Has the Mount Pinatubo eruption cooled the earth?
By Nicholas Booth
IT HAS been described as the world’s greatest climate experiment, but unlike most scientific endeavours, it was not planned. When the tropical tranquility of the Philippines was shattered last June by a volcanic explosion, Mount Pinatubo was a relatively obscure volcano.
Having sent more than 20 million tonnes of dust and ash into the atmosphere, altering its heat balance and accelerating ozone depletion over a large part of the globe, Pinatubo has become the focus of several far-reaching studies.
Climatologists now use the term "Pinatubo effect" to describe how volcanic ash and debris, if sent high enough into the atmosphere, can influence temperature and weather for several years afterwards. Studying this effect is helping scientists assess a key question of the age — to what extent is man-made pollution producing long-term and irreversible climatic change?
The effect of volcanoes on climate has been known for centuries. As far back as the 1780s, Benjamin Franklin suggested that a "dry fog" from an Icelandic volcanic eruption resulted in unusually cold weather. When the Indonesian volcanoes Tambora and Krakatoa erupted in 1815 and 1883, noticeable climatic effects occurred. In both cases, the earth cooled for the decade following each eruption.
It was the eruption of Mount St Helen’s in 1980 that first gave volcanologists the important scientific data on the physical processes involved in a volcanic explosion.
A massive landslide removed a kilometre of material from the volcano’s summit, and a spectacular lateral blast destroyed 600 square kilometres of forest nearby; all of this was documented as it occurred. But Mount St Helen’s did not send much material up into the stratosphere, and so there were no climate disturbances.
But in 1982, a volcano in Mexico, called El Chichon, did send enough ash into the atmosphere for it to affect the climate, thus providing scientists with a benchmark against which they could compare future eruptions. Ash from the El Chichon eruption blocked out enough sunlight to lower the average global temperature by 0.3 C over a few years.
The dust from Pinatubo was ejected as high as 32km above the earth. Satellites observed the plume of volcanic ash as it girdled the globe at a speed approaching 120 kph. A month after the eruption which killed 350 people, a 4,800 km-long cloud of ash and sulphur compound circled the earth.
In the weeks following the eruption, Australian newspapers carried reports of sunset glows lasting more than two hours, caused as blue light from the setting sun was scattered even more than usual in the upper atmosphere by the dust particles, tipping the balance in favour of the sun’s red light.
Satellite temperature measurements confirmed that the dust had effectively shaded the surface of the earth from the sun’s rays, resulting in a lowering of the average global temperature.
A National Aeronautics and Space Administration (Nasa) team at the Goddard Institute for Space Studies in New York, led by Mr James Hansen, tried to assess what effect the cooling caused by Mount Pinatubo would have on global warming caused by man-made emissions of carbon dioxide. They concluded that Pinatubo would in effect delay global warming by several years.
Most climatologists agree that global warming due to the greenhouse effect will increase the mean surface temperature by 0.6 C over the next 30 years.
"All our models, which take into account varying degrees of greenhouse gases, predict a cooling of 0.5 C by the end of this year," says Mr Hansen.
But several climatologists are sceptical that cooling will be observed because of other complications in the global climate system that will work to counteract the volcano’s effect.
For example, another well-known climatic anomaly, the El Nino ocean current off the coast of South America, which occurs once every five to 10 years, is now under way and is expected to warm the ocean surface over the next few months. This warming of the sea, which in turn warms the atmosphere, is likely to counteract the cooling effects of the dust from Mount Pinatubo.
The eruption of El Chichon in 1982 was also followed by an El Nino, which helped to prevent the temperature from falling even further than the 0.3 C drop observed. "If this year’s El Nino is typical, then the effect may get swamped," says Mr Hansen.
While global warming experts argue about the effect of Pinatubo's eruption on average temperatures, ozone specialists are interested in the effect it had and will have on the ozone layer.
The volcano spewed out huge quantities of sulphate aerosols, particles containing sulphur that remain suspended in the atmosphere for several years.
These sulphate particles are important in the chemistry of ozone destruction as they act as sites where ozone-destroying reactions take place, and they mop up nitrogen-containing compounds that prevent ozone destruction.
Last winter, American and European scientists undertook the most intensive investigation of ozone depletion over the northern hemisphere, including Europe and North America. More than 300 scientists from 17 countries were involved, and their work showed that ozone levels fell by 10 to 20 per cent more than expected.
"The eruption of Mount Pinatubo has increased the abundance of natural sulphate particles, potentially enhancing ozone losses due to chemical reactions that occur on particle surfaces," said the Nasa ozone monitoring team.
In the tropics, Mount Pinatubo depleted ozone levels significantly, although further research is needed to determine by exactly how much, said Nasa.
"However, since volcanic particles settle out of the stratosphere in three to five years, any ozone loss caused by Mount Pinatubo will be short-lived in contrast to the chronic effect of long-life chlorofluorocarbons and halons."
The final complication is that removal of ozone is also likely to influence global warming because ozone is, like carbon dioxide, a greenhouse gas.
As Mr Neil Harris of the Ozone Secretariat at Cambridge explains: "There is a worry that when you remove ozone it, will affect the earth’s heat balance."
So the precise influence of Pinatubo on the world’s weather systems may never be known.
Source : The Straits Times, May 19, 1992
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