Beijing had its coldest morning in almost 40 years and its biggest snowfall since 1951. Br

itain is suffering through its longest cold snap since 1981. And freezing weather is gripping the Deep South, including Florida's orange groves and beaches. Whatever happened to global warming？

Such weather doesn't seem to fit with warnings from scientists that the Earth is warming because of greenhouse gases. But experts say the cold snap doesn't disprove global warming at all— it's just a blip in the long-term heating trend.

"It's part of natural variability," said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo. "With global warming", he said, "we'll still have record cold temperatures. We'll just have fewer of them. "

Deke Arndt of the National Climatic Data Center in Asheville, N. C. , noted that 2009 will rank among the 10 warmest years for Earth since 1880.

Scientists say man-made climate change does have the potential to cause more frequent and more severe weather extremes, such as heat waves, storms, floods, droughts and even cold spells. But experts did not connect the current frigid blast to climate change.

So what is going on？

"We ba ically have seen just a big outbreak of Arctic air" over populated areas of the Northern Hemisphere, Arndt said. "The Arctic air has really turned itself loose on us. "

In the atmosphere, large rivers of air travel roughly west to east around the globe between the Arctic and the tropics. This air flow acts like a fence to keep Arctic air confined.

But recently, this air flow has become bent into a pronounced zigzag pattern, meandering north and south. If you live in a place where it brings air up from the south, you get warm weather. In fact, record highs were reported this week in Washington state and Alaska.

But in the eastern United States, like some other unlucky parts of the globe, Arctic air is swooping down from the north. And that's how you get a temperature of 3 degrees in Beijing, a reading of minus-42 in mainland Norway, and 18 inches of snow in parts of Britain, where a member of Parliament who said the snow "clearly indicates a cooling trend" was jeered by colleagues.

The zigzag pattern arises naturally from time to time, but it is not clear why it's so strong right now, said Michelle L'Heureux, a meteorologist at the Climate Prediction Center of the National Oceanic and Atmospheric Administration. The center says the pattern should begin to weaken in a week or two.

What is the relationship between the global warming and the cold snap？