CO2 dissolving in seawater, which reduces its pH. This process is called ‘ocean acidification’, and by the end of this century, acidification may be proceeding at a rate that is 100 times faster and with a magnitude that is 3 times greater than anything experienced on the planet in the last 21 million years. How this will affect marine ecosystems is unknown, but impacts on marine calcifiers could be considerable. Using the pH levels expected by the end of this century, laboratory studies show a significant reduction in the ability of reef-building corals to grown their carbonate skeletons, making them both slower to grow and more vulnerable to erosion. This would also affect the basal structure of coral reef itself.

While the long-term consequences of ocean acidification on corals is not known, corals do not seem to be able to easily adapt to such rapid changes. All predictions from climate change models point to this becoming an equally severe threat to corals and coral reefs as bleaching. Hurricanes and extreme weather events are also predicted to become more frequent and severe as the pace of climate change quickens. There is increasing evidence that the proportion of more destructive hurricanes has increased in recent decades, although the total incidence of tropical storms has not increased. Stronger hurricanes will result in more severe wave damage and flooding from the land, thereby adding an additional stress to already stressed reefs. Low to moderate strength hurricanes can be beneficial during summer, however, by cooling surface waters and reducing the likelihood of coral bleaching.

There is insufficient evidence or indications that the other potential climate change stresses will result in significant damage to coral reefs. There is a potential for negative impacts from possible shifting of ocean currents or associated rises in UV concentrations; however, these are not evident at the moment. Sea level rise will not directly threaten corals, but may render coral reef islands uninhabitable, thereby threatening coral island cultures and nations.


Implications of 2005 for coral managers

Coral reef managers were unprepared for the climate-related destructive events of 1998. Many coral reef managers in the Indian Ocean and Western Pacific reported that massive coral bleaching in mid to late 1998 was devastating their coral reefs, and they asked ‘what have I done wrong to cause the corals to die’. They were perplexed that corals were dying on the same reefs that they were actively managing to remove pollution, sedimentation and over-fishing stresses. The cause of the problems to their reefs was related to climate change via a particularly severe El Niño and La Niña climate switch that raised sea surface temperatures (SSTs) above levels that had ever been recorded on those coral reefs. We now know that no management actions could have prevented the extent of coral death; the only advice the coral reef research and management community could offer was that ‘better managed reefs will recover more rapidly than those under human stresses.

The events of 1998 stimulated the international coral reef community to develop advice for coral reef managers faced with similar circumstances in the future. A Reef Manager’s Guide to Coral Bleaching was developed in 2006 to provide that advice for coral reef managers faced with stresses beyond their immediate control. The report guides reef managers into steps they can take at national and global levels to raise awareness of the potential devastation that increasing global climate change, though the release of greenhouse gases, can have on coral reefs. However, the emphasis is on providing managers with practical advice on how to increase protection of those reefs that are either naturally resistant or tolerant to bleaching, assist in promoting adaptation mechanisms that enhance reef resilience, while simultaneously reducing local pressures on the reefs and nearby ecosystems to enhance chances for natural recovery. Importantly, the Guide advises reef managers on how to engage with local people and assist in maintaining socioeconomic well-being and bringing them on board to assist in the sustainable use of their coral reefs.

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Sadly, for coral reefs, all predictions from the Intergovernmental Panel on Climate Change (IPCC) reports in 2007 indicate that the extreme warming of 2005 will not be an isolated event. It will probably happen again in the future and, when it does, the impacts will be even more severe. The IPCC concluded that human-induced climate change will warm the world by 1.8 to 4.0ºC by the year 2100. This warming will affect most of the wider Caribbean Sea making years like 2005 more common and more devastating for coral reefs. In addition, increasing acidity in the seawater with the solution of more CO2 will result in slower growth of corals that are trying to recover from bleaching and other disturbances.

One other potential consequence of the human-induced warming is an increase in the frequency of more damaging Category 4 and 5 hurricanes in the Caribbean. These storms develop as waters warm over the tropical North Atlantic and Caribbean waters. It is predicted that warmer surface waters with increased amounts of thermal energy will fuel increases in tropical storm strength. The latest predictions are for an increase in the more intense Category 4 and 5 hurricanes that will probably cause significant damage to the coral reefs and the communities that depend upon them. This is a pivotal moment for the coral reefs. The world is already committed to some further warming due to past greenhouse gas emissions and the expected emissions from existing world energy infrastructure. Thanks to more than a century of ‘committed warming’; events like 2005 are expected to occur more frequently by the 2030s. The only possible way to sustain some live coral on the reefs around the world will be to carefully manage the direct pressures like pollution, fishing and damaging coastal developments, and hope that some coral species are able to adapt to the warmer environment. However, a dramatic reduction in greenhouse gas emissions in the next 20 years will be critical to control further warming and dangerously high CO2 levels that will probably reduce the robustness and competitive fitness of corals and limit the habitats for many other organisms living on Caribbean coral reefs.