Melting glaciers in the Himalayas could lead to water shortages for hundreds of millions of people, the conservation group WWF has claimed.
The world's highest mountains hide vast glaciers
In a report, the WWF says India, China and Nepal could experience floods followed by droughts in coming decades.
The Himalayas contain the largest store of water outside the polar ice caps, and feed seven great Asian rivers.
The group says immediate action against climate change could slow the rate of melting, which is increasing annually.
"The rapid melting of Himalayan glaciers will first increase the volume of water in rivers, causing widespread flooding," said Jennifer Morgan, director of the WWF's Global Climate Change Programme.
"But in a few decades this situation will change and the water level in rivers will decline, meaning massive eco and environmental problems for people in western China, Nepal and northern India."
The glaciers, which regulate the water supply to the Ganges, Indus, Brahmaputra, Mekong, Thanlwin, Yangtze and Yellow rivers are believed to be retreating at a rate of about 10-15m (33-49ft) each year.
Hundreds of millions of people throughout China and the Indian subcontinent - most of whom live far from the Himalayas - rely on water supplied from these rivers.
Many live on flood plains highly vulnerable to raised water levels.
And vast numbers of farmers rely on regular irrigation to grow their crops successfully.
The WWF said the potential for disaster in the region should serve to focus the minds of ministers of 20 leading industrialised nations gathering in London for two meetings on climate change.
"Ministers should realise now that the world faces an economic and development catastrophe if the rate of global warming isn't reduced," Ms Morgan said.
She added that a study commissioned for the WWF indicated that the temperature of the Earth could rise by two degrees Celsius above pre-industrial levels in a little over 20 years.
Allowing global temperatures to rise that far would be "truly dangerous", Ms Morgan said.
Farmers in rural China are dependent on regular irrigation
Nepal, China and India are already showing signs of climate change, the WWF report claims.
Nepal's annual average temperature has risen by 0.06 degrees Celsius, and three snow-fed rivers have shown signs of reduced flows.
Water level in China's Qinghai Plateau wetlands has affected lakes, rivers and swamps, while India's Gangotri glacier is receding by 23m (75ft) each year.