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VOLCANOES AND CLIMATE 185

Fig. 12.3 The extent of atmospheric
dispersal of tephra from the 1883
Krakatau Boundary poorly defined
explosive eruption of Krakatau.
(Based on fig. 17.4 in Francis P. (1993)
Volcanoes: a Planetary Perspective.
By permission of Oxford University
Press.)

can cause global cooling of up to ∼0.5°C for periods likely effect of this eruption on climate have been
of 2–3 years after the eruption. The largest erup- made using computer models designed to look at
tions recorded in the geological record are much the climate impact of a nuclear war, and suggest
larger than these historical eruptions (see Chap- that the effects of a Toba-scale eruption are com-
ter 10). It is natural to expect, therefore, that the parable with “nuclear winter” scenarios. Global
effects of these larger eruptions would be propor- cooling of 5–15°C is predicted; this scale of cooling
tionately greater. For instance, the Toba eruption is equivalent to the global temperature difference
75,000 years ago, one of the biggest eruptions in between now and the last Ice Age! The model prob-
the geological record, produced an erupted magma ably overestimates the effect of such an eruption
3
volume of 1500 km (equivalent to ∼4200 Mount St because the residence time of the ash in the atmo-
Helens 1980 eruptions!) and an estimated 3300 Tg sphere is short and because the sulfurous gases
of H SO aerosols (Table 12.2). Simulations of the released could not all be converted to aerosols
2 4
immediately because the stratosphere does not
contain enough water vapor. Climate modeling is
notoriously difficult and it is very hard, therefore,
Table 12.2 The relationship between atmospheric aerosol
to make an accurate assessment of the effects of
loading and erupted volume during various eruptions.
The erupted volume is given as the dense rock equivalent very large volcanic eruptions on climate. It is diffi-
(DRE) value. cult to believe, though, that the effects of such a
large eruption would not be profound.
Eruption Erupted Estimated
volume (DRE) aerosol
3
(km ) loading (Tg) 12.4.4 The effects of magma composition on
climate impact
Toba, 75 ka 1500 3300
Laki, 1783 12.5 90–280 We have seen that the aerosols formed as a result of
Tambora, 1815 50 180 volcanic activity have a far more profound effect on
Krakatau, 1883 10 50 climate than does the ash. Only certain gases form
Katmai, 1912 12 20 aerosols and it is the release of the sulfurous
Agung, 1963 0.3–0.6 16–30 gases, SO and H S, which form the sulfuric acid
2
2
Fuego, 1974 0.1 3–6 aerosols that are most important. For this reason
Mount St Helens, 1980 0.3–0.4 0.3
the composition of the erupting magma, particu-
El Chichón, 1982 0.38 12
Mount Pinatubo, 1991 4–5 30 larly its sulfur content, is very important in deter-
mining the impact of an eruption. Table 12.2 shows