Geoscience Reference
In-Depth Information
key processes (see Chapter 12). The ongoing
uptake of atmospheric CO
2
results in a decrease in
the buffer capacity of ocean water. As a result, the
strength of the ocean sink for CO
2
is going to
decrease in the future, a direct positive feedback of
ocean acidii cation to atmospheric CO
2
levels and
hence the earth system. This positive feedback
may be as large as 30% in the next 100 years for a
business-as-usual scenario. The global rate of cal-
cii cation is likely to decrease in response to
increasing atmospheric CO
2
. Model studies project
a rather modest negative feedback associated with
decreasing CaCO
3
production. If the export of par-
ticulate organic and inorganic carbon is tightly
coupled through ballasting of the organic fraction
by CaCO
3
, then a decrease in the number of CaCO
3
particles will translate into a shallower reminerali-
zation depth for organic carbon, a positive feed-
back that could compensate the effects of reduced
calcii cation. At timescales of several tens of thou-
sands of years, the marine sedimentary reservoir
of carbonates will provide the ultimate buffer
against ocean acidii cation.
There is evidence that phytoplankton groups
with an inefi cient carbon acquisition pathway may
benei t from increased levels of CO
2
. However, in
order to have an impact on the large-scale carbon
cycle and feed back on atmospheric CO
2
, enhanced
photosynthesis needs to translate into a strengthen-
ing of the export of particulate organic carbon to
depth. Changes in elemental stoichiometry, such as
increasing the C:N ratio with increasing atmos-
pheric CO
2
(see Chapter 6) might enhance future
carbon export. The increased delivery of carbon to
depth and its remineralization increases oxygen
consumption and might result in a signii cant exten-
sion of oxygen minimum zones. Oxygen minimum
zones are sites of intense denitrii cation, a suboxic
metabolic pathway yielding N
2
O, a potent green-
house gas. An increase in the oceanic source of N
2
O
would correspond to a positive feedback on the
earth's radiative balance.
Potential impacts of ocean acidii cation on the
marine nitrogen cycle include enhanced nitrogen
i xation and higher rates of water column denitrii -
cation. Both combine to reduce the mean residence
time of i xed nitrogen in the ocean. However, given
that both sources and sinks could be enhanced, it is
not possible to make conclusions about the potential
generation of imbalances which are required to
cause net changes in the ocean's i xed nitrogen
inventory and ultimately changes in the biological
pump. Based on the present understanding, the
magnitude of the feedbacks to the earth system
involving nitrogen i xation and denitrii cation are
unknown. The limited number of studies which
have measured changes in the emissions of climate-
active trace gases (e.g. dimethyl sulphide, halocar-
bon gases) to the atmosphere under elevated CO
2
(see Chapter 11) have yielded contrasting results.
The effects thus remain poorly known and have not
yet been included in coupled climate-marine bio-
geochemical models.
Overall, it is likely that ocean acidii cation will
have biogeochemical consequences at the global
scale but the magnitude is largely unknown and
will remain so until the response of organisms and
communities is better constrained.
15.2.3
Policy and socio-economic aspects
15.2.3.1 There will be socio-economic consequences
As this century progresses ocean acidii cation has
the potential to affect a wide range of marine organ-
isms, food webs, habitats, and ecosystems that sup-
ply important goods and services to humankind
(see Chapter 14). Goods and services provided by
oceans include the provision of food and food prod-
ucts and their signii cant contribution to global food
security, the ocean's capacity for carbon storage and
regulation of gas and climate, and the ability to reg-
ulate nutrients on a global scale. Marine habitats
and ecosystems also provide leisure, recreation and
well-being, and coral reefs, mangroves, and salt
marshes protect our coastlines from inundation and
erosion.
The provision of i sh (including shelli sh) is the
most obvious service, currently supplying 15% of
animal protein for 3 billion people worldwide, and
a further 1 billion rely upon i sheries for their pri-
mary protein. The chemistry of CO
2
in seawater is
such that polar regions, upwelling waters, deep
oceans, and estuaries are likely to be affected i rst,
and these are areas of importance for marine-based
human food resources. Some organisms, many of
which provide food or are key trophic links, will be
