Geology Reference
In-Depth Information
water masses in polar latitudes only partly accelerates the overall
process.
4.2.2.4 Dimethylsulfide and Climatic Implications. The Gaia hypoth-
esis of Lovelock
15
states that the biosphere regulates the global envi-
ronment for self-interest. This pre-supposes that controls, perhaps
poorly understood or unknown, serve to maintain the present status
quo. Charlson et al.
16
have made use of this hypothesis to suggest that
biogenic production of DMS and the consequent formation of atmos-
pheric cloud condensation nuclei (CCN, i.e., small particles onto which
water can condense) acts as a feedback mechanism to counteract the
global warming resulting from elevated greenhouse gas concentrations
in the atmosphere. The cycle is illustrated in Figure 10. Global warming,
with concurrent warming of the ocean surface, leads to enhanced
phytoplankton productivity. This promotes the production and evasion
to the atmosphere of DMS. The DMS undergoes oxidation to form
CCN that promote cloud formation and increase the planetary albedo
(i.e., reflectivity with respect to sunlight) thereby causing a cooling
effect. From a biogeochemical perspective, the two key features are
the controls on the biogenic production of DMS and the formation of
CCN following aerial oxidation of DMS. These will be considered below
in more detail. With respect to the physics, the most important aspects
of the proposed climate control mechanism are that the enhancement of
the albedo is due to an increase in the number and type of CCN, and that
this CCN production occurs in the marine boundary layer. The albedo
of calm seawater is very low (
B
2%) in comparison to vegetated regions
(10-25%), deserts (
90%).
That biological processes within the oceans act as a major source of
reduced sulfur gases is well established,
17
and of particular importance is
the generation of DMS. Surface concentrations, approximately in the
range of 0.7-17.8 nmol L
1
, exhibit large temporal and geographic
variations. Oceanic distributions indicate that DMS is produced within
the photic zone, that is consistent with a phytoplankton source, but
DMS concentrations are poorly correlated with normal indicators of
primary productivity. While Phaeocystis and Coccolithoporidae have
been identified as important DMS producers, there is uncertainty as to
the full potential for biological DMS formation. With respect to climate
modification, questions remain as to the biological response to global
warming. For the model of Charlson et al.
16
to hold, organisms might
either increase DMS formation or biological succession could change in
such a way as to favour DMS producers. Thus, marine biogenic source
B
35%), and snow-covered surfaces (
B
