PerspectiveClimate Change

Threats to Water Supplies in the Tropical Andes

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Science  23 Jun 2006:
Vol. 312, Issue 5781, pp. 1755-1756
DOI: 10.1126/science.1128087

According to general circulation models of future climate in a world with double the preindustrial carbon dioxide (CO2) 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 (46). Indeed, glacier retreat is under way in all Andean countries, from Columbia and Venezuela to Chile (7).

Global warming in the American Cordillera.

Projected changes in mean annual free-air temperatures between (1990 to 1999) and (2090 to 2099) along a transect from Alaska (68°N) to southern Chile (50°S), following the axis of the American Cordillera mountain chain. Results are the mean of eight different general circulation models used in the 4th assessment of the Intergovernmental Panel on Climate Change (IPCC) (15), using CO2 levels from scenario A2 in (16). Black triangles denote the highest mountains at each latitude; areas blocked in white have no data (surface or below in the models). Data from (15).

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, there is often more ablation than during drier, cloud-free periods (6). In some areas, changes in the amount of cloud cover and the timing of precipitation may have contributed to glacier mass loss through their impact on albedo (surface reflectivity) and the net radiation balance (11). As these processes continue and snow is removed, more of the less reflective ice is exposed and absorption of the intense high-elevation radiation increases, thus accelerating the changes under way through positive feedbacks.

The processes involved in mass-balance changes at any one location are complex, but temperature is a good proxy (12) for all these processes, and most of the observed changes are linked to the rise in temperature over recent decades (5). Further warming of the magnitude shown in the figure will thus have a strong negative impact on glaciers throughout the Cordillera of North and South America. Many glaciers may completely disappear in the next few decades, with important consequences for people living in the region (7).

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.

In the High Andes, the potential impact of such changes on water supplies for human consumption, agriculture, and ecosystem integrity is of grave concern. Many large cities in the Andes are located above 2500 m and thus depend almost entirely on high-altitude water stocks to complement rainfall during the dry season. For example, Ecuador's capital Quito currently receives part of its drinking water from a rapidly retreating glacier on Volcano Antizana. Other cities, like La Paz in Bolivia and many smaller population centers, likewise partially depend on glacier sources for drinking water. In many dry inter-Andean valleys, agriculture relies on glacier runoff; for instance, ∼40% of the dry-season discharge of the Rio Santa, which drains the Cordillera Blanca in Peru, comes from melting ice that is not replenished by annual precipitation (13). As these water-resource buffers shrink further (and, in some watersheds, disappear completely), alternative water supplies may become very expensive and/or impractical in the face of increased demand as population and per-capita consumption rise.

Furthermore, in most Andean countries, hydropower is the major source of energy for electricity generation. As these water resources are affected by reductions in seasonal runoff, these nations may have to shift to other energy sources, resulting in large capital outlays, higher operational and maintenance costs, and—most probably—an increased reliance on fossil fuels.

We have focused here on changes taking place in the mountains of the tropical Andes, but the same situation prevails in high mountain regions elsewhere in the Tropics. Glaciers are disappearing rapidly in East Africa and New Guinea, though there is far less reliance on glacier-fed water supplies in those regions. 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 (14).

Practical measures to prepare for, and adapt to, these changes could include conservation of (or price controls on) water supplies in urban areas, a shift to less water-intensive agriculture, the creation of highland reservoirs to stabilize the cycle of seasonal runoff, and a shift to power generation from resources other than hydropower. At the same time, more detailed scenarios of future climate change in these topographically complex regions are urgently needed. High-resolution regional climate models allow for a better simulation of climate in mountain regions than do general circulation models. Coupled with tropical glacier-mass balance models, these regional models will help us to better understand and predict future climate changes and their impacts on tropical Andean glaciers and associated runoff.

Recent high-resolution (grid size ∼10 km) regional climate simulations for the Colombian Andes indicate that even at relatively low altitudes, projected temperature increases and changes in rainfall patterns have the potential to disrupt water and power supplies to large segments of the population (14). Such simulations must be used to inform decision-makers of the steps they need to take to avoid a very problematical future in the region.

References and Notes

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