Blowing hot and cold over the big chill

Experts have yet to pinpoint the causes of the extended period of extreme cold that has disrupted normal life in much of the northern hemisphere.

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No sooner has all that hot air on global warming been spouted at the Copenhagen climate change summit than vast tracts of the world have been thrust into the deepest freeze in 50 years. Brutal winter weather has blanketed large swathes of the northern hemisphere with snow, bringing chaos to Europe's airports and roads, closing schools and office, and forcing power-rationing in China. The bitter snap is even threatening to devastate Florida's orange crop.

The extent and severity of the plunging temperatures has few precedents and has been seized upon by those who question whether global temperatures are rising. "It's very rare to see such an extensive cold event over so much of the northern hemisphere," said Omar Baddour, a climatologist at the World Meteorological Society. "Snow and cold weather are not unusual factors in climate systems, but the extent is certainly unusual."

The big chill caught many nations unaware. In Britain, temperatures plunged as low as minus 21°C, just a few degrees warmer than the South Pole, and the bitter conditions are forecast to last for the next two weeks. The national grid was forced to issue two gas alerts in four days as freezing temperatures caused a surge in consumption. The military were called in after more than 1,000 cars became stranded in snow on a major motorway in southern England and at least 4,000 homes were without electricity as power lines were brought down. Adding to the chaos, thousands of schools closed, hundreds of planes were grounded and cancelled trains and icy roads kept Britons away from work - costing the recession-battered economy an estimated £600 million a day. Hundreds of deaths in Europe are attributed to the extreme conditions. In Poland, 32 people died in three days during December - most were homeless - and the death toll there is now up to 122.

In Switzerland, seven people were killed in one of the country's worst avalanche disasters in a decade as a group of off-piste skiers was buried. A rescue team going to their aid was hit by a second fall. Avalanches killed four more people in Switzerland and France.

In Germany, described by one meteorologist as the "freezer of central Europe", salt for gritting the roads is running low. Another 40cm of snow was forecast there yesterday. Sweden and Norway suffered some of Europe's coldest temperatures with minus 40°C, but are well equipped to handle sub-zero temperatures.

Many Europeans were surprised at Britain's inability to deal with the snow. Roger Hampton told the BBC he had travelled 250km across Norway on Wednesday when it was minus 22°C. "In the town Roros, in central Norway, it has been minus 40°C the last two nights. I have not heard of schools closing and the roads are for the most free," he told the news service. Perhaps Britain had been lulled into complacency by the Met Office's long-range forecast in September of a winter "milder than last year" with only a "one in seven chance of a cold winter".

In the US state of Vermont, also accustomed to snow, a record-breaking 84 centimetres this week still did not stop the district's 3,600 students from attending school. In other parts of the country, however, the snowfall was less expected, and snow and ice reached as far south as Louisiana and South Carolina. Record lows are forecast for two-thirds of the US as the country is buffeted by snowstorms and the Midwest is bracing itself for temperatures to drop this weekend with dangerously low minus 50°C wind chills.

Florida's orange growers, who supply 40 per cent of the world's orange juice, fear the icy conditions will ruin this year's harvest, but so far the bulk of the crop has been spared. Just four hours of temperatures below 2°C can spoil the fruit. Bob Tarr, a meteorologist at AccuWeather, told Bloomberg: "It's a rare pattern, and unusual to see this cold weather affecting a number of major population centres and persisting for about three weeks. The cold weather is hitting a lot of the more populated areas, such as western and northern Europe, a lot of the eastern US."

Ironically, in Salt Lake City, a protest against the result of the climate change talks in Copenhagen was called off because of the weather. "Not many people showed up because of the blizzard conditions," said Clea Major, an international studies student at the University of Utah and one of the organisers. Asia has not escaped the cold, seeing its worst winter weather in half a century and bringing life to a standstill for millions of people. China, which is due to be hit by another cold front this weekend, has put restrictions on electricity consumption because of coal shortages. While power for industry is being rationed, residents are also being urged to limit their gas use.

Having endured an unusually early and cold winter, Beijing ground to a halt last Sunday after more than 33cm of snow fell on suburban areas, the most since 1951. Wednesday's low of minus 16.7°C was the coldest in almost 40 years. Even the central region and east of the country have been hit by rare snow flurries. South Korea had more than 25.5cm of snow, the heaviest since records began, bringing chaos to its roads. In northern India more than 195 people have died due to the extreme weather. With few homeless shelters, those sleeping rough suffered the worst.

So what has happened to global warming? Sceptics have seized on the freezing conditions to debunk the theory. Ann Winterton, a British MP, argued in Britain's House of Commons that the winter weather "clearly indicates a cooling trend" - though her comments were met with jibes and laughter. "This is the 'exceptionally mild winter' that the climate change buffoons warned us would occur as a consequence of global warming. Their credibility is 20 degrees below zero," wrote Gerald Warner in a blog on The Daily Telegraph's website.

Experts say the cold snap does nothing to disprove the science of climate change. They argue that it is wrong to focus on single events, whether heatwaves or cold snaps, when assessing climate change. "To take an event like this over a few days or weeks and draw conclusions about climate change is the worst possible approach you can take," said Mr Baddour. "It's not analysed based on one year or two years, or even 10 years, it's about long-term trends."

Organisations such as Nasa and the Intergovernmental Panel on Climate Change prefer to use the more scientifically accurate term "climate change" over "global warming". Whereas global warming refers only to temperature increases, climate change includes global warming and any other changing trends related to increased greenhouse gases. Britain's Met Office said the cold weather did not disprove the theory that average global temperatures were rising.

"The current cold weather in the UK is part of the normal regional variations that take place in the winter season," the Met office said. "It doesn't tell us anything about climate change, which has to be looked at in a global context and over longer periods of time." While wrong to extrapolate conclusions from single wether events, scientists do not rule out that the cold spell could be the result of climate change.

Guo Hu, the head of the Beijing Meteorological Bureau, went as far as to say that it was the probable cause of the extreme weather in China. "In the context of global warming, extreme atmospheric flows are causing extreme climate incidents to appear more frequently, such as the summer's rainstorms and last year's ice storm disaster in southern China," he told Beijing News. Mr Baddour said the extreme weather could be linked to climate change, but only after extensive analysis.

"At the moment we don't know exactly how to explain this year's colder weather in Europe and other parts of the world," he said. "Such events can happen without being linked to climate change, El Nino, or La Nina - it could just be internal atmospheric factors, or it might be linked." In Britain the cold weather has been caused because the winter winds that normally come from the south-west, travelling over the relatively warm Atlantic, have been "blocked" over the past three weeks by an area of high pressure over Greenland. Instead cold air has been pushing down from the Arctic, according to the UK Met Office.

Mr Baddour said that because climate systems "interact" with each other, "blocking" in one area could lead other linked climate systems into extreme weather conditions. "These events could be linked to each other, but we need to stand back and analyse what has happened in the past few weeks and run computer simulated models before we can say," he said. There is a theory that the collapse in the ocean's circulation, due to an inflow of melting ice from the poles, could lead to cooler winters in the northern hemisphere but "it's just a theory," he added. "We really don't know." And this winter it is not cold everywhere. Record-high temperatures have been recorded in Washington and Alaska, in North Africa, the Mediterranean and south-west Asia. Temperatures in Canada are more than 10°C above normal.

Indeed, despite ending on a cold note in many regions, worldwide, 2009 was still the fifth warmest year since global records began in 1850. Whatever caused the "big freeze", its impact has been wide-ranging and its consequences - such as the potential damage to Chinese wheat crops - will still be felt long after temperatures return to normal.