Geoscience Reference
In-Depth Information
Northern Hemisphere. A number of modelling stud-
ies on the effects of climate change (but not ocean
acidii cation) on DMS production and aerosol for-
mation have revealed large spatial heterogeneities
in both DMS concentrations and l uxes, and the
associated climatic impacts (Gabric
et al.
1998 , 2005 ;
Bopp
et al.
2003 ; Gunson
et al.
2006 ). For example,
Bopp
et al
. (2003) reported a mean decrease in global
seawater DMS concentrations of ~1% in response to
doubled atmospheric CO
2
concentrations, but the
regional impacts were much more pronounced. The
western Equatorial Pacii c and the eastern Equatorial
Atlantic saw a decrease of up to 50%, whilst the east-
ern Equatorial Pacii c saw an increase of up to 50%.
In addition, the subtropical/subantarctic conver-
gence zone experienced a 20% enrichment, with
smaller increases seen in the north Atlantic and
Pacii c. The global DMS l ux showed similar regional
variation, and resulted in heterogeneity in the cli-
matic effects. Whilst the impact on radiative forcing
was -1 W m
-2
in the Southern Ocean, it was calcu-
lated to be +0.5 W m
-2
in the tropics.
with lesser production by dinol agellates and dia-
toms (Malin
et al.
1992 ; Liss
et al.
1997 ). Ecosystem
shifts in response to global changes are likely to
affect DMS production through changing species
dominance. The differing ability of phytoplankton
species to produce these compounds further com-
plicates our capacity to make quantitative predic-
tions about the impact of ocean acidii cation on
DMS production and climate.
11.2.3.4 Atmospheric effects of oxidized sulphur
In terms of climate impacts, it is not the bulk quan-
tity of DMS oxidation that is important, but the abil-
ity of the oxidation products to enhance the CCN
number concentration (Charlson
et al.
1987 ; Andreae
and Crutzen 1997). However, the formation of CCN
from the oxidation products of DMS (SO
2
and
H
2
SO
4
) and the resulting climate sensitivity is com-
plex and non-linear, and cannot be constrained
using a simple, globally applicable model (Carslaw
et al.
2009 ).
11.2.3.5 Impact of other climate change effects on
DMS-CCN-climate feedback
The l ux of DMS to the atmosphere is controlled by
the concentration of DMS in surface seawater and
the magnitude of the transfer velocity, both of which
are inl uenced by climate variables. Seawater DMS
concentrations are controlled by marine biological
activity, which depends on solar irradiance, sea
temperature, and ocean dynamics, whilst the trans-
fer velocity is controlled by air and sea temperature
and wind velocity, and related properties (e.g.
waves and bubbles). Therefore, ocean acidii cation
will not be the only process that has an impact on
DMS production in the future oceans, and a range
of other climatic changes will also have an effect.
Increased solar irradiance and sea-surface tempera-
ture will ultimately result in lowered marine pro-
ductivity in some regions, due to enhanced
stratii cation and reduced upwelling of nutrient-
rich water. Using an atmosphere-ocean general cir-
culation model coupled to a marine biogeochemical
model, Bopp
et al.
(2003) observed such an effect in
the western Equatorial Pacii c in response to dou-
bled atmospheric CO
2
, with a resultant lowering of
particulate DMSP and DMS concentrations. By con-
trast, the modelling study by Gunson
et al.
( 2006 )
11.2.3.2 Number of CCN derived from marine DMS
Atmospheric aerosols come from a range of sources.
Thus, it is difi cult to distinguish the impact of
DMS-derived aerosols on climate from that from
other aerosols. It is also unknown how much the
marine stratus cloud currently in the atmosphere is
affected by them. Watson and Liss (1998) made
attempts to calculate the current inl uence that DMS
has on global albedo. By assuming that around half
of the CCN in typical Southern Hemisphere marine
air are due to DMS, they calculated that a doubling
of the concentrations of CCN would lead to a 6 to
46% increase in marine stratus cloud albedo. If one-
third of the globe is assumed to be covered by
marine stratus clouds, this would result in a ~2%
increase in global albedo, and a 3.8°C cooling of cli-
mate. However, the large projected range highlights
the difi culties involved in quantifying the inl u-
ence of marine DMS on aerosols and climate.
11.2.3.3 Phytoplankton species differ in their ability to
produce DMS/DMSP
Haptophytes, in particular coccolithophorids, are
considered to be the most prolii c planktonic pro-
ducers of DMSP/DMS, followed by
Phaeocystis
,
