One of the most devastating potential climate related impacts is likely to be water related. According to a 2006 United Nations Development Report, as of 2000, 20 percent of the global population had no natural water supply, and 65 percent had only low-to-moderate supplies. In a world in which shortages of potable water already afflict millions of people, further losses from climate related changes will pose enormous difficulties. While human communities may adapt, natural ecosystems may be permanently destroyed.

• Bradley, Raymond S.; Vuille, Mathias; Diaz, Henry F.; and Walter Vergara. "Threats to Water Supplies in the Tropical Andes." Science 312(5781): 1755-1756. 23 June 2006 at .

Warming at higher altitudes has severe implications for glacial melt. Bradley et al. report that warming in the tropical Andes will be more pronounced at higher rather than lower altitudes. Until recently, few measurements had been made above 4,000 meters, and, thus, warming at higher latitudes had not been adequately documented. The scientists obtained measurements from 268 mountain stations, which indicate that tropical Andes' temperatures increased roughly 0.11 degrees C per decade between 1939 and 1998. Extrapolating these trends, as well as predicted CO2 levels and changes in surface reflectivity as glaciers melt, the scientists entered the data into eight general circulation models and found that glaciers in the American Cordillera -- a mountain range extending from Alaska to Chile -- may disappear within the next few decades

Implications: Many cities are almost entirely dependent on water from high altitude sources. For example, La Paz and Quito, rely partly on glacial water sources. In addition, agriculture relies on a steady glacial source, as does hydroelectric power, which many Andean countries depend on for energy sources. Depletion of such resources could lead countries in search of new forms of energy, and they may turn to fossil fuels which will further exacerbate climate change and, in turn, glacial melt. Ecosystems are also dependent on high altitude water stocks, and the loss of this water supply could lead to ecosystem transformation and biodiversity loss, with grave implications for the services generated by ecosystems.

• Maarten de Wit and Jacek Stankiewicz. "Changes in Surface Water Supply Across Africa with Predicted Climate Change." Science 311(5769): 1917-1921. 31 March 2006.

De Wit and Stankiewicz calculate future water supply in Africa by examining the reduction in drainage capability. Even using (as the authors state) a somewhat optimistic model, they find that there is a nonlinear relationship between rainfall and drainage density, which is a ratio of the length of streams divided by the area of the drainage basin. Areas with low levels of rainfall have very little drainage capabilities. The authors discover that future precipitation regimes will lead to a decrease in drainage, and a quarter of Africa's surface water will be left increasingly vulnerable by the end of the 21st century. In some cases, losses will be extremely severe: for example, de Wit and Stankiewicz predict that regions around Cape Town will lose at least half of their perennial supply.

Implications: Not only do water shortages have drastic implications at local and regional scales, but water bodies (both lakes and rives) are also often areas of intense conflict: almost 40 percent of all international border areas are water-based. The flows (or lack thereof) are already exacerbating regional and international tensions, and will likely become worse as other climate impacts limit food production and increase disease.

• Goswami, B. N.; Venugopal, V.; Sengupta, D.; Madhusoodanan, M. S.; and Prince K. Xavier. "Increasing Trend of Extreme Rain Events Over India in a Warming Environment." Science 314 (5804): 1442-1445. 1 December 2006.

Because the average rainfall over the past five decades in India has remained relatively constant despite rising global temperatures, few clues about climate change's impact on India's monsoon season had previously been revealed. However, a new study by Goswami and colleagues was able to differentiate between moderate (5 mm/day to <100 mm/day) and extreme (> or = 100 mm/day) rain events during the monsoon season and discovered that although the mean rainfall remained unchanged, extreme events increased while moderate events declined. The scientists relied upon rain level data from over 1,800 stations throughout the country for the last 50 years of the 20th century, as well as regional climatic data. From 1951 to 2000, extreme monsoon events increased in both frequency and intensity, and smaller monsoon storms decreased in trend during the same period.

Implications: According to the reinsurance company Munich Re, weather-related damages in India have cost more than US$20 billion (http://www.nature.com/news/2006/061127/full/061127-12.html). Extreme monsoon events can have social, economic, and environmental ramifications. Increased rainfall can result in infrastructural damage, as well as potential natural and agricultural species decline, as flooding and landslides become more frequent. And decreased numbers of moderate storms can exacerbate drought throughout the country, as groundwater reservoirs are unfilled while surface runoff and soil erosion increases.