I am sorry to report that your correspondent J E Plumtree appears a little confused about the role of volcanic eruptions and the effect they have on the atmosphere (Problem is, you can't tax volcanoes, Gordon, SA Letters, Tuesday, March 13).

The ozone layer is high in the atmosphere and has the effect of blocking much of the dangerous ultra-violet radiation from the sun. Some years ago it was discovered that a hole was appearing in this layer over the poles and this was caused by the gas then used in aerosols, fridges, freezers and halon fire extinguishers.

This chemical was eventually banned internationally and the ozone hole has started to repair itself, showing just how resilient nature can be. Recent scientific reports suggest that some volcanic eruptions can affect this ozone layer, but again nature allows it to repair itself. Ozone does not have much effect on the climate.

Volcanoes certainly can affect the climate. The biggest effect is the production of clouds of ash and the release of another gas, sulphur dioxide, both of which have the effect of cooling the atmosphere by blocking warmth from the sun. It is because of these effects that Mount Pinatubo caused a small, but measurable drop in temperatures around the world after it erupted in 1991. Not all volcanic eruptions are big or powerful enough to affect the climate in this way, but they can also be much bigger: the eruption of Tambora in Indonesia in 1815, for example, caused two or three years of unusually cold temperatures and crop failures were recorded in Europe. Again, given time, nature recovers.

The problem now is that very large numbers of people are producing vast amounts of greenhouse gases, that do effect the climate, and the natural mechanisms that allow things to recover are just being overwhelmed.

A Read.

Swindon