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:
[EMAIL PROTECTED] (R.S.B.)
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