Climate change. Threats to water supplies in the tropical Andes.

to emphasize those measurements of our local astrophysical laboratory that will best illuminate how these fundamental cosmic entities operate. Moreover, observations to date already indicate that the Saturn system is literally changing before our eyes. We anticipate that even more dramatic transformations in our neighborhood's astrophysical laboratory will be monitored by Cassini's instruments over the next several years. A ccording to general circulation models of future climate in a world with double the preindustrial carbon dioxide (CO 2) concentrations, the rate of warming in the lower troposphere will increase with altitude. Thus, temperatures will rise more in the high mountains than at lower elevations (see the figure) (1). Maximum temperature increases are predicted to occur in the high mountains of Ecuador, Peru, Bolivia, and northern Chile. If the models are correct, the changes will have important consequences for mountain glaciers and for communities that rely on glacier-fed water supplies. Is there evidence that temperatures are changing more at higher than at lower elevations? Although surface temperatures may not be the same as in the free air, in high mountain regions the differences are small (2), and changes in temperature should thus be similar at the surface and in the adjacent free air. Unfortunately, few instrumental observations are available above ~4000 m. The magnitude of recent temperature change in the highest mountains is therefore poorly documented. An analysis of 268 mountain station records between 1°N and 23°S along the tropical Andes indicates a temperature increase of 0.11°C/decade (compared with the global average of 0.06°C/decade) between 1939 and 1998; 8 of the 12 warmest years were recorded in the last 16 years of this period (3). Further insight can be obtained from glaciers and ice caps in the very highest mountain regions, which are strongly affected by rising temperatures. In these high-altitude areas, ice masses are declining rapidly (4–6). Indeed, glacier retreat is under way in all Andean countries, from Columbia and Venezuela to Chile (7). A convergence of factors contribute to these changes. Rising freezing levels (the level where temperatures fall to 0°C in the atmosphere) (8, 9) lead to increased melting and to increased exposure of the glacier margins to rain rather than snow (10). Higher near-surface humidity leads to more of the available energy going into melting snow and ice, rather than sublimation, which requires more energy to remove the same mass of ice. Therefore, during humid, cloudy conditions …