Beijinghad its coldest morning in almost 40 years and its biggest snow fallsince 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?

　　Such weather doesn't seem to fit with warnings from scientists that the Earth is warming because of greenhouse gases. But experts say the coldsnap 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.

　　Scientistssay 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?

　　"Webasically have seen just a big outbreak of Arctic air" over populatedareas of the Northern Hemisphere, Arndt said. "The Arctic air hasreally 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 afence 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 inBeijing, a reading of minus-42 in mainland Norway, and 18 inches ofsnow 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 meteorologistat the Climate Prediction Center of the National Oceanic and Atmospheric Administration. The center says the pattern should begin toweaken in a week or two。

　　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。

　　那么全球变暖又是怎么一回事？

　　科学家一直警告说温室气体排放正导致全球变暖，但这样的天气与这种说法似乎并不相符。但专家们称，寒潮和全球变暖并不矛盾——这只是变暖过程中的一个暂时现象而已。

　　位于美国科罗拉多州博尔德的国家大气研究中心的资深科学家杰拉尔德•弥尔说：“这是大自然易变性的一个表现。”他说，在全球变暖的同时，“我们仍然会遭遇创纪录的低温天气，只是这样的天气会减少。”

　　位于北卡罗来纳州艾西维尔市的国家气象数据中心的德科•阿恩特指出，2009年将是自1880年以来气温最高的十年之一。

　　科学家说，人为的气候变化的确会带来更为频繁和更严重的极端天气，如热浪、暴风雨、洪水、干旱甚至持续的寒冷天气。但科学家没有把目前的严寒天气和气候变化联系在一起。

　　那么这究竟是什么原因呢？

　　阿恩特说：“我们看到的只是北极空气在北半球人口密集地区的一次大爆发，北极空气真的向我们发威了。”

　　在大气中，大股大气环流由西向东在北极和热带地区之间移动。这股气流就像一个屏障，把北极空气给禁锢了起来。

　　不过，最近这股气流明显呈“Z”形走势，向南北方向蔓延。如果你居住的地区有南上气流经过，那么天气将会变暖。事实上，华盛顿州和阿拉斯加州上周的温度都创下历史新高。

　　然而，和其他不太幸运的地区一样，美国东部正遭遇一股从北方呼啸而下的冷空气。这就是为什么北京的气温会低至3度，挪威大陆的气温降到零下42度，英国部分地区会有厚达18英寸的积雪。英国某议员说大雪“表明气候有变冷的趋势”，结果遭到同事的讥笑。

　　国家海洋和大气管理局气候预测中心的气象学家米歇尔•拉赫茹克斯说，这种“Z”形走势不时出现，这很自然，但不清楚为什么现在会这么强劲。该中心说这一走势应该会在一两周后开始减弱。

　　Vocabulary:

　　cold snap: 寒潮

　　disprove: to prove to be false, invalid, or in error 证明……是虚假的；反驳(Evidence has now disproved that theory. 事实现已证明那种理论是不正确的。)

　　blip: a temporary or insignificant phenomenon, especially a brief departure from the normal 偏离正常轨道的一个短暂现象

　　variability: the quality, state, or degree of being variable or changeable 变化性；变化的倾向

　　blast: a powerful hit, blow, or shot 冲击波，气浪

　　turn sb./sth. lose: to give up control of someone or something, so they can do what they want 放开，释放；听凭，放纵

　　pronounced: strongly marked; distinct 明显的，显著的

　　swoop down: to dive or plunge downward 俯冲