17.2.4.1. Coral Reefs

Coral reefs represent one of the most important natural resources of many tropical islands. They are a source of food, beach sand, and building materials and function as natural breakwaters along the coasts of many tropical islands. They also provide habitats for many marine animals and reef fish and generate significant revenues for many small island economies through avenues such as tourism (e.g., snorkeling and scuba diving). On many islands, coral reefs are facing severe threats from climate- and non-climate-related stressors. The total areal extent of living coral reefs has been estimated at about 255,000-1,500,000 km² (Spalding and Grenfell, 1997), of which 58% are considered at risk from human activities, according to a global assessment (Bryant et al., 1997).

Owing to their narrow temperature tolerances, some species of corals currently live at or near their thermal limits (Goreau, 1992; IPCC, 1998). SST projections (based on three variants of the Max Planck Institute ECHAM and CSIRO GCMs) suggest that the thermal tolerance of reef-building corals will be exceeded within the next few decades. Moreover, the incidence of bleaching will rise rapidly, with the rate of increase highest in the Caribbean and slowest in the central Pacific region (Hoegh-Guldberg, 1999).

There is now substantial evidence that indicates that "episodic" warming of the ocean surface, as occurs in El Niño years, leads to significant coral bleaching (Brown and Ogden, 1993; Glynn, 1993; Goreau and Hayes, 1994; Wilkinson and Buddemeier, 1994; Brown, 1997a,b; CARICOMP, 1997; Goreau et al., 1997). The major coral bleaching episodes in the past 20 years were found to be associated with periods when ocean temperature were about 1°C higher than the summer maximum. It also has been suggested that bleaching events could occur annually in most tropical oceans in the next 30-50 years (Hoegh-Guldberg, 1999). Bleaching was particularly severe and widespread during the period of the most recent El Niño episode of 1997-1998, which was considered to be the most intense such event on record. On some islands, more than 90% of all live reefs have been affected (Goreau and Hayes, 1994), with branching species generally most severely impacted (Wilkinson 1998, 1999). An assessment of the literature on coral bleaching is provided in Chapter 6.

The impact of increasing CO₂ concentrations in the oceans on coral reefs is now the focus of an emerging though as yet unresolved debate. Since publication of the SAR and the Special Report on Regional Impacts of Climate Change (IPCC, 1998), it has been suggested that the ability of reef plants and animals to make the limestone skeletons that build the reefs is being reduced by rising atmospheric CO₂ concentrations. Indeed, some authors suggest that based on projected CO₂ concentration in the atmosphere, the calcification rate of corals would decline by approximately 14-30% by 2050 (Gattuso et al., 1999; Kleypas et al., 1999). Again, see Chapter 6 for a full evaluation of the main issues in this debate.

Chapter 6 points out that earlier IPCC assessments have concluded that the threat of sea-level rise to reefs (as opposed to reef islands) is negligible. This conclusion was based on projected rates of global sea-level rise from Warrick et al. (1996) on the order of 2-9 mm yr^-1 over the next 100 years. It has been suggested that healthy reef flats will be able to keep pace with projected sea-level rise, given an approximate upper limit of vertical reef growth during the Holocene of 10 mm yr^-1 (Schlager, 1999). However, the prognosis is far less positive in many small island states (e.g., in the Caribbean Sea and the Indian Ocean), where reef structures have been weakened by a variety of anthropogenic stresses. This concern also is applicable to many island countries, where reefs in close proximity to major settlements have been severely stressed. The ability of reefs to keep pace with sea-level rise also will be adversely affected by more frequent coral bleaching episodes and by reduced calcification rates resulting from higher CO₂ concentrations.

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