Understanding global climate change: paleoclimate perspective from the world's highest mountains.

Glaciers are among the world's best recorders of, and first responders to, natural and anthropogenic climate change and provide a time perspective for current climatic and environmental variations. Over the last 50 years such records have been recovered from the polar regions as well as low-latitude, high-elevation ice fields. Analyses of these ice cores and of the glaciers from which they have been drilled have yielded three lines of evidence for past and present abrupt climate change: (1) the temperature and precipitation histories recorded in the glaciers as revealed by the climate records extracted from the ice cores; (2) the accelerating loss of the glaciers themselves; and (3) the uncovering of ancient fauna and flora from the margins of the glaciers as a result of their recent melting, thus illustrating the significance of the current ice loss. The current melting of high-altitude, low-latitude ice fields is consistent with model predictions for a vertical amplification of temperature in the tropics. The ongoing rapid retreat of the world's mountain glaciers, as well as the margins of the Greenland and Antarctic ice sheets, is not only contributing to global sea level rise, but also threatening fresh-water supplies in many of the most populous regions. More recently, strong evidence has appeared for the acceleration of the rate of ice loss in the tropics, which especially presents a clear and present danger to water supplies for at-risk populations in South America and Asia. The human response to this issue, however, is not so clear, for although the evidence from both data and models becomes more compelling, the rate of global CO2 emissions continues to accelerate. Climatologically, we are in unfamiliar territory, and the world's ice cover is responding dramatically. The loss of glaciers, which can be viewed as the world's water towers, threatens water resources that are essential for hydroelectric power, crop irrigation, municipal water supplies, and even tourism. As these glaciers are disappearing, we are also losing very valuable paleoclimate archives.