Climate change - Atmosphere, Weather and Climate - page 395

VOLCANIC ERUPTIONS AND
CLIMATE

box 13.3 topical issue

The eruption of Krakatao volcano in Indonesia in 1883 demonstrated the global significance of large explosive events in
andesitic volcanic cones. The eruption was followed by dramatic red sunsets around the world, as a result of the injection
of aerosols into the stratosphere, and by cooler conditions. However, after the 1912 eruption of Katmai in the Aleutian
Islands there was a lull in volcanic activity until Agung in Bali erupted in 1963. Aerosols from equatorial eruptions can
disperse into both hemispheres whereas those in middle and high latitudes cannot be transferred equatorward due to
the upper circulation structure. Non-explosive eruptions of basaltic shield volcanoes of the Hawaiian type do not inject
material into the stratosphere. Volcanic aerosols are often measured in terms of a dust veil index (DVI), first proposed
by H. H. Lamb, that takes account of the maximum depletion of monthly average direct incoming radiation measured
in the middle latitudes of the hemisphere concerned, the maximum spatial extent of the dust veil, and persistence of the
dust veil. However, this cannot be calculated directly for historical events.
The largest DVI values are estimated for 1835 and 1815 to 1816. Volcanologists use a Volcanic Explosivity Index
(VEI) to rank eruptions on a scale of 0-8. El Chichon (1982) and Agung (1963) are rated 4, but the index may not
necessarily be a good indicator of climatic effects.
The atmospheric effects involve both micro-particles of dust as large particles rapidly settle out and gaseous sulphur,
which forms sulphate aerosols. These lead to acidity in the snow falling on ice sheets and this can be measured by
determing the signal of electrical conductivity in an ice core.
Sulphate aerosols play a significant radiative role by increasing atmospheric turbidity and therefore reducting the
transmission of incoming solar radiation. Temperatures in mid-latitudes in the year following a major eruption are reduced
on average by 0.5 to 1.0°C. Dramatic evidence of such effects was provided by the 'year without a summer' in 1816,
following the eruption of Tambora in 1815. This seriously impacted on societies in many parts of the world. However,
it also followed a series of cold winters in Europe. Mount Pinatubo caused a larger seasonal response over the northern
continents amounting to 2°C cooling in summer 1992 and up to 3°C warming in the winters of 1991 to 1992 and 1992
to 1993.
It appears that repeated major eruptions are required in order for there to be long-term climatic effects. Ice
core records provide long histories of volcanic eruptions through the late Pleistocene and do show episodes of more
frequent eruptions.

Table 13.2 The four categories of climatic variable subject to change.

Variable changed Scale of effect Sources of change

Atmospheric composition Local-global Release of aerosols and trace gases

Surface properties; energy budgets Regional Deforestation; desertification; urbanization

Wind regime Local-regional Deforestation; urbanization

Hydrological cycle components Local-regional Deforestation; desertification, irrigation; urbanization