SCIENCE / A threat, or just more hot air?: A recent eruption cooled the planet, giving ammunition to critics of the greenhouse 'theory'. Fred Pearce reports

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GLOBAL warming is a serious threat. Or is it? The past few months have been the planet's coolest for several years. The reason is the eruption of Mount Pinatubo two years ago, which sprayed a thin veil of dust across the upper atmosphere. Nonetheless, the cooling has encouraged claims that the 'theory' of the greenhouse effect is a scare story got up by grant-hungry scientists - or, as one recent book in the US has it, 'a new McCarthyism'.

So how much in the greenhouse debate is known, how much is conjecture and how much is mythmaking? Are we all going to fry or not?

First, the facts that nobody contests:

Human activities, mainly burning fossil fuels and destroying forests, have increased the amount of carbon dioxide gas in the atmosphere by about a quarter in the past 200 years. And if we go on as we are, some time after the middle of the next century the concentrations will have doubled.

This doubling of carbon dioxide will have some warming effect on the atmosphere, because the gas traps the heat of the Sun. The direct effect will increase global temperatures by around 1 degree Celsius. However, the actual extent of warming will depend upon whether the planet responds to this change in a way that amplifies the warming or damps it down.

Nobody is sure whether global warming has begun. Global average temperatures have risen by about half a degree Celsius in the past century. But this could be a natural variability.

Those are the bare bones on which the opposing arguments are built. It is a rather lopsided debate. On one side there is an impressively broad consensus reached by researchers from around the world who have contributed to the reports of the Intergovernmental Panel on Climate Change (IPCC). Chaired by Sir John Houghton, former director of Britain's Meteorological Office, the IPCC was set up by the UN in 1988 to find out more about climate change.

On the other side are a few noisy mavericks. The most quoted of these is Richard Lindzen, professor of atmospheric science at the Massachusetts Institute of Technology in the US. He appears regularly on platforms for organisations such as the National Coal Association and the World Coal Institute. Houghton calls Lindzen 'the most serious' of the IPCC's critics, and Lindzen gave a lecture to the Royal Meteorological Society (RMS) in London last year.

In essence, the IPCC and most of the world's experts in the field argue that the planet's natural systems will amplify the initial warming caused by extra carbon dioxide in the atmosphere by a factor of

between two and four. Lindzen believes the 'sensitivity' of the atmosphere to global warming is very low, which could reduce the warming almost to nothing. 'In my view you could turn the Sun's heat down by 10 or 20 per cent, something like that, without much effect,' he told the RMS last December.

Using statistical models of the atmosphere, run on supercomputers, Sir John Houghton and his colleagues predict that a doubling of carbon dioxide will raise global average temperatures by between 1.5 and 4.5 degrees Celsius. Lindzen says the rise will be 0.5 to 1 degree. Such changes may not sound a lot, but small alterations in averages have a big impact. Current average temperatures are only 5 degrees above those of the last ice age, when most of Britain was covered in more than a kilometre of ice.

Though IPCC scientists are cautious about making forecasts for individual countries, they believe warming on a similar scale would cause deserts in the interior of continents to grow, engulfing some of the world's grain belts, while melting ice caps would raise sea levels, flooding low-lying areas.

If Lindzen is right, we could ignore the greenhouse effect, for a while at least. If the IPCC is right, global warming will be one of the most disruptive changes of the coming century. The global community has given a cautious nod to the IPCC view, agreeing at last year's Earth Summit to stabilise emissions of carbon dioxide at 1990 levels by the year 2000, but making no commitments beyond that date.

So how good are the IPCC's models? Do they accurately represent how the atmosphere works? Or is it a case of 'garbage in; garbage out'? Here the scientists who run the models are caught between their natural caution and the desire to present a clear message.

Away from the public platform, they admit they have much to be cautious about. Take just one variable in the equation: forests. Trees play a crucial role in the carbon dioxide story, absorbing it as they grow and yielding it up to the atmosphere when they die. Will they amplify or damp down global warming?

