Coral Reefs & Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems

Coral reefs are striking, complex, and important features of the marine environment.  Reefs are geologic formations constructed from the accumulated skeletons of limestone-secreting animals and plants.  The intimately linked plant-animal communities that create them are representative of an ecosystem that occurs in tropical and subtropical waters across the planet, most commonly in shallow oceanic water, and often close to land.  Coral reefs have the highest biodiversity of any marine ecosystem, and they provide important ecosystem services and direct economic benefits to the large and growing human populations in low-latitude coastal zones.

The natural habitat of coral reefs near the junction of land, sea, and air is both varied and variable, and is a potentially stressful environment.  Reef organisms have evolved adaptations over hundreds of millions of years to cope with recurring disturbances:  damage or destruction, followed by recovery or regrowth.  These are natural features of coral reef history.  However, recent global increases in reef ecosystem degradation and mortality (the “coral reef crisis”) appear to be sending a clear message that the rate and nature of recent environmental changes are frequently exceeding the adaptive capacity of coral reef organisms and communities.

The coral reef crisis is almost certainly the result of complex and synergistic interactions among local-scale human-imposed stresses and global-scale climatic stresses.  Both can produce direct and indirect chronic and acute stresses, leaving few, if any, parts of the ocean truly hospitable for healthy coral reef communities. Documented human stresses include increased nutrient and sediment loading, direct destruction, coastal habitat modification, contamination, and the very important chronic indirect effects of overfishing.  The major climate change factor that is becoming increasingly important for coral reefs is rising ocean temperatures, which have been implicated in chronic stress and disease epidemics, as well as in the occurrence of mass coral bleaching episodes.  Also of concern are the effects of increasing atmospheric carbon dioxide (CO₂) on ocean chemistry, which can inhibit calcification—the deposition of the calcium carbonate minerals that are the structural building materials of coral reefs.

Coral reef communities usually recover from acute physical damage or coral mortality if chronic environmental stresses (such as reduced water quality) are weak, and if the acute stresses are not strong or overly frequent.  Coral reefs also withstand chronic stresses in the absence of acute stresses.  The combination of acute and chronic stress, however, often results in the replacement of the coral reef community by seaweeds or some other nonreef system.  Such ecosystem shifts are well advanced in the Caribbean region, where two of the major reef-building coral species have been devastated by disease.  In the Indo-Pacific region, the repeated and lethal episodes of “bleaching” associated with unusually high water temperature raise concern that reefs cannot sufficiently recover between such events.

Whereas remote oceanic reefs will be affected primarily by climate change, reefs close to human populations will continue to be affected by combinations of additional stresses (e.g., reduced water quality, physical damage, and overharvesting) that must be considered together to be understood and managed.   Predictions of the future of coral reefs are difficult because current environmental changes are leading to a combination of surface ocean chemistry and temperature conditions that have almost certainly never occurred over the evolutionary history of modern coral reef systems.

This report reviews the published literature in an effort to analyze the current state of knowledge regarding coral reef communities and the potential contribution of future climate change to coral reef degradation and loss.  The major conclusions of the review are summarized as follows:

1. Climate and localized nonclimate stresses interact, often synergistically, to affect the health and sustainability of coral reef ecosystems.   Stresses associated with climate change, such as high-temperature episodes that promote coral bleaching, reduced calcification, and changes in ocean and atmospheric circulation, present one set of challenges to coral reefs.  However, these stresses may exacerbate other stresses not directly related to climate, such as disease, predation, and the cumulative effects of other nonclimate stresses.  Thus, it is difficult to separate the effects of global climate and local nonclimate influences when considering reef condition or vulnerability.
   
   
2. Coral reef alteration, degradation, and loss will continue for the foreseeable future, especially in those areas already showing evidence of systemic stress.   As we enter an unprecedented climatic state, recent geological and biological history gives us little on which to base predictions regarding the future of coral reef ecosystems. Key uncertainties include the extent to which human activities will continue to alter the environment, how climate variability such as the frequency and intensity of El Niño-Southern Oscillation (ENSO) events will change relative to global temperature, and the biological and ecological responses of coral reef communities to unprecedented future conditions.    However, there is no realistic doubt that continued climate change will cause further degradation of coral reef communities, which will be even more devastating in combination with the continuing nonclimate stresses that will almost certainly increase in magnitude and frequency.
   
   
3. The effects of climate change on global coral reef ecosystems will vary from one region to another.   Although climate change has the potential to yield some benefits for certain coral species in specific regions, such as the expansion of their geographic ranges to higher latitudes, most of the effects of climate change are stressful rather than beneficial.  Reef systems that are at the intersection of global climatic and local human stresses will be the most vulnerable.  Remote, deep, or well-protected reef communities are more likely to provide reserves and refuges for future generations of coral reef organisms, and aesthetic and scientific resources for future generations of humans.
   
   
4. While the net effects of climate change on coral reefs will be negative, coral reef organisms and communities are not necessarily doomed to total extinction.   The diversity of coral species comprising existing reefs, the acknowledged adaptation potential of reef organisms, spatial and temporal variations in climate change, and the potential for human management and protection of coral reef ecosystems all provide possibilities for survival. Nevertheless, coral reefs of the future will be fewer and probably very different in community composition than those that presently exist, and these changes will cause further ecological and economic losses.    
   
   
5. Research into adaptation and recovery mechanisms, and enhanced monitoring of coral reef environments will permit us to learn from and influence the course of events rather than simply observe the decline. Most local (and some regional) nonclimate stresses have the potential to be mitigated and managed more readily than global climate change itself.   A significant step would be a distributed international network of coral reef refuges and marine protected areas, selected on the basis of biological and environmental diversity, connectivity, potential threats, and enforcement feasibility.  Yet, even with such efforts, recent degradation of coral ecosystems combined with future climate change will still pose a significant challenge to the global sustainability of coral reefs.