Environmental Engineering Reference
In-Depth Information
models. These model simulations suggested a global mean temperature response
about three times weaker (change of about
-
3.5K)thanpreviouslyestimated
using climate models that did not consider microphysical processes. Pinto
et al
.
(
1989
)andBekki(
1995
) noted that oxidant depletion of OH
could prolong
the climate response producing sulfuric acid aerosol more slowly and increasing
the lifetime of SO
2
, but Robock
et al
.(
2009
) suggested that this would be a
small effect.
A reasonable rule of thumb might be that a minimum amount of 5 Tg of SO
2
injected into the stratosphere is likely to signi
is radiative
balance on timescales of months to years; however, there are complicating
factors such as the season and latitude of an eruption as well as what temperature
change we consider signi
cantly alter Earth
'
cant. In any case, simply extrapolating the magnitude
of the climatic effects of present-day explosive volcanism to, for example,
CFB volcanism is bound to be
flawed due to the non-linear relationship
between eruption magnitude and climatic effect, as well as the differences in
eruption style.
13.5.1 Ancient large-scale CFB eruptions
Five Phanerozoic periods of CFB volcanism in particular have been the subject of
a long-standing debate about their association with major environmental changes
as evident from the proxy record of, for example, the abundances and diversity of
planktonic foraminifera. However, the mechanisms by which CFB volcanism
may have triggered these environmental changes remain enigmatic (for reviews
see Officer
et al
.,
1987
and Wignall,
2001
; see also
Chapter 11
).
Most studies on CFB volcanism suggested, by analogy to present-day volcan-
ism, a short-term cooling effect (lasting years to decades) from SO
2
and the
formation of sulfuric acid aerosol; together with a long-term warming effect
(lasting tens to thousands of years) from the seemingly
'
high
'
volcanic CO
2
emissions. However, the annual volcanic CO
2
flux during an eruptive phase
equates to about 4% of the current annual anthropogenic
ux
(
Section 13.3
).
To date, there has been only one carbon-cycle modelling study, and it found a
negligible effect of volcanic CO
2
emissions from the Deccan Traps on Late
Cretaceous temperatures (Caldeira and Rampino,
1990
). Whether volcanic CO
2
affected long-term weathering rates and/or ocean acidity could be addressed in
future using carbon-cycle models. Alternative explanations for the long-term
warming trend observed at the time of, for example, the Siberian Traps revolve
around the dissociation of gas hydrates (see also
Chapters 10
,
12
and
20
).
Release of isotopically light carbon from gas hydrates may explain the negative
