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

Coral reefs represent some of the most biologically diverse ecosystems on Earth, providing critical habitat to approximately 25 percent of marine species. In addition, these ecosystems provide economic benefits through tourism and fisheries. One recent estimate valued the annual net economic benefits of the world’s coral reefs at $30 billion. However, human activities including development in coastal areas, over-fishing, and pollution have contributed to a global loss of over 10 percent of these valuable ecosystems. An additional 15 percent have been lost due to warming of the surface ocean, and climate change will further contribute to coral reef degradation in the decades ahead.

Coral Reefs and Global Climate Change is the tenth in a series of Pew Center reports examining the potential impacts of climate change on the U.S. environment. It details the likely impacts of climate change over the next century to coral reef ecosystems both in U.S. waters and around the world. Report authors Drs. Robert W. Buddemeier, Joan A. Kleypas, and Richard B. Aronson find:

• Increases in ocean temperatures associated with global climate change will increase the number of coral bleaching episodes. High water temperatures stress corals leading to “bleaching” — the expulsion of colorful, symbiotic algae that corals need for survival, growth, and reproduction. While coral species have some capacity to recover from bleaching events, this ability is diminished with greater frequency or severity of bleaching. As a result, climate change is likely to reduce local and regional coral biodiversity, as sensitive species are eliminated.
• Increases in atmospheric concentrations of carbon dioxide (CO₂) from fossil fuel combustion will drive changes in surface ocean chemistry. The higher the concentration of CO₂ in the atmosphere, the greater the amount of CO₂ dissolved in the surface ocean. Higher dissolved CO₂ increases ocean acidity and lowers the concentration of carbonate which corals and other marine organisms use, in the form of calcium carbonate, to build their skeletons. Thus, continued growth in human emissions of CO₂ will further limit the ability of corals to grow and recover from bleaching events or other forms of stress.
• The effects of global climate change will combine with more localized stresses to further degrade coral reef ecosystems. Although climate change itself will adversely affect coral reefs, it will also increase the susceptibility of reef communities to degradation and loss resulting from natural climate variability such as El Niño events as well as disease, over-fishing, disruption of food webs, and pollution from neighboring human communities.
• Multiple environmental management strategies, from local to global, will be necessary to ensure the long-term sustainability of the world’s coral reef ecosystems. Efforts to reduce emissions of greenhouse gases that contribute to global climate change can reduce the risk of future bleaching events and moderate changes in ocean chemistry. Meanwhile, the establishment of marine protected areas may help protect coral reefs from non-climate stresses as well as enable coral reefs to better adapt to the effects of global climate change.

The authors and the Pew Center gratefully acknowledge the input of Drs. Janice Lough and Peter Glynn on this report. The authors thank Drs. Laurie Richardson and Terry Done for additional reviews and comments and Mark Schoneweis for figure preparation. The Pew Center also thanks Joel Smith of Stratus Consulting for his assistance in the management of this Environmental Impacts Series.