Causes and mechanism

The impact of volcanic eruptions on the climate is determined by two quite separate factors.

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  The chemism of the original magma: magmas that are rich in silicates, so-called acid magmas, contain more highly volatile gases than basaltic magmas like sulphur dioxide, hydrogen sulphide, carbon dioxide, nitrous oxide, and water vapour. This chemism depends on the plate tectonic situation. Acid magmas are formed over the subduction zones of ocean plates, e.g. over the circum-Pacific "ring of fire" with the Andes and the North American Cordilleras in the east and the island arcs of Japan, the Philippines, and Indonesia in the west. The high fluid content of these magmas makes them highly explosive and their eruption columns extend much further into the atmosphere than in the case of basaltic volcanoes like Etna or Kilauea. The very rare flat basaltic eruptions are an exception. Compared with the sulphur fraction, other gases generally have much less impact on the climate.
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  The geographical position and the composition of the atmosphere: its lowest stratum (troposphere) only reaches a height of 7 to 10 km in the higher latitudes, compared with 15 to 18 km near the equator. For this reason, in the higher latitudes, eruption columns are much more likely to break through the boundary zone (tropopause) and penetrate the layer above (stratosphere). This is decisive because, unlike the troposphere, the stratosphere is for the most part dry, so that aerosol clouds can survive there much longer — for several years, in fact. In the moist troposphere they are soon washed out.

The aerosol particles produced by photochemical reaction form clouds up to several kilometres thick. These begin to drift under the influence of the stratospheric circulation and can travel around the earth several times. This is illustrated in Fig. 2, which takes the eruption of El Chicón in 1982 as an example. Their influence on the energy balance of the atmosphere is complex. First, the aerosol clouds absorb solar radiation, which logically reduces the amount of heat radiation reaching the surface of the earth. At the same time, however, the reflected earth radiation is absorbed and the lower stratosphere is heated up.

This leads to substantial changes in the atmospheric circulation, with corresponding effects on the temperature pattern. The specific regional effects vary widely, depending on these circulation changes. Following the eruption of Pinatubo, for example, the winter temperatures were higher than usual in Europe, Siberia, and North America, but much lower in Alaska, Greenland, the Middle East, and China. The global mean temperature signal nevertheless showed a distinct cooling. A further effect is the temporarily reduced ozone content of the stratosphere.

As mentioned above, normal basaltic eruptions have a much smaller climatic effect. The volumes and speed are not sufficient to produce eruption columns that can reach the stratosphere. It is quite a different matter in events like the eruption of the Laki fissure on Iceland. In such cases, sulphur dioxide can be transported into the stratosphere by convection streams of hot air over very hot lava fields.

This must have been the case on a much greater scale when the great flood basalts were formed in Siberia, the Paraná Basin in South America, the Karroo Desert in South Africa, the Indian province of Deccan, and on the Columbia Plateau in North America. They cover an area of up to several hundred thousand square kilometres, but on average this kind of event occurs only once in tens of millions of years. In the Deccan event, it is possible that large volumes of carbon dioxide were released as well, which would have caused global warming and atmospheric instability.