Climate change and coral bleaching
Sea surface temperature anomalies and observations of coral bleaching, 1998
Threat Description
Many corals and other reef organisms have become highly adapted to local conditions and are extremely sensitive to change. When corals are stressed, they eject their zooxanthellae or cause them to lose their chlorophyll. Withouth zooxanthellae, corals become pale or turn completely white -- a response known as coral bleaching. A variety of factors can trigger bleaching, including temperature extremes, sedimentation, pollution, air exposure, or changes in salinity. However, temperature-correlated bleaching is the most widely reported.
The range of temperatures tolerated by reef-building corals worldwide is relatively narrow, usually between 16 degrees C and 36 degrees C. On any particular coral reef, the range is even narrower. Studies have shown that even temperatures of only 1-2 degrees C above the normal threshold temperature for a few weeks are sufficient to drive a bleaching event. Corals often recover from bleaching, but extreme or prolonged temperature anomalies can cause significant mortality.
Scientific studies now confirm that the earth's surface has warmed 0.6 degrees C during the past hundred years, a rate unprecedented in the past thousand years.[1] Evidence suggests that increases in both air and sea temperatures are mostly a direct result of anthropogenic activities such as burning fossil fuels and land clearing, which release greenhouse gases into the atmosphere.[2] In some places, changes may be even more dramatic, as in Phuket, Thailand, where the temperature increased between 1981 and 1999 at a rate of more than 2 degrees C per hundred years. Sea-surface temperatures have now moved so close to coral thermal limits that the fluctuations of temperatures within natural climatic events such as the El Niño Southern Oscillation (ENSO) can cause massive coral bleaching.[3] In fact, reports of mass coral bleaching have increased greatly since 1979.[4]
Most episodes of mass coral bleaching can be attributed to ENSO events. The most severe ENSO event since statistics have been recorded occurred in 1997-98. [5] Although effects from the 1997-98 event were most severe in the central Indian Ocean, major bleaching was also reported across Southeast Asia, where an estimated 18 percent of reefs were damaged.[6] These patterns of bleaching and coral mortality can be linked to high sea-surface temperature anomalies that were caused by the ENSO event.
Despite the severity of bleaching in the region, recovery is occurring. New coral growth has been observed, but patterns of recovery are site specific. Local turbulence, temperature, salinity, and levels of ultraviolet radiation affect how severely specific sites are impacted and how well they recover.[7] The rate of recovery may also be influenced by other factors including existing levels of human disturbance.
Owing to the lack of data and intense and unpredictable local variations, the RRSEA project was not able to incorporate coral bleaching into the Reefs at Risk threat model. However, observations of bleaching reports from throughout the region are presented on the map above and are summarized by country in the Reefs at Risk case studies database.
Outlook
The wide global extent of coral bleaching observed during the 1997--1998 ENSO foreshadows the likely serious consequences of rising sea-surface temperatures associated with global climate change. The extent and productivity of coral reefs in coming decades may depend on how fast corals can adapt to increased temperature extremes, in terms of both physiological adaptation and evolutionary change. Because corals have generation times that range from decades to centuries, some scientists believe they could take centuries to millennia to adapt-too slow to respond to the current pace of global climate change. Other researchers have pointed to the wider range of temperature tolerances shown by the same species in different areas. They hypothesize that individual corals may be able to adapt but also that the right conditions of currents could allow heat-resistant larvae and zooxanthellae from corals occurring in naturally warmer waters to recolonize newly warming areas.
In addition to the problems associated with rising sea-surface temperatures, corals may also be placed under stress by projected increases in atmospheric CO2. Some scientists believe that elevated atmospheric CO2 levels will reduce the alkalinity of surface waters, thereby reducing the calcification rate and skeletal strength of corals. Increases in atmospheric CO2 thus could cause the rates of reef growth to fall behind rates of natural erosion. The balance of many reefs may shift from that of gradually accreting structures to that of gradually eroding structures. This change might eventually compromise the effectiveness of some coral reefs in providing shoreline protection and other benefits.[8]
Although scientists and others continue to monitor coral reef growth and recovery following bleaching events, it remains unclear whether coral reefs will be able to adapt with sufficient speed to adjust to the dramatic changes predicted under climate change scenarios. Where direct human impacts already threaten coral reefs, resilience may be lower and recovery rates may be slower.
Notes:
1. O.F. Canziani et al., eds., Climate Change 2001: Impacts, Adaptation, and Vulnerability, Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change (New York: Cambridge University Press, 2001), p. 5.
2. S. Levitus et al., "Anthropogenic Warming of the Earth's Climate System," Science 292, 5515 (2001): 267-70; and T. Barnett, D. Pierce, and R. Schnur, "Detection of Anthropogenic Climate Change in the World's Oceans," Science 292, 5515 (2001): 270-74.
3. O. Hoegh-Guldberg, "Climate Change, Coral Bleaching and the Future of the World's Coral Reefs," Marine and Freshwater Research 50 (1999): 839-66.
4. J.M. Lough, "Perspectives on Global Climate Change and Coral Bleaching: 1997-1998 Sea Surface Temperatures at Local to Global Scales," in Proceedings of the Japanese Marine Science & Technology Center (JAMSTEC) International Coral Reef Symposium (Yokosuka, Japan: JAMSTEC, 2000), pp. 215-29.
5. M. McPhaden, "Genesis and Evolution of the 1997-1998 El Niño," Science 283 (1999): 950-54.
6. Wilkinson, Status of Coral Reefs of the World: 2000, p. 18.
7. Wilkinson et al., "Ecological and Socioeconomic Impacts," pp. 188-96.
8. A. Kleypas et al., "Geochemical Consequences of Increased Atmospheric Carbon Dioxide on Coral Reefs," Science 284 (1999): 118-20.