By Alex Kirby BBC News Online environment correspondent

A US environment group says future Winter Olympics could be in jeopardy because of global warming.

The group, the World Resources Institute (WRI), says winters will be shorter and warmer, with less snow falling.

It says the problem will affect Europe as well as north America.

The 2006 Winter Olympic Games are due to be held in the Italian city of Turin.

Jonathan Lash, WRI's president, spoke at a news conference in Salt Lake City organised by WRI, the environment programme of the Salt Lake Organising Committee (Sloc), and the Earth Communications Office, based in Los Angeles.

Green for stop

He said: "Global warming threatens future Winter Olympic Games. Just as Salt Lake has done, we urge potential host cities to seriously consider the consequences of global warming.

"There is no more weather-dependent event than the Winter Games, and they are at risk.

Montana is feeling the warmth

"Ski areas that previously could be depended on for fabulous conditions by December and January now have green slopes where formerly giant slaloms took place."

WRI's website says: "The International Olympics Committee added environment to sport and culture as the third principle of Olympism in 1994.

"Salt Lake City was the first city to win the bid after the element was added. Climate protection is a key goal of the 2002 Winter Games.

"Sloc will be using more energy-efficient systems, using buses powered by natural gas, planting trees at hot spots in the Salt Lake area, and offsetting the rest using unused emission credits donated by national and local businesses."

WRI says climate scientists estimate that half the glaciers in the world's alpine regions could disappear in the next century.

In the US, it says, Montana's Glacier national park, which had 150 glaciers in 1850, will have none by 2030.

False hopes

The Intergovernmental Panel on Climate Change (IPCC) expects changing snowfall patterns in Europe will harm winter tourism in Austria, Switzerland and France.

Some winter sports areas in the US have turned to snowmaking machines to make good the shortfall.

Alpine glaciers are in trouble

Dr Nancy Kete of WRI said: "The machines are gluttons for water, a resource that may be in short supply in the next 25 years.

"Global warming's impact on winter sports and the economies that depend on them give us a good opportunity to face the unimaginable."

Some scientists challenge the IPCC's forecasts, saying global warming is a natural process unaffected to any great degree by human activities.

No worries

These sceptics agree that some melting of the Earth's mid-latitude glaciers is taking place. But they represent only 6% of the planet's total ice mass.

In the remaining 94%, represented by the Antarctic and Greenland, the ice sheets are expanding, they say.

Adlai Amor of WRI told BBC News Online: "We don't know if the Turin games are safe, because we haven't done those studies yet.

"But what's clear is that there's a question mark over anything held after 2006."