Environmental Engineering Reference
In-Depth Information
(a)
(b)
Figure 16.3 (a) Additional load (in %) of a future average CAVA eruption to the
projected anthropogenic halogen loading (Daniel
et al
.,
2011
). (b) Scenarios (1%,
10% and 25%) for stratospheric loading of future EESC (in ppb), total Cl (Cl
y
,in
ppb), total Br (Br
y
, in ppt) applying anthropogenic halogen burden and average CAVA
eruptions during 2015, 2050 and 2100. A black and white version of this
figure will
appear in some formats.
For the colour version, please refer to the plate section
.
from anthropogenic emissions in 2015. Towards the end of the twenty-
rst century
the potential role of volcanic Br increases compared to volcanic Cl resulting in a
total (anthropogenic and volcanic) EESC abundance of 1 to 8 ppb in 2100 com-
pared to 2 to 10 ppb in 2015.
If we want to evaluate the impact of large explosive subduction-zone eruptions on
the stratosphere, we need realistic estimates of their global eruption frequency.
A possible approach is to compare the two probably best studied subaerial arcs
worldwide, the CAVA and Japan, and scale the results to the global situation.
For the long-term eruption frequency, we selected the last 191 000 years of the CAVA
history, which covers 92 eruptions (Metzner
et al
.,
2012
) probably larger than M
6,
leading to an eruption frequency of one every 2100 years on average for this region.
The largest uncertainty for the global extrapolation is probably the poor coverage of
absolute dates for large eruptions. We thus use the comparatively well-studied
geological record of Japan, which covers about 10% of the subaerial arc volcanoes
worldwide (Mori
et al
.,
2013
), and is distributed along about 2100 km of arc length. If
we assume that the time-averaged volcanic production rates (Völker
et al
.,
2011
,
2014) and the amounts of large explosive eruptions per unit arc length are comparable
to CAVA, the CAVA arc length of 1200 kmwould account for about 6%of the global
arc large explosive eruptions. Taking these relations as representative, we get a global
eruption frequency of large explosive eruptions of 2100 years
¼
126 years.
This is consistent with the study of Deligne
et al
.(
2010
), which assumes a magnitude
6.5 to 7 eruption every 100 to 200 years. If we further follow Lallemand
et al
.(
2005
)
and note that about half of the global subduction zone length (about 28 000 km of
55 000 km) occurs within the tropical belt (30
Nto30
S), we can assume that 50%of
(6/100)
¼
