Impacts and Risks of Global Change - Impact of Global Changes on Mountains: Responses and Adaptation - page 60

Risks related to global climate change—global change
In the Fourth Assessment Report the Intergovernmental Panel Climate
Change (IPCC) stated that most of the observed global warming over the
last 50 years has very likely (>90 to 99% probability, see Manning et al. 2004,
IPCC 2005) been caused by greenhouse gas forcing, while it is cited to be
very unlikely (1 to 10% probability) that it is due to known natural external
causes alone (Hegerl et al. 2007). These fi ndings altered the perception
of climate change fundamentally and consequently led to a worldwide
acceptance that global warming and dependent changes of further climate
elements as well as multiple effects on nature and society are no longer
disputable—they have become a fact (see, e.g., Oreskes 2004). Prior to the
World Climate Conference held in Copenhagen in December 2009 a group
of IPCC authors highlighted that in recent years multiple evidence has been
produced for even more drastic warming in the 21st century.
Due to the complex topography as well as specifi c and spatially intensive
variability of human-environmental sub-systems, mountains tend to become
regions affected by Global Climate Change far beyond average. In some
mountain areas, it can be shown that warming trends and anomalies are
elevation dependent, where increasing temperature has a steeper positive
gradient in higher altitudes (e.g., in the Alps, see Böhm 2009).
The impact of intensifi ed climate change on the natural mountain
environment has become especially apparent in the shrinking water storages
of the cryosphere, i.e., snow and ice cover (see, e.g., Lemke et al. 2007,
UNEP and WGMS 2008). Being key components of the hydrological cycle
this causes further radical changes of the seasonal character (regimes) and
amount of runoff of mountains and adjacent lowlands (see, e.g., Viviroli et
al. 2007, Bates et al. 2008). In fact mountains are the source for 50% of the
world's rivers (Beniston 2003). The Himalaya and Hindu Kush alone feed the
Indus, Ganges, Brahmaputra, Irrawaddy, Salween, Mekong, Tarim, Yangtse
and Yellow Rivers, the Alps supply the Rhine, Po, Rhône and Danube
tributaries. Mountains are i) water pumps, which extract moisture from
the atmosphere through the orographic uplift of air masses and they are ii)
water towers due to their water storage capacities in glaciers, permafrost,
snow, soil and groundwater . Much of the inter- and intra-annual variation
of discharge is compensated by discharge from mountains. In semi-arid
areas mountain discharge accounts for 50-90%, in extreme cases (e.g., Nile,
Colorado, Rio Negro) for more than 95% of the total river discharge (Viviroli
et al. 2007). Roughly 23% of China's 1.3 billion people depend on glacier
discharge from the Himalayas. The Alps supply a signifi cant proportion
of freshwater for the population of Europe (Braun et al. 2000). This water
means freshwater, irrigation and hydropower.