Even though heavy snow this year will guarantee a white Christmas in many parts of Asia, Europe and North America, an 0.7-degree Celsius (1.3 Fahrenheit) rise in world temperatures since 1900 and projected bigger rises by 2100 suggest an inexorable trend.

"The probability of snow on the ground at Christmas is already lower than it was even 50 years ago but it will become an even greater rarity many places by the latter half of the century," said Friedrich-Wilhelm Gerstengarber, climate researcher at the Potsdam Institute for Climate Impact Research. In the northern German city of Berlin, for instance, the chances of snow on the ground on December 24, 25 and 26 have fallen from 20 percent a century ago to approximately 15 percent in 2008, he said. By 2100 the odds will be less than 5 percent.

Berlin last had snow on the ground at Christmas in 2001, and even though the German capital is due a festive snowfall, from a statistical point of view, meteorologists say it will not be white in 2008 either.

In cities with more maritime climates, such as London, and mild continental climates like Paris, snow on Christmas is even now fairly rare and will only be a freak occurrence within 100 years, he said. No snow is expected in either city this year.

"The yearning for snow at Christmas seems to grow stronger the rarer it becomes," Gerstengarber told Reuters, noting cities at low altitudes such as Berlin (30 meters above sea level) will probably almost never see snow surviving on the ground by 2100.

Betting on the fabled "white Christmas" is a pastime in some countries, like Britain, and oddsmakers will increasingly have to factor in global warming's impact, climate researchers said.

Evidence continues to mount that mankind is to blame for climate change, according to the UN Climate Panel. Drawing on the work of 2,500 experts, it says greenhouse gases, mainly from burning fossil fuels, are blanketing the planet.

Emissions of the gases, led by carbon dioxide, have surged by about 70 percent since 1970 and could in the worst case more than double again by 2050, it says. Rising temperatures will bring more floods, heatwaves, stronger storms and rising seas.

Paal Prestrud, director of the Center for International Climate and Environmental Research in Oslo, said the sort of "white Christmas" in the 1940 Irving Berlin song made famous by Bing Crosby will be rare in the decades ahead -- even in Oslo.

"The probability of snow on Christmas has declined even faster in places like Oslo, where average winter temperatures are closer to 1 degree warmer and the early part of the winter is especially warm," Prestrud said.

"The conditions for cross-country skiing have deteriorated. There is now an average of 100 days (a year) with at least 25 cm snow. In 1900 that was 150," he said. Oslo's streets were clear of snow on Monday.

The U.S. National Snow and Ice Data Center has satellite data collected since 1978 showing northern hemisphere snow cover for the March-April period has declined by about 2 million square km (772,300 sq mile) to 36 million square km.

But Gerhard Mueller-Westermeier, a climatologist at the German Weather Service, pointed out there will still be lots of snow in many temperate zones for decades to come -- and there are some areas where the probability has barely changed.

Cities like Munich, to say nothing of Alpine areas, will have high probabilities of snow on December 25 beyond 2100.

"Winters have become milder but at some weather stations, like Frankfurt, the already relatively low chance of snow on December 25 aren't much lower than before," he said. "There will still be the odd white Christmas for quite some time."