Raymond Bradley has a paper in tomorrow’s Science laying out the significant threats to water supplies in the Andes as a result of global climate change. This is the sort of thing Roger Pielke Sr. wisely argues that we need – a better understanding of climate change effects on a regional scale “focused on regional and local societal and environmental resources of importance.”
Bradley and colleagues argue that melting glaciers threaten water supplies in places like Quito and La Paz, where water flowing off of glaciers serves as a critical dry season buffer:
Although an increase in glacier melting initially increases runoff, the disappearance of glaciers will cause very abrupt changes in stream-flow, because of the lack of a glacial buffer during the dry season. This will affect the availability of drinking water, and of water for agriculture and hydropower production.
Adaptation will be required, they say. Apparently no way to mitigate this on any reasonable time scale:
It is in the tropical Andes that climate change, glaciers, water resources, and a dense (largely poor) population meet in a critical nexus. Some glaciers have already reached the threshold at which they are destined to disappear completely; for many more, this threshold may be reached within the next 10 to 20 years. Therefore, governments must plan without delay to avoid large-scale disruption to the people and economy of those regions.
 Threats to Water Supplies in the Tropical Andes, Raymond S. Bradley, Mathias Vuille, Henry F. Diaz, Walter Vergara, Science 23 June 2006: Vol. 312. no. 5781, pp. 1755 – 1756, DOI: 10.1126/science.1128087