Sizing the impact: Coral reef ecosystems as early casualties of climate change

The environment surrounding the world's coral reefs is rapidly changing. Already tropical oceans are almost one degree warmer than a century ago and current rates of warming are among the highest recorded (e.g. possibly as high as 5 degreesC per century, Northern tropical Pacific Ocean, NOAA 2000). Mass bleaching events (known only since 1979) are caused by small excursions in sea temperature above average summer maxima. These events are increasingly followed by the widespread mortality of reef-building corals and other symbiotic invertebrates. Mass coral bleaching and mortality begin with the destruction of the dark reactions of the photosynthetic reactions of the symbiotic algae. In some cases (e.g. recent events in NW Australia, Okinawa, Seychelles), communities of corals have been reduced from 50-80 % to less than 5 % living cover. Thermal mortality events have also increased in scale and intensity over the past two decades. There is little evidence that corals or their symbionts are acclimating or adapting fast enough to these changes. Projections from General Circulation Models of tropical sea temperatures reveal that the thermal tolerance of these organisms will be exceeded on an annual basis by 2030-2050. This leads to the inescapable conclusion that reef-building corals will become rare on coral reefs within the next 50 years. Evidence of the scale of impact expected within the next few decades is already available. In the warm conditions of 1998 alone, 10-16 % of the world's reef-building corals died. The impacts of thermal stress, coupled with carbon dioxide induced reductions in seawater alkalinity (causing reduced calcification by corals) and the impact of non-climate related stresses, suggest that even mild global climate change will have major impacts on the health of tropical marine ecosystems and coastlines. These changes in turn are expected to have major influences on the fisheries, coastal protection and tourism associated with the nations that depend on these ecosystems