VIENNA, April 15 (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.
"This is just a time bomb," said hydrologist Carmen de Jong 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."
De Jong was referring to parts of the Mediterranean where her research is focussed 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.
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