Vulnerability of freshwater resources to climate change in the tropical Pacific region

El Nino events and associated droughts adversely affect freshwater resources on islands in the tropical Pacific region. Particularly vulnerable are low-lying atolls because rainwater collection is the main freshwater source on such islands. During El Nino-related droughts, water can be drawn only from the limited freshwater lenses beneath the islands. If drought conditions such as these intensify, the depletion of freshwater resources could affect the habitability of atolls. Average climate change in the pacific region from increased anthropogenic carbon dioxide in a global coupled climate model resembles present-day El Nino conditions as well as the decadal time-scale sea surface temperature and precipitation anomalies observed during the 1980s and early 1990s. These anomalies are a consequence of greater warming of sea surface temperatures in the eastern equatorial Pacific than over the western Pacific warm pool with increased carbon dioxide in the climate model. Attendant increases in precipitation in the central equatorial pacific are also accompanied by precipitation decreases in the northern and southern tropical Pacific (roughly 5°N to 15°N and 5°S to 15°S), as well as in the Australasian and eastern Indian Ocean regions. Associated effects in the midlatitude North Pacific also resemble El Nino conditions and the decadal time-scale signals from the 1980s. Future possible increases of drought conditions in certain tropical Pacific regions, as indicated by the climate model results, could limit the sustainability of atoll populations in those regions, causing migration and increased urbanization, with all the attendant problems, on larger high islands with more stable water supplies.