The United Nations forecast an increase in global temperatures of 1.4-5.8 degrees Celsius (34.5-42.4 degrees Fahrenheit), and implications for mountain water resources could be massive, the experts warned at the four-day conference in the French ski resort of Megeve.

The effects are already evident in the reduction in size of glaciers, with close to half of those in France forecast to disappear by the end of the century, according to Pierre Etchevers from the French weather office.

"We add eight to 10 meters (26 to 33 feet) of ladder every year to get to the Mer de Glace (glacier) in Chamonix," said Martial Saddier from the French Association of Mountain Water.

And a reduction in the volume of snow has been noted over the past 20 years, as well as a shortening of the period when snow falls, threatening the future of ski resorts below 1,800 metres and prompting the increased usage of snow cannons, machines turning water in snow which is then sprayed onto the pistes.

For ski resorts, the recourse to man-made snow has obvious economic advantages, attracting more and more visitors and extending the season -- despite complaints from purists.

Resorts now want to "guarantee that everyone who comes to the mountains has the possibility to ski from December to March/April," said Jean-Claude Domenego, head of the French Alpine Club.

But both the increase in the number of winter sports tourists and the greater recourse to snow machines have also added to pressure on mountain water resources, depleting resources and leaving less for other human uses such as agricultural irrigation downstream and hydro-electric power stations.

As a result around 20 artificial water reservoirs are being constructed in the Alps, said Alain Marnezy, professor at University of Savoie, including one for 400,000 cubic metres (14 million cubic feet) at Grand Bornand.

With mountains covering around a third of Europe's surface, there were also calls for greater support from European Union authorities.

The scientists also discussed the European directive aiming for a "good ecological state" of Europe's water by 2015, although there were differences over the definition of such a term.

"No one is in agreement on the definition of a good ecological state of water," said Jean-Marie Wauthier, international director at the water ministry in the Walloon region of Belgium.

There has to be a distinction between the biological state, characterised by a minimum presence of animal and plant life, and a good chemical state, meaning a lack of pollutants in the water, Wauthier said.

Further difficulties are created by the fact that many of Europe's rivers flow through more than one country, making cooperation between states imperative. The Danube, for example, flows through 18 countries.

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