Beijing had its coldest morning in almost 40 years and its biggest snowfall since 1951. Britain is suffering through its longest cold snap since 1981. And freezing weather is gripping the Deep South, including Florida's orange groves¹ and beaches.

北京遭遇了近40年来最寒冷的清晨和1951年以来最大的降雪；英国正遭遇自1981年来持续时间最长的寒潮；美国包括佛罗里达的橘园和海滩在内的南部地区也未逃过严寒袭击。

An undated file photo shows a man skis. Beijing had its coldest morning in almost 40 years and its biggest snowfall since 1951. Britain 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?

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寒流，寒潮 doesn't disprove global warming at all - it's just a blip标志 in the long-term heating trend.

"It's part of natural variability," said Gerald Meehl, a senior scientist at the National Center for Atmospheric² Research in Boulder³, 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., noted⁴ 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春寒期. But experts did not connect the current frigid寒冷的，严寒的 blast to climate change.

So what is going on?

"We basically 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 bent⁶ into a pronounced断然的，显著的 zigzag之字形 pattern, meandering漫步，漫谈 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突然袭击，突然下降 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 jeered¹⁰嘲笑，戏弄 by colleagues.

The zigzag⁷ 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气象学者 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.