Conclusions

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

Global climate change poses a broad range of challenges, both acute and chronic, to coral reef ecosystems, particularly when climate change is integrated with the more localized nonclimate stresses coral reefs are currently experiencing.   Table 2 summarizes the stress factors in terms of origin (locally human-induced or global climate change), spatial scale (global, regional, or local), and whether they are predominantly acute or chronic.  Most (but not all) of the global factors are of climatic origin, but many stresses have manifestations at a variety of spatial scales.  We have emphasized the ways in which the various types of stresses interact, as well as the geographic and temporal diversity of reefs, reef organisms, reef habitats, and environments.  Future changes in coral reef condition will reflect this diversity of stresses, present conditions, and biota, but will almost certainly be in the direction of further loss and degradation.

Drawing on the information assembled, we offer the following major conclusions:

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 attempt 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 existing coral species, the acknowledged adaptation potential of reef organisms, the 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 widely 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.