Vulnerability, Impacts and Adaptation of Coastal Zones to Global Change - Vulnerability of Coastal Ecosystems and Adaptation - page 154

Ocean acidification is a direct consequence of the CO 2
anthropogenic emissions and not of the climate change resulting from
such emissions. The ocean functions as an enormous sink for CO 2
which, since the industrial revolution, has absorbed about 30% of
the total anthropogenic emissions of this gas. If this sink did not exist,
the concentration of CO 2 in the atmosphere would be much higher.
We find here another example where the ocean plays the role of
climate regulator. Contrary to other effects of climate change, ocean
acidification is immediately reversible and could be directly controlled
by limiting the CO 2 emissions into the atmosphere .
All RCP scenarios project a decrease in the ocean's surface pH for
2100, with values situated between 0.06 and 0.32; for the extreme
scenarios RCP2.6 and RCP8.5, these decreases range from 0.06 to
0.07 and from 0.30 to 0.32, respectively.
Ocean acidification results mainly from a global systemic change
but also from the presence of nitrogen and sulfur of anthropogenic
origin in coastal waters, associated with cumulative global changes.
Even if this last process has a weak impact on a global scale, of the
order of a few percent in relation to the contribution of atmospheric
CO 2 [DON 07], it is nevertheless responsible for significant temporal
and spatial variations that have harmful effects on biodiversity and
coastal and marine ecosystems.
4.3.4.5. Ocean deoxygenation
In the first 100-200 m of the ocean's water column, oxygen is
generally abundant as a result of dissolution from the atmosphere.
Moreover, photosynthesis in the phytoplankton produces large
amounts of oxygen with seasonal variations. The oxygen dissolved in
the ocean is used by organisms that consume organic matter, which
comes directly or indirectly from phytoplankton. In the intermediary
waters, the oxygen concentration decreases, but increases again
beyond 1,000-2,000 m.
Today, several coastal zones, called hypoxic zones, have such low
oxygen concentration that animal life becomes impossible, and as a
result are also called “dead zones”. The number of dead zones has