Vulnerability, Impacts and Adaptation of Coastal Zones to Global Change - Vulnerability of Coastal Ecosystems and Adaptation - page 153

depth. The average rate of warming is higher close to the surface
where it has increased by 0.1 C per decade, in the first 75 m, over the

period 1971-2010 and decreases in depth [IPC 13]. Below 700 m, a
systematic warming of the ocean has already been observed up to
2,000 m in depth. The warming of the ocean is on average stronger
near the coast.

°

Extreme values of the sea surface temperature, or “maritime heat
waves”, have been recorded on the coasts of West Australia with
temperatures of 5

C above average [PEA 13]. This warming of the
upper layers of the ocean currently pushes marine and coastal species
nearer to the poles at an average estimated speed of 72 km per decade
[POL 13]. This rate of migration is considerably larger than that of
land species, which migrate toward the poles at an average speed of
six kilometers per decade. It has also been shown that on average the
seasonal cycle of marine life begins four days earlier and that the algal
blooms occur six days earlier than in the past, whereas fish larvae
appear eleven days early [POL 13].

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The warming of the ocean also affects the coral reefs and increases
the frequency of algal blooms that are, very often, intensified by the
supplies of nutrients of anthropogenic origin; these blooms can upset
the equilibrium of the food chain at a local level and can lead to the
prohibition to harvest or sell some seafood species.

Finally, let us remember that the increase in the temperature of the
upper layers of the ocean contributes to the water stratification and
modifies the dynamics of the ocean currents and of the ice in the polar
regions.

4.3.4.4. Ocean acidification
With the increase in the atmospheric concentration of CO 2 , the flux
of absorption into the ocean becomes larger than the inverse flux of
emission into the atmosphere. The dissolution of some of the excess
CO 2 into the ocean leads to its acidification [GAT 13]. The pH at the
ocean surface, which had the value of 8.25, has decreased by around
0.1 since the industrial revolution and is currently 8.14, which
corresponds to an increase in concentration of the ion H + by 26%.