Anselm Smolka

Climate change and volcanism

Mighty volcanic eruptions can severely interfere with the global climate and influence it for many years. This was illustrated very strikingly by the eruption of Tambora in Indonesia. 1816 went down in history as the year without a summer.

History

The landscapes painted by the English artist J.M.W. Turner in the first half of the 19th century are admired throughout the world. Their connection to a natural hazard event, the largest volcanic eruption in history, is less well known: the origins of the brilliant colours in Turner's sunsets may be traced to the eruption of Tambora on the island of Sumbawa in Indonesia on 11 April 1815. In this mammoth event the top 1,200 metres of the volcano were flung into the air. Altogether, 50 km3 of tephra and ash particles were blown into the atmosphere within the space of a few days. The effects were global, with the sunsets being only one — aesthetically pleasing — side. 1816 went down in history as the year without a summer. Mean temperatures for that year in Europe and eastern parts of the United States were much lower than in any of the previous 200 years. These low temperatures were accompanied by excessive rainfall in Europe and unusually dry weather in New England, leading to substantial crop failures and famine. The monsoon cycle in India was severely disrupted, which had negative repercussions on farming and the food situation.

The first scientist to establish a link between atmospheric phenomena and a volcanic eruption was Benjamin Franklin, later president of the United States of America. In 1783, the eruption of the Laki fissure on Iceland produced the coldest winter in the northern hemisphere for several decades and led to devastating floods in central Europe the following year. Similar connections were then established with numerous volcanic eruptions, which were finally collated and interpreted in H.H. Lamb's ground-breaking investigation of 1970. The Pinatubo eruption on the Philippines in 1991 provided confirmation that such observations are by no means subjective. It lowered the measured mean global temperature by 0.5°C and for two years interrupted the long-term trend towards global warming due to the anthropogenic greenhouse effect. On the basis of Lamb's findings, it was the ash ejected into the atmosphere that blocked out the sunlight and caused the resultant cooling effect.

The eruptions of Mount St. Helens in the United States Cascade Mountains in 1980 and El Chichón in Mexico in 1982 together with advances in instrumentation shed new light on the climatic effects of volcanism. Systematic analyses of the aerosols circulating in the atmosphere by aircraft and satellites have shown that it only takes a few days for the ash particles to be washed out. The bulk of these airborne particles that remained in the atmosphere for comparatively long periods of time consisted of droplets of sulphuric acid, formed by large volumes of emitted sulphur dioxide reacting with the water vapour in the atmosphere. This is visualised in Fig. 1.