CORAL REEFS AND GLOBAL CLIMATE CHANGE: ANTHROPOGENIC IMPACTS VERSUS RAPID SEA LEVEL RISE

The world’s coral reefs are in decline. Coral cover has diminished during the last thirty years by at least 30% and in some place such as the Caribbean by up to 90%. Climatic stresses such as cyclones and unusually warm ocean temperatures, acting together with high nutrient loads from soil erosion and fertilizers, is proving to be a lethal combination. In 1997-98, exceptionally warm global sea surface temperatures caused mass coral bleaching and reef degradation. The effects were compounded by the inability of many coral reefs to regenerate due to algal blooms from high levels of nutrients. It is predicted that the frequency of coral bleaching will increase dramatically during this century as a result of increased global warming and that coral calcification rates will decrease due to high pCO2 from fossil fuel burning. Thus, the effects of climate change on the world’s already highly stressed coral reefs may well be terminal.

The adaptability of coral reefs however is often underestimated. On longer timescales extreme variations in climate have occurred. Over the past 500,000 years marine isotope stages (or MIS) 11, 9 and 5e were among the warmest and/or longest interglacial periods identified from deep-sea records. Multiple oscillations of sea level during MIS 9 (ca. 320,000 years ago) were recently documented on Henderson Island and during the Last Interglacial (MIS 5e) sea levels were up to 4 m to 6 m higher. During these periods coral growth was generally prolific, a result of rising sea levels providing space for corals to grow and warmer ocean temperatures allowing expansion of reefs to sub-tropical regions. Thus the survival of coral reefs may well depend on the relative rates and timing of sea level rise, global warming and anthropogenic impacts on reefs. These factors will be examined using both the fossil as well as modern coral records.