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12 Volcanoes and climate

12.1 Introduction climate, at the factors which control the impact of
individual eruptions and, finally, at links between
It has long been thought that volcanic eruptions the largest volcanic eruptions and mass extinc-
can affect climate for some time after an eruption. tion events preserved in the geological record.
For instance, contemporary accounts relate how
the 44 BC eruption of Mount Etna caused a dimming
12.2 Evidence for the impact of volcanic
of the Sun, which was blamed for crop failure and
eruptions on climate
famine in Rome. In 1783 the American scientist/
diplomat Benjamin Franklin described a “dry fog”
which spread across much of Europe and reduced Investigating climate change is notoriously difficult,
the amount of sunlight reaching the ground. He as is evident from the current debate about whether
linked this dry fog and reduced sunlight to the very human activity is causing global warming. Studies
severe winter which occurred in 1783–4 and sug- aimed at investigating links between volcanic ac-
gested that the 1783 Laki eruption in Iceland was tivity and climate change have similar difficulties
responsible. Similarly the year 1816 is known as to overcome. Until recently, such studies relied on
the “Year Without a Summer”. The unusually cold comparing records of volcanic activity with exis-
weather of 1816 followed the April 1815 eruption ting climate records. Since the late 1970s it has

of Tambora, a volcano on the island of Sumbawa become possible to investigate the climate effects
in Indonesia. Accounts from 1816 talk about a “dry of volcanic eruptions directly using satellite mon-
fog” or haze in the atmosphere which dimmed the itoring. Detailed study of two eruptions – the 1982
Sun, just as Franklin described in 1783. As will be El Chichón eruption in Mexico and the 1991 Mount
seen, this is a feature of the injection of ash and gas Pinatubo eruption in the Philippines – have been
into the atmosphere during volcanic eruptions. the focus of very detailed study which has signif-
Other similar accounts exist which suggest that icantly improved our understanding of how vol-
there is a link between volcanic eruptions and canic eruptions affect climate. These findings will
periods of abnormal weather. be discussed later. We will start, though, by looking
During the past 100 years a number of studies at some of the evidence that scientists used to link
have been carried out to investigate the scientific volcanic eruptions to climate change before the
validity of anecdotal accounts linking volcanic activ- advent of satellite monitoring.
ity to climate change. These have shown that some A number of studies have used records of known
volcanic eruptions do indeed cause a short-term volcanic eruptions which have been classified in
change in climate, usually lasting ∼2–3 years after such a way as to allow the most “significant” erup-
the eruption. This chapter looks at some of the evi- tions to be highlighted. The volcanic records are
dence which has been used to arrive at this conclu- then compared with climate records to see if the
sion. We will look at how volcanic eruptions affect “significant” eruptions coincide with periods of