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182 CHAPTER 12

Years more reflection of light and therefore less light
–4 –3 –2 –1 E +1 +2 +3 +4 reaching the surface. The satellites detected an
+0.3 increase in albedo immediately following the erup-
tion indicating that the amount of sunlight reaching
the surface had decreased. Furthermore, satellite in-
+0.2
Temperature deviation (°C) +0.1 absorbing some of the incoming sunlight and that
struments showed that the acidic aerosols were
the stratosphere was warming as a result. Overall
the eruption caused a detectable cooling of the
0
Earth’s surface with an average global decrease in
temperature of ∼0.5°C. The effects when examined
–0.1
in detail, however, proved to be considerably more
complex, with some regions experiencing net cool-
–0.2
ing but others actually experiencing net warming.
–0.3 For instance, North America, Europe, and Siberia
were warmer than normal during the winter of
Fig. 12.2 Typical example of global temperature changes
1991–2 while Alaska, Greenland, the Middle East,
for a few years before and after a major explosive volcanic
and China were cooler. In fact, it was so cold in
eruption, showing the small but significant amount of
cooling caused by such events. (Based on fig. 2a in Self, Jerusalem that it snowed there, which is extremely
S., Rampino, M.R. and Barbera, J.J. The possible effects of unusual. These effects resulted from changes in
large 19th and 20th century volcanic eruptions on zonal and normal weather patterns induced by the strato-
hemispheric surface temperatures. J. Volcanol. Geotherm. spheric warming caused by the eruption.
Res., 11, 41–60, copyright Elsevier (1981).)

12.4 The effects of volcanic eruptions
age, however, it has become possible to detect the
on climate
climate impact of volcanic eruptions directly. The
1982 El Chichón eruption in Mexico and the 1991
eruption of Mount Pinatubo in the Philippines (one In most cases the dominant effect of eruptions on
of the largest eruptions of the 20th century) were climate is one of overall cooling, but the Mount
the first eruptions for which detailed satellite mon- Pinatubo eruption shows that the actual effects,

itoring of their effects on climate was possible. Study when they can be recorded in detail, are more com-
of these two eruptions has demonstrated unequiv- plex, and vary through time and with geograph-
ocally that volcanic eruptions can induce rapid, ical location. This section discusses why volcanic
short-term climate change and has allowed detailed eruptions affect climate and why different eruptions
study of how eruptions cause climate change. have different effects.
The 1991 Pinatubo eruption, for example, gener- Accounts of many volcanic eruptions describe
ated an eruption plume greater than 30 km high how a cloud of ash from the eruption blocks out the
and injected large quantities of ash and gas into the Sun and causes it to become as dark as night in the
atmosphere. Satellites were able to detect the con- middle of the day. During the 1815 Tambora erup-
version of sulfurous gases from the eruption into tion, for example, areas as much as 600 km from
sulfuric acid aerosols (see section 11.2.7) and to the volcano experienced 2 days of darkness as a
detect the dispersal of the ash and aerosols within result of the spread of the ash downwind. The injec-
the stratosphere. This showed that the ash and tion and progressive dispersal of ash in the atmo-
aerosols had circled the globe within 22 days of the sphere causes a less obvious but still significant
eruption. Satellites detected how the presence of blocking of some sunlight, with the reduction in
the aerosols affected the Earth’s albedo. The albedo the amount of sunlight reaching the surface thus
is a measure of how much incoming sunlight is causing surface cooling. While the effect of the
reflected back into space; a higher albedo means release of ash is the most obvious immediate effect