Optimists say that in an atmosphere rich in carbon dioxide, trees will grow faster. They will suck up more of the gas and so stabilise the greenhouse effect. This is sometimes called a negative feedback.

Pessimists believe global warming will alter regions where forests grow and the forests will die, releasing carbon dioxide into the atmosphere and amplifying the greenhouse effect - a positive feedback.

The most abundant natural greenhouse gas - the one principally responsible for keeping our planet from being a frozen waste like Mars - is water vapour. Everybody agrees that the impact of changes in carbon dioxide on water vapour is crucial. The IPCC argues that a warmer world will evaporate more moisture from the oceans, creating more water vapour and providing a strong positive feedback. But Lindzen believes that increased cloud formation would both shade the Earth and change the balance of radiation to stabilise temperatures. There are abstruse questions about ice crystals in clouds high up in the atmosphere that all agree could be crucial.

One way of solving the argument is to look at historical data. Both sides point to the ice ages. The IPCC scientists note the astoundingly precise correlation over at least the past 160,000 years between global temperatures and the amount of carbon dioxide in the atmosphere. Their data comes from chemical analysis of snow and bubbles of air preserved in the ice caps of Antarctica and Greenland.

Lindzen argues that the world plunged into and out of the ice ages even though there were only minute changes in the amount of solar radiation reaching the Earth - changes caused by 'wobbles' in the Earth's orbit. Those wobbles changed climate so dramatically, he says, because they reduced the amount of radiation reaching the poles and so started the growth of ice sheets and triggered the ice ages. 'Changing carbon dioxide levels won't do that,' he says.

The IPCC scientists say carbon dioxide can do that. Ice caps reflect much of the Sun's heat right back into space. As they melt, the planet will absorb more heat - a classic positive feedback that is included in their models.

Even climate modellers accept that the jury is still out on questions such as the role of clouds. Tom Wigley, head of the Climatic Research Unit at the University of East Anglia and publicly a stalwart of the IPCC consensus, says: 'The problem of assessing the climate's sensitivity is the biggest uncertainty we face.'

Tim Palmer, of the European Centre for Medium-Range Weather Forecasts at Reading, uses climate models every day in his work, but shares some of Lindzen's scepticism about their reliability and agrees that simulating the movement of heat in the atmosphere and the influence of clouds are the key problem for

the modellers. 'You cannot even assume that the greenhouse effect and global warming are synonymous,' he says. But he criticises Lindzen for his outright rejection of statistical models. 'They are the only way to bring all the processes together.'

The certainty that the modellers strive for seems to be receding. Four years ago, the IPCC scientists agreed that it would be a decade before they could tell how sensitive the climate was to the greenhouse effect. Now, says Wigley, it may be 2025 before they are sure.

For most of us the proof will be in the pudding. Has global warming begun yet? The truth is that, despite the long warming trend, there is too much natural variability to be sure.

Paradoxically, one of the strongest indicators of the sensitivity of the atmosphere to global warming could have been provided by the cooling that has followed the eruption of Mount Pinatubo. Pinatubo's volcanic shroud has reduced the solar heat reaching each cubic metre of the Earth's surface by about 4 watts - rather more than the warming caused so far by the man-made greenhouse effect, and compared to a total average solar heating of about 240 watts per square metre. The world has cooled as a result by about 0.4 degrees, an effect likely to wear off later this year. This cooling is in line with the top end of the range of changes predicted by climate models. 'Thus,' says Nasa climatologist James Hansen, 'it appears that the Pinatubo cooling favours high climate sensitivity.'

He predicts that there will be a 'rapid recovery and new record temperatures within the 1990s'. Even that wouldn't be proof that global warming has begun. But the problem in deciding how the world should respond to the scientists' fears is that we are likely to be a lot warmer before they can be sure.

(Photograph omitted)