[aymara] Amenaza al abastecimiento del agua en los andes tropicales (en ingles)

["Aitor Gonzalez" <larrinaga@xxxxxxxxxxxxxxxxx>]

Science 23 June 2006:
Vol. 312. no. 5781, pp. 1755 - 1756
DOI: 10.1126/science.1128087
http://www.sciencemag.org/cgi/content/full/312/5781/1755

Threats to Water Supplies in the Tropical Andes
Raymond S. Bradley,1* Mathias Vuille,1 Henry F. Diaz,2 Walter Vergara3

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 (4-6).
Indeed, glacier retreat is under way in all Andean countries, from Columbia and
Venezuela to Chile (7).

    Figure 1 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

   1. R. S. Bradley, F. T. Keimig, H. F. Diaz, Geophys. Res. Lett. 31, L16210 (2004).
[CrossRef]
   2. D. J. Seidel, M. Free, Clim. Change 59, 53 (2003). [CrossRef]
   3. M. Vuille, R. S. Bradley, M. Werner, F. T. Keimig, Clim. Change 59, 75 (2003).
[CrossRef]
   4. E. Ramirez et al., J. Glaciol. 47, 187 (2001).
   5. B. Francou, M. Vuille, P. Wagnon, J. Mendoza, J.-E. Sicart, J. Geophys. Res. 108,
4154 (2003). [CrossRef]
   6. G. Kaser, C. Georges, I. Juen, T. Molg, in Global Change and Mountain Regions: A
State of Knowledge Overview, U. Huber, H. K. M. Bugmann, M. A. Reasoner, Eds. (Kluwer,
New York, 2005), pp. 185-195. [publisher's information]
   7. A. Coudrain, B. Francou, Z. W. Kundzewicz, Hydrol. Sci. J. 50, 925 (2005). [CrossRef]
   8. J. F. Carrasco, G. Casassa, J. Quintana, Hydrol. Sci. J. 50, 933 (2005). [CrossRef]
   9. H. F. Diaz, N. E. Graham, Nature 383, 152 (1996). [CrossRef]
  10. B. Francou, M. Vuille, V. Favier, B. Cáceres, J. Geophys. Res. 109, D18106 (2004).
[CrossRef]
  11. P. Wagnon, P. Ribstein, B. Francou, J.-E. Sicart, J. Glaciol. 47, 21 (2001).
  12. From 1999 to 2002, some glaciers had neutral or slightly positive mass balance as
a result of a prolonged La Niña episode. Since 2002, glaciers are again retreating
everywhere, and temperatures have rebounded upwards.
  13. B. G. Mark, J. M. McKenzie, |. J. Gómez, Hydrol. Sci. J. 50, 975 (2005). [CrossRef]
  14. W. Vergara, Adapting to Climate Change. Latin America and Caribbean Region
Sustainable Development Working Paper 25 (World Bank, Washington, DC, 2005).
  15. www-pcmdi.llnl.gov/ipcc/about_ipcc.php
  16. Nebojsa Nakicenovic, Rob Swart, Eds., Special Report on Emissions Scenarios
(Cambridge Univ. Press, Cambridge, U.K., 2000). [publisher's information] [Full text]
  17. We acknowledge the international modeling groups for providing their data for
analysis; the Program for Climate Model Diagnosis and Intercomparison (PCMDI) for
collecting and archiving the model data; the Johnson Space Center/Climate Variability
and Predictability (JSC/CLIVAR) Working Group on Coupled Modelling (WGCM) and their
Coupled Model Intercomparison Project (CMIP) and Climate Simulation Panel for organizing
the model data analysis activity; and the IPCC WG1 Technical Support Unit for technical
support. The IPCC Data Archive at Lawrence Livermore National Laboratory is supported by
the Office of Science, U.S. Department of Energy (DOE). This research was supported by
the Office of Science (Office of Biological and Environmental Research), U.S. DOE, grant
DE-FG02-98ER62604 and NSF grant EAR-0519415. 

10.1126/science.1128087
1R. S. Bradley and M. Vuille are at the Climate System Research Center, Department of
Geosciences, University of Massachusetts, Amherst, MA 01003, USA. 2H. F. Diaz is at the
Earth System Research Laboratory, National Oceanic and Atmospheric Administration,
Boulder, CO 80303, USA. 3W. Vergara is in the Latin America Environment Department,
World Bank, 1850 I Street, NW, Washington, DC 20433, USA. *E-mail:
rbradley@xxxxxxxxxxxxx (R.S.B.)




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