VIENNA (Reuters) - 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 warned on Monday.

Mount Everest, the highest peak in the world, with an altitude of 8,848 metres (29,028 feet), is seen in this aerial view March 25, 2008. REUTERS/Desmond Boylan

“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 United States, 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.”

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 percent 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 percent 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, let alone to water crops, which use about 70 percent of melt-water.

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 U.S. Geological Survey researcher Bruce Molnia.

“Glaciers are getting smaller and smaller,” he said adding that this was leading to more frequent flooding.

In some valleys snow has completely disappeared during months when it usually blankets the mountains and many basins have drained, 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.

It has also been difficult to collect data in the region with scientists preferring to rely on satellite imagery rather risk fieldwork in the Taliban-occupied mountains.

Buytaert points out that because only a handful of scientists study the hydrology of mountains, what they don’t know about them could be just as concerning as what they do.

“Mountains are seen as having water all the time and everywhere so people think they can take it all the time,” she said.

“But mountains are black boxes in the scientific sense, there is so much data missing for our models. We don’t quite know what is going on.”