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Challenges ahead Data availability and accessibility

Improved understanding of the effect of climate change in high mountains on downstream communities and water re- sources will require significant efforts to develop databases and analytical capacities. Currently most projections of cli- mate change are based on general circulation models (GCM) which attempt to mathematically model interactions between oceans, atmosphere and large land areas, typically on the order of hundreds to thousands of square kilometres. The models present two significant challenges. Firstly, the models have a low resolution and have a very limited capability to pro- vide detailed predictions for smaller areas. Large mountain regions like the Himalayas and Andes are extremely diverse and complex in terms of geography, ecology and socio-cul- tural conditions, and there is a great need for higher resolu- tion models with better predictive capacity at smaller scales. Secondly, the current models are in themselves uncertain as our understanding of climate change is limited, i.e. we have limited knowledge of the key natural processes and of which choices we will make in the future with regards to energy con- sumption and greenhouse gas emission.

Extensive coordination and cooperation among mountain nations and institutions is required in order to fill data gaps and develop regional assessment models. In some cases rel- evant data sets exist on a national level, but are not acces- sible for regional analysis and cooperation due to strategic or other reasons. In any case current databases and capacity is limited and several factors play a part; the number of years of observation (often short term), quality and availability of data, distribution of observation networks, capacity to ana- lyze and compute data, financial constraints, and lack of time to achieve required results. Currently the paucity of data in many areas, the lack of institutional capacity to analyze, cor- rect and verify data and short duration of data records limit the validity of current models.

Modelling water flow is complicated

Mountain regions contribute a substantial proportion of the glob- al river runoff (Viviroli et al., 2003; Viviroli and Weingartner, 2004), but modeling this runoff and future variability in time and space due to climate change is highly complicated. The pro- cesses that determine the change from precipitation into run- off are many and complex. As mentioned earlier in this report runoff from melting ice is often a relatively small component in the total runoff regime, but still highly important as a long term, relatively stable supply of water. As noted above, the broad picture indicates that in the coming decades many large glaciers will retreat and a high number of small ones will disappear completely. This could mean that the supply of water will be favorable to agriculture and livelihoods in the short term with increasing amounts of water, but it could also contribute to excess water levels in some areas.

In the long run however, i.e. after a few decades when water levels may be drastically reduced due to the diminishment of high mountain glacier systems, impacts on downstream communities could become dramatic in some of the arid ar- eas (ICIMOD, 2009a,b; UNEP, 2009; 2010a,b).

However, climate change does not only indicate higher tem- peratures, but also changes in overall precipitation, evapo- transpiration, and changes in the balance between rain and snow which has great implications for runoff rates and stor- age of water. Intensity, amount and distribution of precipi- tation over time are all factors of importance for modeling runoff and impacts to ecosystems and human populations.

Seasonality is another factor that will affect mountain regions around the world differently. Most mountain areas have sea- sonal patterns to annual runoff regimes. In areas with mon- soons, runoff from glacial melting is particularly important in the shoulder seasons. There are already many signs of

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