Glaciers and mountain snow are melting earlier in the year than usual, meaning the water has already gone when millions of people need it during the summer when rainfall is lower, scientists warn .
"This is just a time bomb," said hydrologist Wouter Buytaert at a meeting of geoscientists in Vienna.
Those areas most at risk from a lack of water for drinking and agriculture include parts of the Middle East, southern Africa, the US, South America and the Mediterranean.
Rising global temperatures mean the melt-water is occurring earlier and faster in the year and the mountains may no longer be able to provide a vital stop-gap.
"In some areas where the glaciers are small they could be gone in 30 or 50 years time and a very reliable source of water, especially for the summer months, may be gone."
Ms Buytaert, from Britain’s Bristol University, was referring to parts of the Mediterranean where her research is focused, but she said this threat also applies to the entire Alps region and other global mountain sources.
Daniel Viviroli, from the University of Berne, believes nearly 40 per cent of mountainous regions could be at risk, as they provide water to populations which cannot get it elsewhere.
He says the earth’s sub-tropic zones, which are home to 70 per cent of the world’s population, are the most vulnerable. And with the global population expected to expand rapidly, there may not always be enough water to drink.
In Afghanistan, home to some 3,500 of the world’s glaciers, the effects of global warming are already being felt in the Hindu Kush, said US Geological Survey researcher Bruce Molnia.
In some valleys snow has completely disappeared during months when it usually blankets the mountains and many basins have drained, Mr Molnia said.
"And what I am talking about here is adaptable to almost every one of the Himalayan countries that’s dependent on glacier-melted water," he said.