Environmental Engineering Reference
In-Depth Information
not coincide with the greatest cumulative DVI.
Thus, although increased volcanic activity and
the associated dust veils can be linked to
deterioration between 1430 and 1850, it is likely
that volcanic dust was only one of a number of
factors which contributed to the development of
the Little Ice Age at that time.
Major volcanic episodes in modern times have
usually been accompanied by prognostications
on their impact on weather and climate, although
it is not always possible to establish the existence
of any cause and effect relationship. The Agung
eruption produced the second largest DVI this
century, but its impact on temperatures was less
than expected, perhaps because the dust fell out
of the atmosphere quite rapidly (Lamb 1970). It
is estimated that it depressed the mean
temperature of the northern hemisphere by a few
tenths of a degree Celcius for a year or two
(Burroughs 1981), but such a value is well within
the normal range of annual temperature
variation. The spectacular eruption of Mt St
Helens in 1980—enhanced in the popular
imagination by intense media coverage—
promoted the expectation that it would have a
significant effect on climate, and it was blamed
for the poor summer of 1980 in Britain. In
comparison to other major eruptions in the past,
however, Mt St Helens was relatively insignificant
in climatological terms. It may have produced a
cooling of a few hundredths of a degree Celcius
in the northern hemisphere, where its effects
would have been greatest (Burroughs 1981). The
eruption of El Chichón in Mexico in 1982
produced the densest aerosol cloud since
Krakatoa, nearly a century earlier. Within a year
it had caused global temperatures to decline by
at least 0.2°C and perhaps as much as 0.5°C
(Rampino and Self 1984). However, the cooling
produced by El Chichón may have been offset
by as much as 0.2°C as the result of an El Niño
event which closely followed the eruption, and
effectively prevented cooling in the southern
hemisphere (Dutton and Christy 1992) (see
Figure 5.5). Past experience suggested that the
eruption of Mount Pinatubo would also lead to
lower temperatures. It was blamed for the cool
summer of 1992 in eastern North America, and
by September of that year it was linked with
reductions in global and northern hemisphere
temperatures of 0.5°C and 0.7°C respectively
(Dutton and Christy 1992). Estimates by
modellers studying world climatic change
indicate that such cooling would be sufficient to
reverse—at least temporarily—the global
warming trends characteristic of the 1980s
(Hansen
et al.
1992).
Although volcanic activity is most commonly
associated with cooling, there is some evidence
that it may also cause short-term, local warming.
Figure 5.5
Monthly mean global temperature anomalies obtained using microwave sounding units (MSU).
The events marked are: A—La Niña; O—El Niño; E—El Chichón; P—Pinatubo
Source:
After Dutton and Christy (1992)
