Environment

FROM the start of 2008, something seemed amiss with the weather. During an unseasonably warm January, two tornadoes struck Kenosha County, Wisconsin, damaging 105 homes. It was only the second known January tornado strike in Wisconsin. Just a few days later, a tornado struck Vancouver in Washington - only the third January tornado in that state since 1950.

Then on 5 and 6 February, a storm system spawned 84 tornadoes across several southern states as their primary elections took place. Five of the tornadoes were powerful EF4s on the so-called Enhanced Fujita scale. The Super Tuesday outbreak was the worst for 23 years, and left 57 dead. Once again, it came unusually early in the year. Unusually warm weather seemed partly to blame.

By the end of that month, 2008 already seemed to be one for the record books. Altogether there were 148 tornadoes across the US in February, more than double the previous record for this month, set in 1971.

$200-million damage

March seemed more normal, apart from an EF2 tornado hitting downtown Atlanta for the first time. Its 10-kilometre rampage racked up $200 million in damage and struck a CNN building, ensuring massive media coverage.

In May the drama continued. It is usually a very active month, but for this May the monthly count was a staggering 447 tornadoes. With half the year gone, tornadoes have already killed 119 people. That is nearly as many as in the whole of 1998, when 129 deaths made it the deadliest year since the 1970s.

Inevitably, in the face of such extreme weather, many are wondering what part global warming might have to play. Interviewed after the deadly Super Tuesday tornado outbreak, Senator John Kerry cautiously drew a connection (and was promptly criticised by the conservative right for doing so). Some climate bloggers have been far less hesitant in pointing the finger.

After all, it's not the first time global warming has been linked to tornadoes. The record number of US tornadoes in 2004 - 1819 - was mentioned by Al Gore in An Inconvenient Truth. So was he right to hint at a connection? Was Kerry?

There is no doubt that any increase in the strength or frequency of tornadoes could have dramatic implications. In the US, tornadoes kill 60 people per year and cause $400 million in damage, on average. It has been estimated that the US can expect a catastrophic $1 billion tornado once per decade.

Killer tornado

One recent study concluded that if one of the most powerful tornadoes, rated EF4 or EF5, were to rip through Chicago, it could kill 13,000 to 45,000 people and rack up over $40 billion in damage. Other exposed major cities besides Atlanta include Oklahoma City, St Louis and Houston. We definitely don't want to raise the odds on these worst-case scenarios.

Of course, the US is certainly not the only country at risk. Of the 10 deadliest tornadoes on record, just two occurred there. Six of them occurred in Bangladesh, including one in 1989, reported to have killed as many as 1300 people, which would make it the deadliest on record.

Determining the impact of ongoing climate change on weather events remains one of the knottiest aspects of global warming, and within this murky arena, the tornado issue is among the murkiest. It is entirely possible, for instance, that as the planet warms we could see an increase in severe thunderstorms but not in the kind that produce tornadoes. As politicians and bloggers stampede to link the 2008 tornado season to global warming, it remains questionable whether science can supply much justification.

The first question is whether these storms are changing in number or intensity. It is not an easy one to answer. While their size varies dramatically, in the US the average tornado is just 150 metres across and rakes along the ground for 8 kilometres. They can easily go unnoticed, especially in remote regions.

Relatively small

Even the severe thunderstorms that spawn tornadoes only measure tens of kilometres across and last for just a few hours. When it comes to such small-scale and relatively rare meteorological phenomena, historical records cannot be taken at face value. "Tornado reports, even more so than hurricane reports, are target-of-opportunity observations," says Harold Brooks of the National Severe Storms Laboratory in Norman, Oklahoma.

Over the past half-century the number of tornado reports in the US has grown rapidly, by an average of 14 per year, meaning that the total number of reports has doubled. No tornado expert thinks this huge rise reflects a real increase, though. Instead, it is due to more people living in tornado-prone regions, and to improved observation and reporting systems. These have led to the more frequent detection of the commonest and weakest tornadoes, rated EF0 and EF1 (see "More tornadoes?").

If you look only at EF4 and EF5 tornadoes, the number reported has, if anything, fallen slightly. But these numbers are also questionable. The original Fujita scale was only adopted in the 1970s, and retrospectively applied to earlier tornadoes based on written reports. It was based on damage to structures - a tornado that missed buildings would be classed as an F0 no matter what its wind speeds - yet did not fully take into account differences in the strength of buildings. In an attempt to solve such issues, the Enhanced Fujita scale was introduced last year.

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