In June 1991 Mt Pinatubo erupted with terrifying force, killing 800 people and forcing thousands to flee. It thrust 30m tonnes of dust into the stratosphere, forming a cloud around the world which many scientists believe slowed down global warming - the steady rise in the earth's temperature caused by man-made gases - until, more recently,it fell back to earth.
Until 1991 there had been a steady progression of hotter and hotter years, which backed up computer predictions that the atmosphere was starting to warm. Global warming became a regular topic of conversation: 1989 was the hottest year ever recorded. 1990, which set the record temperature in Britain - 990F in Gloucestershire - was hotter still. Many scientists expected 1991 would continue the upward trend, until Pinatubo exploded, remoulding the mountain's upper slopes into dark slippery ash.
The eruption threw dust and sulphur dioxide some 12 miles into the upper atmosphere, above the clouds, where it could not be removed by rain or winds. This "aerosol", as climate researchers call it, encircled the earth in three weeks, and covered 42 per cent of the planet's surface after just two months. "If you have a volcano in the Arctic or the Antarctic, the dust from it doesn't spread so far from the poles," said Dr Philip Jones of the Climate Research Unit at the University of East Anglia. "But if you have a tropical volcanic eruption, the dust spreads outwards towards the poles." At 150 north of the equator, Mt Pinatubo was well placed to have a global effect on the weather.
In the two years following Pinatubo's eruption, scientists working on computer models to predict climate change found that their numbers refused to add up: it should have been hotter than it was.
Global warming is caused when "greenhouse" gases such as carbon dioxide in the atmosphere absorb and trap heat, rather than allowing it to radiate into space. Without this effect, the earth would be cold and barren. Scientists fear that over the past century, the burning of fossil fuels such as coal and oil has led to an accumulation of gases which are slowly raising the planet's average temperature.
The disagreement between their computer models and the recorded figures stymied some. But the dust from the eruption was having a subtle effect on the world's weather. According to David Parker, a researcher at the Hadley Centre for Climate Prediction in Bracknell, Berkshire, "the dust reflects the sunlight back into space, rather than letting it through the stratosphere to heat up the earth".
The sulphur dioxide also reacts with water vapour and other chemicals in the upper atmosphere to create sulphate compounds which also cool the surface by absorbing heat. Mr Parker estimates that the overall effect of the volcano was to lower average global temperatures by 0.50C - tiny in human terms, but significant to scientists who are trying to measure rises in global temperature of a tenth of a degree each year.
A sufficiently large volcanic eruption can have a dramatic effect on the weather. When Toba, in Sumatra, exploded into life 73,500 years ago, it put out enough material to accelerate the onset of the last ice age. In July 1783, Mt Laki in Iceland blew up. Benjamin Franklin suggested that the cold winter that followed in the US could have been caused by it.
In 1815, Tambora, in Indonesia, erupted, the second biggest (after Toba) in the past million years. It is reckoned to have thrown out five times more dust than Pinatubo. In 1816 was called "the year without summer". There were June frosts in New England and the French wine harvest was the latest for five centuries.
Krakatoa, perhaps the most famous and apocalyptic volcano, was followed by cold, wet summers through Europe for years - but also by beautiful sunsets, caused by high-level dust scattering light from the setting sun to create spectacular violet and purple afterglows. The biggest eruption in the 20th century was Mt Katmai in Alaska in 1912.
By the end of 1993 the billions of dust particles ejected by Mt Pinatubo had fallen to earth and the cooling effect had worn off.
The dry, hot summer in Britain, along with other conditions such as Spain's worst-ever drought and water restrictions elsewhere in Europe, are early indications that climate change is back on track, said Dr Jones.
"I expect that 1995 will be the warmest year on record," he said. "It is already ahead of 1990, which was the previous warmest year, on average temperature. It only remains to be seen what happens in the autumn and the end of the year." The rest of this decade could see the world's hottest years ever, he estimates.
New figures to be published later this month from the WorldWatch Institute in Washington indeed forecast that it will be even hotter in the years to come. Their figures show that the global temperature is between 0.30C and 0.60C higher than in 1880; the 10 warmest years on record have all occurred since 1980. The trend is forecast to continue with world temperatures increasing by a fraction of a degree every year, barring further volcanic interventions.
Mr Jones said: "The different computer models for computer warming all say the same things about the average temperatures in the future. But they can't predict rainfall. You might get fewer days with heavier rain, making it seem that the weather is worse, although you're getting the same rainfall as before." Few people remember that September 1976 was exceptionally wet in Britain.
One of the strongest indicators that the theories for global warming are correct comes from a monitoring station based atop the Mauna Loa observatory in Hawaii, which has been measuring the concentration of carbon dioxide in the atmosphere since 1958. It is the longest continuous record of atmospheric carbon dioxide levels in the world and has been moving relentlessly upwards every year.
Carbon dioxide, the principal greenhouse gas, is generated by most industrial activities. Yet the Hadley Centre's research suggests that even though the indicator at Mauna Loa could keep rising, some parts of the world which generate the most carbon dioxide could actually become cooler because of a localised version of the Pinatubo effect. "Human-induced sulphate aerosols actually slow heating," said Mr Parker. "When you burn coal or wood you get tiny particles and sulphates in the troposphere [the bottom of the atmosphere] which absorb heat from the sun and actually cool the areas beneath them. Then they will be dispersed by the winds." Rapidly industrialising countries such as China could suffer markedly from this effect.
Meanwhile, villagers living in the shadow of Mt Pinatubo are still in fear of deadly mudflows caused by heavy rains, which sweep down river valleys and bury roads and houses under a mixture of mud and volcanic ash.
In contrast, Britain's tales of warm weather woes might seem rather small.Reuse content