9.3.3. Biodiversity and Small Islands

The biodiversity of islands also could be adversely affected by climate change. A wide range of changes might be expected, including alterations in population size, species distribution, species composition, and the geographical extent of habitats and ecosystems, as well as an increase in the rate of species extinction. One of every three known threatened plants are island endemics; among birds, approximately 23% of island species are threatened, compared with only 11% of the global bird population (McNeely et al., 1993). Although this situation is believed to be linked closely to increasing population pressures and habitat alteration, additional stressors-such as projected climate change effects-could further adversely affect island biodiversity.

Small islands are variable in their marine, coastal, and terrestrial biodiversity. Some are very rich; for example, coral reefs have the highest biodiversity of any marine ecosystem, with some 91,000 described species of reef taxa. Table D-7 of Annex D shows that endemism among terrestrial flora is high in Fiji (58%), Mauritius (46%), Dominican Republic (36%), Haiti (35%), and Jamaica (34%). In the case of Cuba, 42.6% of the known flora and fauna is endemic (Vales et al., 1996). In contrast, other island ecosystems such as low-reef islands tend to have both low biodiversity and low endemism.

The impacts of climate change, in association with human-induced stresses, probably would result in a loss of biodiversity. Bleaching of coral reefs as a result of changes in sea-surface temperature may deplete one of the world's most species-rich ecosystems (IPCC 1996, WG II, Section 9.4.5). In addition, the capacity of species and ecosystems, such as mangroves, to shift their ranges and locations in response to climate change will be hindered by land-use practices that have fragmented existing habitats. The establishment of nature reserves (terrestrial as well as marine) therefore is worth consideration as a viable option for arresting the decline in terrestrial, marine, and coastal biodiversity.

9.3.4. Hydrology and Water Resources

It is highly probable that the effects of climate change will lead to adjustments in the global hydrological cycle, which could affect the distribution and availability of regional water resources. Climate variability on seasonal to interannual time scales can cause changes in precipitation-which can affect the magnitude, rate, and timing of runoff and the frequency and intensity of floods and droughts. Temperature variations can result in changes in evapotranspiration, soil moisture, and infiltration rates (IPCC 1996, WG II, Sections 10.1, 10.3, 10.4).

In many small island states (such as the islands of the eastern Caribbean), the annual rainfall regime often is characterized by pronounced wet and dry seasons. In some countries (e.g., Antigua and Barbuda, Barbados, Grenada), as much as 65% of the annual rainfall may occur during the wet season (June to December); this rainfall is associated largely with the northerly migration of the Inter-Tropical Convergence Zone (ITCZ) and the passage of major weather systems such as easterly waves, tropical depressions, storms, and hurricanes (Nurse, 1985; Gray, 1993). Therefore, to the extent that the availability of water resources in these islands is dependent on heavy rainfall events, changes in the occurrence of these phenomena inevitably will impact water supplies. In the south Pacific, the SPCZ plays a similar role (Salinger et al., 1995).

There is growing evidence that hydrological variability might be associated with the occurrence of mega-scale climate anomalies, such as those associated with the ENSO phenomenon (IPCC 1996, WG II, Section 10.2.1). In the tropics and low-latitude regions of the Southern Hemisphere, the ENSO phenomenon is a major factor in year-to-year climate variability, with a marked effect on rainfall patterns (Pittock, 1984; Philander, 1990; Whetton et al., 1996). Similarly, floods experienced in the Gulf states, Cuba, and other parts of the Americas in the early 1980s and hurricanes in Tahiti and Hawaii during the same period have been linked to a major ENSO event in 1982-83 (Shea, 1994). Possible climate changes associated with ENSO events could have serious consequences for water supplies and agriculture in many nations. As with sea-level rise, atoll communities dependent on rainfall for fresh water could be at risk from precipitation variability associated with anomalies such as the ENSO phenomenon (Meehl, 1994).

Coral islands and atolls are particularly sensitive to changes in groundwater recharge because almost all of their water supply comes from groundwater sources. In The Bahamas, for instance, freshwater lenses are the only exploitable groundwater resources; these lenses are affected periodically by salinity intrusions caused by overpumping and excess evapotranspiration (Cant, 1996). Sea-level rise may precipitate the intrusion of saltwater into the freshwater lens, reducing the quality and quantity of potable water, if the recharge rate or the width of the island is reduced. In contrast, if recharge and island-width remain constant or expand with rising sea level, the freshwater lens may increase in size (Buddemeier and Oberdorfer, 1990).

Various options have been suggested for minimizing the effects of climate change on water resources (IPCC 1996, WG II, Section 12.5.5), and many of these strategies are worthy of consideration by small island states. Options that these countries may wish to evaluate include the harvesting of rainwater, more efficient and extensive use of surface water, artificial recharge of aquifers with rainwater or treated wastewater, and more efficient management of existing supplies and associated infrastructure (e.g., use of various water-saving devices, reduction of leaks, replacement of worn pipes, and recycling).

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