Pinatubo's summer washout: Volcanoes erupt and the world's weather changes. Bill Burroughs explains the link between forecasting such effects and global warming

IF YOU want to know why the weather has been so bad over the past couple of months, you may have to look as far afield as the Philippines and as far back as June last year, to the eruption of Mount Pinatubo.

Although it may be pitching it a bit too strongly to link our wet and disappointing weather to the volcano, the way the world's climate has behaved may be a strong piece of evidence that global warming is a reality. For the climate has reacted to the dust and sulphur dioxide from Pinatubo in just the way that the computer models predicted. These same models are predicting global warming, so if they are right about the volcanic eruption, they may well be right about the greenhouse effect.

Since Benjamin Franklin suggested that the cold winter of 1783-84 in northern Europe was caused by the dust cloud produced by the massive eruption of Laki in Iceland in July 1783, it has been widely accepted that volcanoes play an important role in climatic change.

Explosive volcanic eruptions inject huge quantities of dust and, more significantly, sulphur dioxide, into the upper atmosphere, where the sulphur dioxide is converted into sulphuric acid aerosols. At altitudes of 15 to 30 kilometres, where there is no significant vertical motion, these tiny particles remain in the stratosphere for several years. A dust veil in the upper atmosphere absorbs sunlight. This heats up the stratosphere, but causes a compensating cooling at lower levels, as less solar radiation reaches the Earth's surface.

Analysis of past eruptions suggests that not only do they cause a general cooling of the global climate, but they also produce a shift in weather patterns. In particular, six to 18 months after major volcanic eruptions, summers over eastern North America and north-western Europe tend to experience a disproportionate cooling.

The eruption of Pinatubo provides an excellent opportunity to check these theories. This eruption, by far the biggest this century, injected some 20 million tons of sulphur compounds into the stratosphere. These spread all round the world and the effect of the dust veil could be clearly seen in Britain last autumn and winter in the form of spectacular sunsets.

It may be coincidental that we have had such dismal weather over the past month or so. Overall it was not a bad summer, with June being the sixth hottest this century and July also being warm. More intriguing has been the fact that, in keeping with the consequences of past volcanoes, much of Canada has had a particularly miserable summer. But most important is clear evidence that there has been a significant cooling of the average global temperature of a few tenths of a degree Celsius since Pinatubo erupted.

These observations are consistent with the most widely quoted example of volcanic disturbance of the climate - the eruption of Tambora in Indonesia in 1815, which is estimated to have injected five to ten times as much sulphur into the stratosphere as Pinatubo. This was followed in 1816 by the renowned 'year without summer', which brought June frosts to New England and the latest European wine harvest in records that go back to 1482.

But even this catastrophic event pales into insignificance against the eruption of Toba in Sumatra 73,500 years ago. Reckoned to be the largest volcanic eruption in the past million years, it was at least five times the size of Tambora. In a paper published in Nature on 3 September, Michael Rampino of New York University and Stephen Self of the University of Hawaii propose that this volcano accelerated the onset of the last ice age.

Working on the basis of both the experience of Tambora and computer models of the global climate, they estimate that such a massive eruption could have led to a 'volcanic winter'. This would have cooled the northern hemisphere by 3C to 5C for about a year. More crucial is that for two or three years it could have led to a drop in summer temperatures over northern Canada of 10C to 15C (compared to 3C to 5C after Tambora), enough to produce perennial snow cover over the central plateau of Quebec and Labrador.

At a time when the Earth was slipping into an ice age, this additional snow cover would have reinforced the cooling effect by reflecting more sunlight back into space. This could have been the last straw in triggering the ice age. Geological records, such as the composition of ocean sediments and changes in sea-level, clearly show that the climate did undergo a rapid and long-lasting cooling around 73,000 years ago.

But with all this talk of cooling, it is easy to lose sight of what could be the most important consequence of the accurate monitoring of the changes caused by Pinatubo. It could show that predictions of global warming resulting from the build-up of man-made greenhouse gases in the atmosphere can be made with greater confidence.

When Pinatubo erupted and the scale of its atmospheric impact became clear, predictions were made of how much cooling this would produce. So far the forecasts based on the computer models of the global climate appear to be broadly in line with what has happened. So, instead of being concerned about the cooling effect of Pinatubo, we should be more worried about how current weather events are providing support for the performance of global climate models - and hence the threat of global warming.

(Photograph omitted)