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Shan glacier area decreases continuously, the annual river discharge has been growing over the last
decade, mainly due to precipitation increase.
The sharp change in river runoff suggests the non-linear system response. In fact, the
evapotraspiration process does not respond linearly to air temperature and precipitation changes.
As reported by Aizen et al. (2006d), the evapotranspiration process seems to be independent of
amount of precipitations. This phenomenon is accelerated both when there is a surplus of
precipitation and when there is a deficit of precipitation, and air temperature increases.
The increase in aridity of continental interiors may cause a massive aeolian aerosol spread in the
troposphere, which could affect the Earth’s heat balance and generate direct biospheric and societal
impacts that extend thousands of kilometers away from the origins of dust storms. The deposition of
dust on seasonal snow cover and glacier ice will decrease the albedo of these surfaces and may
accelerate melting (Adhikary et al., 2002; Hansen and Nazarenko, 2004). Current glacier recession,
initially considered a positive factor that increased river flow, ultimately causes the runoff to
decrease.
2.5.3 North America Mountains
The Western Cordillera is the most prominent mountain range in North America (NA), extending
from Alaska to Central America. It includes a number of ranges along the western coast (Alaska
Range, Coast Range, the Cascades, the Sierra Nevada and Sierra Madres) and the Rocky
Mountains, just west of the Great Plains. In these regions also, snowpack represents an important
component of the water cycle in much of western NA and snowmelt generates roughly 60-90% of
streamflow in western North America. Many snowmelt-fed rivers originating in western mountain
ranges provide water to arid and semi-arid regions downstream, as is the case of the Colorado River
basin.
Important changes were observed in the NA mountain hydroclimate during the 20th century (IPCC,
2007), including the decrease in snow cover above all in spring over western NA (Groisman et al.,
2004) and the decline of spring mountain soil water equivalent (SWE) in western NA since 1950
(Stewart et al., 2005). Another important hydrological change associated to recent warming is the
tendency for precipitation to occur more in the form of rain rather than as snow (Knowles et al.,
2006). Regarding NA glaciers, it is estimated that the fraction of glacier area lost in the Western
United States since 1900 is on average roughly 40%.
Leung et al. (2003) investigated factors determining cold season hydroclimate anomalies in western
NA mountainous regions, demonstrating that there are interactions between large scale circulation
changes and regional topography. The current climate changes produce anomalies that are difficult
to forecast. For this reason both the ENSO (El Nino Southern Oscillation) and the Pacific Decadal
Oscillation models have great difficulty in simulating and predicting the MJO (Madden Julian
Oscillation).
The North American Monsoon system (NAM) during the warm season is of particular importance to
water resources in the American Southwest. The Sierra Madre Occidental mountains of western
Mexico on average receive in excess of 60-70% of their annual rainfall in June-September, as part of
the NAM (Douglas et al., 1993). The mountainous terrain of southwestern North America and its
juxtaposition to seasonally warm water bodies of the eastern tropical Pacific, the Gulf of California,
and the Gulf of Mexico, serve as a driving component of the regional climate circulation, which
helps to initiate and sustain extra-tropical moisture during the NAM (Gochis, 2007).
Since 2004, a continental-scale process study called the North American Monsoon Experiment
(NAME) has been operative. Its aim is to understand the NAM (Higgins et al., 2006) and study the
characteristics of the precipitation-elevation relationship along the Sierra Madre Occidental
Mountains from the Gulf of California coast to the Mexican Plateau (NERN; Gochis et al., 2004).
Currently, further research is required to study microphysical processes and the structure of
diurnally-forced terrain circulations (Gochis, 2008).
Many factors condition the NA climate. For instance, drought occurrence over NA, due to a
combination of remote SST influences and regional land feedbacks (McCabe et al. 2004; Schubert et
al., 2004), has significant impacts on mountain environments, with reduced precipitation amounts
directly impacting snowpack formation, soil moisture and streamflow. Indirect hydrologic effects on
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