Hurricane Gustav: Fuzzy Science Confounds Predictions

Meteorologists are predicting a more active hurricane season than usual this year, but there is no way to know whether global warming has caused an individual event such as a hurricane, or whether it has made such storms worse.

On the other hand, some scientists argue that severe storms such as Gustav are more likely in a warming world, because warmer seas make more powerful storms. The issue was hotly debated after Hurricane Katrina struck in 2005 and was subsequently featured in former US vice-president Al Gore's film documentary, An Inconvenient Truth. Publicity material for the film showed a tornado emerging from the chimney of a power station.

If anything, the science has become fuzzier in the years after Katrina, with studies suggesting that future storm strength could increase in places but decrease in others - studies seized on by both sides of the debate.

Last year's report from the Intergovernmental Panel on Climate Change (IPCC) said it was "likely" that global warming would make future cyclones more intense. Studies of hurricane records suggest that this trend can already be seen. A high-profile paper in 2005 from Kerry Emanuel, professor of meteorology at the Massachusetts Institute of Technology, showed that tropical cyclones in the west Pacific and Atlantic have become more powerful in the past 50 years. Another study concluded that the frequency of the strongest tropical cyclones has almost doubled globally since the early 1970s.

Some scientists have also linked an increasing number of hurricanes in the north Atlantic to global warming. Although, globally the number of tropical storms each year has hovered around 90 over the past century, in the north Atlantic there has been a clear increase. From 1850-1990, the overall average number of tropical storms in the north Atlantic was about 10, including about five hurricanes. Since 1995, the 10-year average has risen, with the 1997-2006 average at about 14, including about eight hurricanes. The IPCC report said this was "more likely than not" down to global warming.

A number of factors prevent more definitive conclusions. It is difficult to use climate models to simulate the conditions that allow a hurricane to form. Although sea surface temperature is important, so are other variables, including the difference between the temperature of the sea and air, as well as the formation of high level winds that stop storms developing.

It is also unclear how reliably historical records of hurricane strength can be compared, while storm activity in some regions seems to rise and fall in natural cycles over several decades. In December 2006, an expert group from the World Meteorological Organization concluded: "This variability makes detecting any long-term trends in tropical cyclone activity difficult." On the issue of whether climate change is influencing tropical cyclones, the group said: "No firm conclusion can be made at this point".

The number of people killed and injured by hurricanes has risen significantly in recent decades. This is a result of more people living and building in regions prone to extreme weather.