Geoscience Reference
In-Depth Information
indirectly inl uence climate through involvement in
CCN and cloud formation, further impacting on the
rel ection of solar radiation (Andreae and Crutzen
1997). Aerosols originate from both natural and
anthropogenic sources. Anthropogenic sources
include sulphate and carbonaceous materials pro-
duced during fossil fuel combustion and biomass
burning. Such pollutants generally show localized
distribution, more concentrated in the industrialized
Northern Hemisphere (Ramanathan
et al.
2001 ).
Natural aerosols originate from both terrestrial and
marine environments, most notably non-methane
hydrocarbons such as terpenes that are emitted from
forests and DMS from the oceans. It has been esti-
mated that marine DMS contributes from 20 to 80%
of the sulphates in air over the Northern Hemisphere,
and more than 80% over most of the Southern
Hemisphere (Chin and Jacob 1996).
When DMS is emitted to the atmosphere, it under-
goes rapid oxidation in the marine boundary layer
(MBL) via two major pathways (addition and/or
abstraction), with their respective roles being depend-
ent on environmental conditions such as atmospheric
temperature, solar intensity, and cloud cover (von
Glasow and Crutzen 2004; Vogt and Liss 2008).
Through the addition pathway, DMS is initially oxi-
dized to dimethyl sulphoxide (DMSO), followed by
methane sulphinic acid (MSIA) and MSA. The
growth of smaller particles is encouraged, as all
intermediate and end products are taken up onto
pre-existing particles. The abstraction pathway
involves the transformation of DMS to SO
2
, MSA,
and H
2
SO
4
. Again, SO
2
and MSA are taken up by pre-
existing particles, whilst formation of H
2
SO
4
may
lead to the production of new particles (von Glasow
and Crutzen 2004; Vogt and Liss 2008). The extent of
climate regulation by the oxidation products of DMS
is dependent on a number of processes, but ulti-
mately relies on there being an overall increase in the
number concentration of CCN (particles ~0.05 μm in
diameter) (Charlson
et al.
1987 ; Andreae and Crutzen
1997 ; von Glasow and Crutzen 2004 ).
The CLAW hypothesis states that changes to oce-
anic DMS emissions would cause corresponding
changes to atmospheric [SO
4
2-
] and hence to the
number of particles that grow to the size of CCN
( Charlson
et al.
1987). Therefore a decrease in DMS
production in the oceans as a result of ocean acidii -
cation, as observed in the mesocosm studies dis-
cussed earlier, may ultimately lead to a reduction in
CCN and marine stratus cloud albedo, and there-
fore produce a positive feedback on climate that
would increase the warming that will occur as a
result of anthropogenic greenhouse gases (GHGs).
The l ux (
F
) of DMS to atmosphere can be
described as follows:
FAk c
=××D
(11.2)
where
A
is the total ocean surface area,
k
is the trans-
fer velocity, and Δ
c
is the concentration difference
across the air-sea interface. Due to the highly super-
saturated nature of the ocean relative to the atmos-
phere, Δ
c
can be considered to be identical to the
concentration of DMS in the surface oceans.
Therefore, assuming no changes to other parame-
ters, a signii cant decrease in seawater DMS concen-
trations, e.g. ~50% (Hopkins
et al.
2010 ), as a result
of ocean acidii cation, would be equivalent to a 50%
decrease in Δ
c
. This would lead to a proportional
decrease of the l ux of DMS to the atmosphere.
Despite these assumptions, it is likely that the cli-
mate response would be substantial if such a change
were seen over extensive ocean areas. For example,
using a coupled ocean-atmosphere general circula-
tion model, Gunson
et al
. ( 2006 ) predict a 1.6°C
increase in surface air temperature in response to a
halving of ocean DMS emissions.
11.2.3 Large uncertainties
However, despite such results, and for a variety of
reasons discussed below, it is highly problematic to
make quantitative predictions about the climatic
implications of changes in marine DMS emissions.
11.2.3.1 Effects vary regionally
The effect of the number of CCN on albedo is more
prominent at low particle numbers, resulting in a
greater climatic effect in oceanic areas away from the
inl uence of terrestrial air heavily laden with aero-
sols (Twomey 1991). As a demonstration of this,
Twomey (1991) calculates that if equal quantities of
sulphur were to enter the atmosphere in the two
hemispheres, the impact on albedo would be 25
times more pronounced in the Southern than in the
