The Himalayan mountain ranges—extending 2,400 km through six nations (India, Pakistan, Afghanistan, China, Bhutan, and Nepal)—make up the largest cryosphere region and fresh water source outside the poles. Rapid climate-induced changes in the region directly affect the water resources of more than 1.3 billion lives, as well as services such as electricity, and the food supplies of 3 billion. Projected and observed impacts include disruption of the annual monsoon, changes in runoff from river basins, and an increased risk of flooding and landslides.
Annual mean surface temperature across the Himalayan region has increased by 1.5º C over pre-industrial average temperatures—similar to increases seen in the Arctic and Antarctic Peninsula (Shrestha, Gautam, and Bawa 2012). Measuring the impacts of this temperature rise on the Himalayan cryosphere has proved challenging because of the complicated topography that makes each glacier and region unique and difficult to study, even using satellite technology (Fujita and Nuimura 2011).
Despite the complexity of observations and the lack of on-site measurements, an overall pattern of warming and melting has been apparent, with evidence of glacier and snow cover decrease recorded across most of the Himalayan region (Bolch et al. 2012; Armstrong 2010; Bamber 2012; Kang et al. 2010). The most extreme melting has occurred in the eastern Himalayas, where the mean glacial thickness of Chinese glaciers decreased by nearly 11 meters from 1985-2005. A more mixed pattern is evident in the far Northwest and the Karakoram region, which are further north, colder, and more remote from large human populations and from monsoon precipitation impacts, receiving greater humidity from the west and the winter monsoon season (UNEP-GRID 2012; Kaab 2012; Yao et al. 2012).
Many glacial lakes have formed or expanded during the rapid melt process in the Eastern and Central Himalayas. These have led to catastrophic floods — so-called glacial lake outbursts (GLOFs) — especially in Nepal and the Tibetan region. Other GLOFs have been narrowly averted there and in Bhutan by implementing measures such as siphoning off melt water, as occurred with Tsho Rolpa in Nepal (Liu et al. 2013).
The importance of melt water from greater Himalayan glaciers and snowpack to human water supplies varies widely, with the semi-arid regions of western China, Pakistan and Central Asia most clearly dependent on a regular, predictable melt season (Immerzeel, van Beek, and Bierkens 2010). Estimates range from 80-percent dependency of overall river flow on melt water in these western regions (especially the Indus and Tarim river basins) to under 20 percent in the Yangtze, Ganges, and Yellow Rivers (see Figure 1) (Xu, Shrestha, and Eriksson 2009) . A 2013 report by the Asian Development Bank categorized Pakistan as one of the most water-stressed nations in the world, largely due to changes already seen in the supply to the Indus River (Asian Development Outlook 2013). In such situations of water stress, even seemingly small changes can have large impacts on human populations, where changes in timing or just a few percentage points in flow may make the difference between adequate irrigation and crop loss for that season.
This dependency on regular water supply occurs to an even greater extent with the Asian monsoon rains, around which local populations have based their agricultural practices for millennia. The past decade has seen a general decrease in overall rainfall during the monsoon and a later date of onset. At the same time, the region has seen an increase in extreme events, such as the flooding in northern India in June 2013 that killed nearly 6,000 people, and in which rainfall-induced heavy melting of the Chorabari Glacier was also implicated. Any direct relationship between Himalayan conditions and the monsoon is, however, highly uncertain; the monsoon is more likely driven by conditions in the Indian Ocean and El Nino events than by conditions over the Himalayas. At the same time, studies of air-pollution-related climate impacts and the Atmospheric Brown Cloud (ABC) seem to indicate a large impact from regional sulfate and particle emissions on monsoon precipitation levels and timing (Turner and Annamalai 2012).
While the long-term impacts of rapid regional climate change and air quality on the monsoon may continue to be uncertain for some time, the very introduction of much greater uncertainty in water supply for local agriculture – in many cases, marginal to begin with – is itself an impact to be avoided.
As in the Arctic, permafrost melt is also of concern in the greater Himalayas, especially on the Tibetan Plateau, where much of the infrastructure (e.g., highways, rail lines, and dams) has been built over frozen ground. The permafrost of this region is unique and ancient, formed over the past two million years through uplift of the Himalayas and the Tibetan Plateau. Due to the arid nature of this region, the permafrost is relatively fragile, and its extent has decreased by nearly 20 percent over the past 30 years (Cheng and Jin 2103), with an increase in average permafrost temperature documented since 1996 (Wu and Zhang 2008).
 All temperatures in this report are in degrees Celsius.