Are we to blame for this?
By Robert Matthews, Science Correspondent

WHAT links the following: tornadoes in Sussex, the worst floods in England for more than half a century, and the highest global temperatures since records began?

 The Government at least seems in little doubt. John Prescott, the Deputy Prime Minister, told the Commons that the devastating weather of the past few weeks is a "wake-up call to everyone" over global warming.

 It is a wake-up call that many environmentalists insist is long overdue. Since the late 1980s, they have been claiming that everything from melting glaciers in Iceland to vanishing coral in the Caribbean and Indian Ocean points to the same awful conclusion: that the Earth is over-heating - and it's all our fault.

 Last week, Tony Blair told a a crowd of Worcestershire flood victims of his concern that they might have been victims of global warming. On the face of it, such suspicions seem well-founded. Some of those communities now under water were inundated in appalling floods barely two years ago. Add in the storms that have lashed Britain over the past decade and the predictions that global warming would mean more extreme weather, and the argument seems rock solid. 

No sooner had the Prime Minister voiced his suspicions, however, than climate scientists were warning of the dangers of seeing patterns in the British climate that don't exist.

 Researchers from the universities of Newcastle and Exeter unveiled a record of British rainfall dating back to the Norman Conquest. It showed that the bizarre weather of the past few years is entirely consistent with the natural variations in the climate that have taken place over the past 1,000 years.

 Yet environmentalists argue that events in Britain are only part of the picture, and point to recent storms in Taiwan, floods in Bangladesh and fires in the United States as further evidence for global warming.

 Even so, the scientific consensus - as expressed recently by the UN Intergovernmental Panel on Climate Change (IPCC) - is that the world's weather has not become significantly more extreme during the past 100 years.

 What most scientists do accept is that the Earth is warming up: temperature records from around the world point to the 1990s as being by far the hottest decade of the century. Many scientists also insist that the chief cause of this warming is the rising level of carbon dioxide (CO2) in the atmosphere, generated by burning fossil fuels such as coal and petrol. 

The solution to global warming would thus seem clear: dramatic cuts in fossil-fuel emissions. Finalising the nature of those cuts will be the purpose of of an international meeting in The Hague later this month.

 The science behind the proposed cuts is, however, far from finalised. Not everyone agrees on how big a cut is needed, how it should be effected, or whether it would do more harm than good.

 Some scientists argue that global warming will lead to more extreme weather - worse floods and stronger winds, for example - essentially because, if any gas is heated, it becomes less stable. During the past five years, however, climate experts have found that all the "filth" that industry puts into the air along with CO2 actually combats global warming by reflecting the Sun's heat back into space. 

While the precise effect of these so-called aerosols on global warming is still hotly debated, some climate experts think it might even cancel out the warming effect caused by greenhouse gases.

 If true, this would have profound implications for those trying to decide how to combat global warming. The demand for a clean-up from industry might actually accelerate global warming, with who knows what consequences.

 It is a possibility that has prompted one leading climate researcher to call for a shift away from controls on CO2 and towards controls on methane, a potent greenhouse gas generated largely in the wetlands and rice paddies of developing nations.

 It is a call all the more striking given that it comes from Dr James Hansen of the US Goddard Institute for Space Studies, the scientist credited with putting climate change on the political agenda in June 1988, when he told Congress: "It is time to stop waffling so much" and accept the reality of global warming.

 Dr Hansen now argues that industrialisation might not, in fact, be the source of all climatic evil. In highlighting the role of methane, he has also focused attention on the role of the Third World in combating global warming - a highly politically incorrect move that could lead to Dr Hansen's reputation among eco-warriors sinking from hero to zero.

 While the arguments continue over who is to blame and who should pay, some scientists question the very notion that humans are even responsible for global warming. Instead, they point the finger of blame at the most obvious potential culprit: the Sun.

 Attempts to implicate the Sun have repeatedly been howled down by climate experts, but refuse to go away. Earlier this year, a meeting of space scientists sponsored by the EU was told of a striking correlation between the temperature of the Earth and the strength of the Sun's magnetic field. The correlation stretches back more than a century, and suggests that about half of the measured global warming might have been caused by the Sun.

 While many climate researchers dismiss the correlation as a statistical quirk, space scientists are taking it seriously. One explanation, put forward in 1997 by Dr Henrik Svensmark of the Danish Space Research Institute, is that the changes in magnetic field alter the levels of cosmic rays reaching the Earth's atmosphere. According to Dr Svensmark, cosmic rays might trigger cloud formation, and thus change the amount of solar heat that reaches the Earth's surface.

 Satellite images of the Earth taken between 1978 and 1996 do suggest a link between cosmic ray levels and cloud cover. Whether the link is coincidence is about to be tested in experiments at Cern, the European centre for particle research in Geneva.

 As with the cooling effect of aerosols, it is only in the past few years that climate experts have begun to take seriously the potential role of the Sun in global warming. Yet, once again, it could have major implications for attempts to stem climate change. 

The biggest problem facing climate researchers is that they still lack computer models sophisticated enough to predict the Earth's climate with any certainty. Nothing shows this more clearly than the IPCC's estimate for the most basic figure in the global-warming debate: the likely rise in temperature in the next 100 years.

 Five years ago, the IPCC forecast a rise in the range of 1.5C to 3.5C. In most scientific disciplines, the range of uncertainty shrinks with time as more data and insight accumulates. With climate scientists discovering only ever more complexity, the IPCC's next report is set to forecast a rise of anywhere between 1C and 6C - more than double the uncertainty of five years ago. 

With so little certainty surrounding so basic a figure, precise predictions of the impact of global warming on specific countries are still years away. Even so, broad-brush pictures are emerging - and, once again, they confound simplistic views of climate change. 

Of all the unspoken assumptions about global warming, none is more pervasive than the view that it will visit untold misery on the Earth. According to a study published last week of the possible effects on European countries, however, the truth is likely to be different.

 The European Acacia study suggests that, while some parts of southern Europe could become intolerably arid, Britain and other northern European countries could see their climates improve, with better summers, milder winters and longer growing seasons.

 Because predictions of even world-scale effects of climate change are still far from certain, any regional forecasts must be treated with caution. Nevertheless, this latest study highlights the dangers of presuming that one global climate change policy will fit all. 

In the end, it hardly matters whether last week's floods were the result of man-made pollution, an overheating Sun or just the vagaries of the British climate. The real challenge facing scientists and politicians lies in doing what humans have always done: using their ingenuity to find ways of enjoying the advantages of a changing climate, while ducking its disadvantages.