Volcanism and climate

Large, explosive volcanic eruptions commonly inject huge amounts of silicate microparticles and acid gases into the stratosphere. This normally leads to warming of the stratosphere due to absorption of incoming solar radiation. Sulphate aerosols are formed, causing a cooling of the lower troposphere by back-scattering of solar radiation (e.g. Rampino and Self, 1982). The volcanic particles have a residence time in the stratosphere of at least one to three years and are distributed by stratospheric winds to form a veil over much of the planet. This aerosol veil prevents the passage of incoming solar radiation to the Earth's surface, causing a lowering of surface temperatures.

The Greenland Ice Sheet Project 2 (GISP2) and the Greenland Ice Core Project (GRIP) from Summit, Greenland, provide records of aerosol (H2S04) and tephra particles from past volcanic activity (Zielinski et al, 1997). These continuous records are helpful in producing a hemispheric and global chronology of explosive volcanism and assessing the climatic effects of volcanism. The volcanic S042 records for the last 110,000 years show a strong connection between periods of

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Years before 2000 AD

Figure 3.20 The Holocene record of electric conductivity measurements (ECM) and volcanic sulphate in the GISP2 ice core. The records are presented as standard deviation units. Adapted from Taylor et al (1997) and Zielinski et al (1997)

Years before 2000 AD

Figure 3.20 The Holocene record of electric conductivity measurements (ECM) and volcanic sulphate in the GISP2 ice core. The records are presented as standard deviation units. Adapted from Taylor et al (1997) and Zielinski et al (1997)

enhanced volcanism and periods of climate change. An increasing number of explosive volcanic eruptions at 27,000-36,000 and 79,000-85,000 years bp may reflect initial ice sheet growth. The largest number of volcanic events occurred during the period between 17,000 and 6000 years bp, probably reflecting increased crucial stress due to variations in ice-sheet loading and volume changes of ocean water (Sejrup et al., 1989). The Holocene records of electric conductivity measurements (ECM) and volcanic sulphate in the GISP2 ice core are shown in Figure 3.20.

The majority of the largest volcanic eruptions have been in South East Asia: Tambora in ad 1815, Krakatoa in ad 1883, Agung in ad 1963, and Pinatubo in ad 1991. Normally, the climatic effect of a volcanic eruption is greatest the following year. The 'year without summer' in 1816 is believed to have resulted from the 1815 Tambora eruption.

It has been proposed that volcanism has had a profound effect on climate and glacier variations during the last millennium (Lamb, 1970; Baldwin et al, 1976; Mass and Schneider, 1977; Bradley, 1978, 1988; Miles and Gilder-sleeves, 1978; Hammer et al, 1980; Porter 1981a, 1986; Self et al, 1981; Gilliland, 1982;

Rampino and Self, 1982; Kelly and Sear, 1984; LaMarche and Hirschboeck, 1984; Sear et al, 1987; Oerlemans, 1988; Bailie, 1989; Scuderi, 1990). Porter (1986) discussed the pattern and forcing of northern hemisphere glacier variations during the last millennium and found a close relationship between glacier fluctuations and variations in Greenland ice core acidity (Hammer et al, 1980), indicating that sulphur-rich aerosols were a primary forcing mechanism of recent glacier fluctuations. Local and regional temperature drops related to the largest eruptions were calculated to 0.5-1.2°C. By keeping (winter) precipitation constant, this corresponds to a depression of glacier equilibrium line altitudes in the range of 80-200 m, which coincides with values found for the culmination of the Little Ice Age (Porter, 1986; Oerlemans, 1988).

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