Glaciers in the Himalayas, a 2,400-kilometre (1,500-mile) range that sweeps through Pakistan, India, China, Nepal and Bhutan, provide headwaters for Asia's nine largest rivers, lifelines for the 1.3 billion people who live downstream.

But temperatures in the region have increased by between 0.15 and 0.6 degrees Celsius (0.27 and 1.08 degrees Fahrenheit) each decade for the last 30 years, dramatically accelerating the rate at which glaciers are shrinking.

As world leaders gather in Copenhagen this month for a crucial climate change summit, campaigners warn that some Himalayan glaciers could disappear altogether within a few decades.

"Scientists predict that most glaciers will be gone in 40 years as a result of climate change," said Prashant Singh, leader of environmental group WWF's Climate for Life campaign.

"The deal reached at Copenhagen will have huge ramifications for the lives of hundreds of millions of people living in the Himalayan drainage systems who are already highly vulnerable due to widespread poverty."

The Intergovernmental Panel on Climate Change (IPCC), a UN body regarded as the world's top authority on climate change, has warned Himalayan glaciers could "disappear altogether by 2035" and experts say the effects of global warming are already being felt in the region.

In Nepal and Bhutan, the receding glaciers have formed vast lakes that threaten to burst, devastating villages downstream.

Nepalese mountaineer and environmental campaigner Dawa Steven Sherpa said he first became interested in climate change after a close call when part of the Khumbu icefall above Everest base camp collapsed during an expedition in 2007.

Sherpa, who has scaled Everest three times, was walking on the glacier minutes before the collapse, and said his near miss alerted him to the dramatic toll that global warming is already taking on the Himalayas.

"Every time I go to the mountains the older Sherpas tell me this is the warmest year yet," Sherpa, who will take part in a special "summiteers' summit" in Copenhagen, told AFP.

"Initially it struck me how much more dangerous mountaineering would become. But then I realised it was much bigger than that. Entire villages could be wiped out if one of the glacial lakes burst."

In China, studies have shown that the rapid melting of the glaciers will result in an increase in flooding in the short term, state news agency Xinhua has reported.

In the longer term, it said, the continued retreat of glaciers would lead to a gradual decrease in river flows, severely affecting large parts of western China.

Experts say the resulting water shortages could hit the economic development of China and India, with potentially dire consequences for development in two of the world's most populated countries.

Even in low-lying Bangladesh, prone to severe floods, the IPCC has said rivers could run dry by the end of the century.

But research on the impact of global warming on the rugged and inaccessible Himalayas remains sparse, with the IPCC describing the region as a "blank spot" due to a lack of scientific data.

Even the experts disagree on the issue, with some arguing that some of the Himalayan glaciers are actually advancing.

India's Environment Minister Jairam Ramesh recently came under fire for denying that climate change was causing Himalayan glaciers to melt, citing research by the Indian geologist Vijay Kumar Raina.

The Nepal-based International Centre for Integrated Mountain Development (ICIMOD) has studied the Himalayan region for more than three decades and warns of an "urgent need" for more research on the impact of climate change.

"There are so many uncertainties surrounding where, how and to what extent the Himalayan region will be affected by climate change," ICIMOD climate change expert Arun Shrestha told AFP.

"But most experts accept that temperatures are changing, and this is happening more rapidly at altitude."

ICIMOD has warned that the current trends in glacial melt suggest flows in major Asian rivers including the Ganges, Indus and Yellow Rivers will be "substantially reduced" in the coming decades.

"The situation may appear to be normal in the region for several decades to come, and even with increased amounts of water available to satisfy dry season demands," it said in a recent report on the Himalayas.

"However, when the shortage arrives, it may happen abruptly, with water systems going from plenty to scarce in perhaps a few decades or less."

Shrestha added: "When the glaciers get hotter, you get more water, but there comes a point when the water will run out.

"It's like a bank balance, if you're not putting money in, you can't take it out."