Most of us already know that climate change is shrinking glaciers, but two recent articles paint an alarming picture of how quickly glaciers are receding – and what that means for millions of people relying on them.

An Economist article cites a World Bank team prediction that runoff from Andean glaciers may dry up altogether in just 20 years. Peru will be particularly hard hit, since two-thirds of its residents live on the desert coast (Lima, with a population of 8 million, is the world’s second-largest desert city, after Cairo) and the country’s agriculture relies heavily on irrigation. Plus, 70% of Peru’s electricity comes from hydroelectric dams on glacier-fed rivers. In Bolivia, glacial melting threatens water supplies of La Paz and is likely to aggravate existing conflicts between farmers and miners over other water supplies.

In the New York Times, Somni Sengupta follows glaciologist D.P. Dobhal, who studies the Chorabari glacier in India. Although India’s glaciers are not well studied, the retreat Dobhal has measured thus far fits the Himalayan pattern. As the Himalayan glaciers melt, the more than one billion people who rely on glacier-fed rivers face the prospect of flooding and mudslides, followed by severe droughts.

A 2005 report from the WWF Nepal Program (PDF) describes the Himalayan situation:

The Himalayas have the largest concentration of glaciers outside the polar caps. With glacier coverage of 33,000 km2, the region is aptly called the “Water Tower of Asia” as it provides around 8.6 X 106 m3 of water annually (Dyurgerov and Maier, 1997). These Himalayan glaciers feed seven of Asia’s great rivers: the Ganga, Indus, Brahmaputra, Salween, Mekong, Yangtze and Huang Ho. It ensures a year round water supply to millions of people.

Climate change has impacted the glacial ecosystem tremendously. Sixty-seven percent of glaciers are retreating at a startling rate in the Himalayas and the major causal factor has been identified as climate change (Ageta and Kadota, 1992; Yamada et al., 1996; Fushinmi, 2000). Glacial melt will affect freshwater flows with dramatic adverse effects on biodiversity, and people and livelihoods, with a possible long-term implication on regional food security.

The flooding risk comes not just from rivers, but from glacial lakes:

Glacial lake outburst floods (GLOFs) are catastrophic discharges of water resulting primarily from melting glaciers. An accelerated retreat of the glaciers in recent times has led to an enlargement of several glacial lakes. As the glaciers retreat they leave a large void behind. The ponds occupy the depression earlier occupied by glacier ice. These dams are structurally weak and unstable and undergo constant changes due to slope failures, slumping, etc. and run the risk of causing GLOFs.

Principally, a moraine dam may break by the action of some external trigger or self-destruction. A huge displacement wave generated by rockslide or a snow/ice avalanche from the glacier terminus into the lake may cause the water to top the moraines and create a large breach that eventually causes dam failure (Ives 1986). Earthquakes may also be one of the factors triggering dam break depending upon magnitude, location and characteristics. Self-destruction is a result of the failure of the dam slope and seepage from the natural drainage network of the dam.

Characterized by sudden releases of huge amounts of lake water, which in turn would rush down along the stream channel downstream in the form of dangerous flood waves, GLOF waves comprise water mixed with morainic materials and cause devastation for downstream riparian communities, hydropower stations and other infrastructure. In South Asia, particularly in the Himalayan region, it has been observed that the frequency of the occurrence of GLOF events has increased in the second half of the 20th century. GLOFs have cost lives, property and infrastructure in India, Nepal and China.

As usual, the countries affected most will not be the ones that have emitted the most greenhouse gases.