Two people died in Japan after inhaling toxic fumes expelled from the Mount Aso volcano while they were sightseeing on its lower slopes. Earlier this year three Japanese soldiers were killed by the toxic volcanic fumes of Mount Hakkoda. We have also recently seen Montserrat devastated by a volcanic eruption, yet while the local effects of volcanoes such as these may grab the headlines, the long-term effects of a major eruption are potentially still more serious.

An erupting volcano may throw huge amounts of dust and gases - particularly sulphur dioxide - into the upper atmosphere. Energy from the sun transforms the sulphur dioxide into sulphuric acid and sulphate particles which remain in the stratosphere for several years. As they disperse around the earth, they cut out some radiation from the sun, thus leading to a general cooling of the lower atmosphere.

When El Chichon erupted on 3 April 1982, strong winds carried the dust cloud at an average speed of 45 mph so that it encircled the earth in less than three weeks. Major eruptions can result in a drop in average temperatures of about 0.2C at the surface for as long as two years after the event. There is even a suggestion that volcanic activity may have led to the "Little Ice Age" that began in the 16th century.

The effects of the last huge volcanic eruption - that of Mount Pinatubo in the Philippines on 12 June 1991, are still being assessed, but it is known that it expelled some 20 million tons of sulphur dioxide into the stratosphere, and vast quantities of dust. The amount of solar radiation reaching the lower atmosphere dropped by about 2 per cent, and average temperatures around the world are believed to have fallen by between a quarter and half of a degree as a direct consequence.

Some success has been had recently with mathematical models of climate change that incorporate the effect of volcanoes. After Pinatubo, exceptionally cold winters in the Middle East and mild winters in western Europe were successfully predicted from an increase in volcanic dust in the atmosphere.

For once, however, it seems that the long-term effects are more accurately predicted than the short-term. There is a theory that the ash particles thrown up by a volcano are good at stimulating the formation of water droplets - which is believed to be why volcanic activity is associated with heavy rain and thunderstorms, though quite why there is more lightning during volcanic activity than in comparable storms at other times is still not understood.

Since small changes in temperature can have a large effect on the circulation of the air, it is even possible that major volcanic eruptions may be connected with large-scale changes in the weather pattern such as El Nino. Some recent research in Hawaii has indicated a correlation between seismic activity on the East Pacific Rise and the strength and timing of El Nino cycles. For the most part, however, volcanoes are just another unknown in the great weather equation.