Volcanoes and climate
High sulphur levels have greatest effect

by Philip Eden

The ongoing volcanic eruption in Iceland has had a dramatic impact on air traffic, with all the knock-on effects which follow in our increasingly globalised society. However, there may yet be further developments, with a possible impact on the climate of parts of the northern hemisphere over the next few years.

It has been known for many years that major volcanic eruptions can have a significant impact on the global climate. Indeed, the link between volcanic activity and the weather in a given region (for instance in western Europe following the eruption of Laki in Iceland in 1783) was made by scientists centuries ago, although at that time they were unaware of the global climatic response.

The mechanism by which the biggest eruptions affect the world's climate is now fairly well understood. The ejection of debris and gases needs to reach the stratosphere to have the greatest impact, for this is a region of the atmosphere (more than 10km up) which is above the weather, so the dust and other material is not 'rained out'. Just as important, stratospheric winds blow in such a way that ejected matter from a volcano close to the equator is gradually spread across the planet to form a veil which usually lasts for three years, and occasionally as long as seven years or more. An eruption in relatively high latitudes, like this one, will produce a geographically much more restricted veil of material in the upper atmosphere.

The dust veil reduces the strength of the sun, but does not limit the amount of heat energy which escapes from the Earth to outer space at night and during the winter. But the solid particles in the dust veil gradually drop out of the atmosphere under the influence of gravity while gaseous matter lasts much longer. It is now known that eruptions with a high sulphur content have the greatest impact on climate. Sulphur dioxide gas reacts with water vapour in the atmosphere to form billions of tiny droplets of sulphuric acid which are much more effective than ordinary dust at diminishing the energy coming from the sun.

All this tells us that the greatest cooling effect comes from a volcano in tropical or equatorial latitudes which erupts explosively to send its plume of debris high into the stratosphere, and which has a high sulphur content. It is not yet clear whether the present eruption has produced much sulphur dioxide.

The last climate-changing eruption was that of Pinatubo, in the Philippines, in 1991. It is calculated that the global temperature dipped by between 0.5 and 1.0C during subsequent years as a result. The Central England temperature in 1992 was 0.4C below, and in 1993 0.7C below, the average for the whole of 1990s. Some readers will remember the hazy brown ring which encircled the sun during those two summers - a direct consequence of the veil of sulphuric acid droplets.