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184 CHAPTER 12

extent on the pre-existing state of the atmosphere, the troposphere are only likely to affect climate on
and this is why even if an eruption, such as the Pina- a regional scale. If, however, the plume reaches the
tubo eruption, causes net global cooling the effects stratosphere, the area affected can be very much
are more complex when examined in detail, and greater. For example, Fig. 8.3 shows satellite data
some regions may actually warm rather than cool. on the spread of the ash from the 1982 El Chichón
The effect of any given eruption on climate eruption. Winds in the stratosphere disperse the
depends on a number of factors such as the height ash and aerosols and areas far from the eruption site
of the eruption plume, the geographical location of can be affected. Stratospheric injection allows ash
the eruptive vent, the composition of the erupting and aerosols to affect climate within a whole hemi-
magma, and the volume and duration of the erup- sphere, or even on a global scale if the eruption site
tion. These effects are discussed below. is located close to the Equator (see below).
In general, then, it is only eruptions which involve
stratospheric injection which are likely to have
12.4.1 The influence of plume height on
widespread effects on climate. The consequences
climate change
of eruptions confined to the troposphere may be
The height to which ash and gas are carried in a significant, though, under certain circumstances,
volcanic eruption has a profound influence on the such as if the eruption is of long duration (see
impact of the eruption because it determines how below), or if the effects in one region are severe
long the climate may be affected and also the size enough to cause “knock-on” effects on the atmo-
of the region affected. spheric circulation and weather systems in the
The maximum height to which material is carried rest of the affected hemisphere.
depends primarily on the mass flux of the eruption
(see section 6.5.2). In relatively low mass-flux erup-
12.4.2 The effect of geographical location
tions the ash and gas stay within the troposphere
on climate impact
(the lowest layer of the atmosphere), but if the mass
flux is high enough both ash and gas can be injected The geographical location of the eruption site also
into the stratosphere (Fig. 6.7). The height of has an effect on the size of the region affected by an
the boundary between the troposphere and strato- eruption. The atmospheric circulation of the north-
sphere, called the tropopause, varies with geogra- ern and southern hemispheres is such that little
phical position and season but is typically 10–15 km. mixing of air occurs between the two. This means

The ash and aerosols have a limited residence time that if an erupting volcano is located at middle to high
which depends on their size and the height they latitudes, the ash and aerosols it releases into the
reach within the atmosphere. As we have seen, stratosphere will be spread only within that hemi-
ash and aerosols reaching the stratosphere have sphere; there will be very little mixing of the mater-
residence times of a few months and a few years, ial into the opposite hemisphere. If, however, the
respectively. Ash and aerosols confined to the eruption occurs in an equatorial region, ash and gas
troposphere, however, have far shorter residence can be released into both hemispheres and spread
times because they are rapidly “rained-out” by the by the winds in each hemisphere so that the ash and
tropospheric weather systems. There is only likely aerosols can affect climate on a truly global scale.
to be a significant long-term effect if ash and For instance, Fig. 12.3 shows the widespread dis-
aerosols are injected into the stratosphere and so persal of tephra during the 1883 Krakatau eruption.
have a relatively long residence time.
The height of the eruption plume has another
12.4.3 The effects of eruption volume on
important effect. In eruptions which are confined
climate impact
to the troposphere, the rapid removal of erupted
ash and gas means that the material typically only It is expected that the larger the eruption volume,
spreads out in the atmosphere to affect areas on a the greater the effect on climate will be. Study of
regional scale. So eruptions which are confined to the largest historical eruptions has shown that they