Environmental Engineering Reference
In-Depth Information
of preventing non-volcanic sources of dust from
being included in the index. The use of climatic
parameters in the calculation of certain index
values may introduce the possibility of circular
reasoning. For example, falling temperatures are
taken as an indication of an increasing DVI, yet
a high DVI may also be used to postulate or
confirm falling temperatures.
In an attempt to deal with some of these
problems a number of researchers reassessed
Lamb's index, but introduced only limited
refinement and modification (Mitchell 1970;
Robock 1981). Other indices have also been
proposed, although none has been as widely used
as the original DVI. One example is the volcanic
explosivity index (VEI), developed as a result of
research sponsored by the Smithsonian Institute
into historic eruptions (Chester 1988). It is based
only on volcanological criteria, such as the
intensity, dispersive power and destructive
potential of the eruption, as well as the volume
of material ejected. It also includes a means of
differentiating between instantaneous and
sustained eruptions (Newhall and Self 1982).
Being derived entirely from volcanological
criteria, it eliminates some of the problems of
the DVI—such as circular reasoning—for
example, but it does not differentiate between
sulphates and dust, nor does it include corrections
for the latitude or altitude of the volcanoes
(Chester 1988). Thus, although the VEI is
considered by many to be the best index of
explosive volcanism, it is not without its problems
when used as a tool in the study of climatic
change.
A glaciological volcanic index (GVI) has also
been proposed (Legrand and Delmas 1987).
Based on ice cores, it would reveal acidity levels
in glacial ice, and therefore give an indication of
the SO
2
levels associated with past volcanic
eruptions. Since neither the DVI nor the VEI deal
adequately with volcanic SO
2
emissions, the GVI
has the potential to improve knowledge of the
composition of volcanic debris. However, current
ice core availability is limited, and the GVI adds
little to the information available from
established indices (Bradley and Jones 1992).
Volcanic activity, weather and climate
Many of the major volcanic eruptions in
historical times have been followed by short-term
variations in climate which lasted only as long
as the dust veil associated with the eruption
persisted. The most celebrated event of this type
was the cooling which followed the eruption of
Tambora in 1815. It produced in 1816 'the year
without a summer', remembered in Europe and
North America for its summer snowstorms and
unseasonable frosts. Its net effect on world
temperature was a reduction of the mean annual
value by 0.7°C, but the impact in mid-latitudes
in the northern hemisphere was greater, with a
reduction of 1°C in mean annual temperature
and average summer temperatures in parts of
England some 2-3°C below normal (Lamb
1970). The eruption of Krakatoa in 1883 was
also followed by lower temperatures which made
1884 the coolest year between 1880 and the
present (Hansen and Lebedeff 1988).
Increased volcanic activity may have been a
contributing factor in the development of the
Little Ice Age—which persisted, with varying
intensity from the mid-fifteenth to the
midnineteenth century. The eruption of Tambora
falls within that time span, and other eruptions
have at least a circumstantial relationship with
climatic change. A volcanic dust veil may have
been responsible for the cool, damp summers and
the long, cold winters of the late 1690s in the
northern hemisphere, which ruined harvests and
led to famine, disease and an elevated death rate
in Iceland, Scotland and Scandinavia (Parry
1978). Lamb (1970) has suggested that dust veils
were important during the Little Ice Age because
their cumulative effects promoted an increase in
the amount of ice on the polar seas, which in
turn disturbed the general atmospheric
circulation. He also points out, however, that
contemporaneity between increased volcanic
activity and climatic deterioration was not
complete. Some of the most severe winters in
Europe—such as those in 1607-08 and 1739-
40—occurred when the DVI was low, and the
period of lowest average winter temperatures did
