Page 10 - climate-change2013

Impact of climate change on

marine and coastal ecosystems

Global changes in climate of ocean acidification is not the total amount of CO en-
2
tering the oceans but the fact that the rate of pH change
is faster than anything experienced before.
There is widespread agreement in the scientific com-
munity about the ongoing changes to the earth’s cli- Variable ocean circulation
mate (Trenberth et al., 2007). The causes of this process
are manifold: changes in land use and carbon emis- Possible changes in atmospheric circulation, the posi-
sions, alterations in both stratospheric and tropospheric tion of high and low pressure systems and other factors
ozone, aerosol emissions and other factors. Neverthe- could alter the main wind circulation patterns and af-
less, the main factor inducing global warming seems to fect the intensity and/or position of upwelling systems.
be the emission of greenhouse gases. There are many These are places where wind action brings nutrients up
of these gases — nitrous oxide, methane, CFCs, etc. — to the well-lit surface layers of the sea, where they fer-
but it is the emission of carbon dioxide (CO ) from the tilize the water and support primary production, which
2
burning of fossil fuels that is primarily responsible for the forms the base of the marine food web.
rise in the planet’s temperature.
Seawater temperature and salinity are the variables
Climate and sea warming that control the density of the oceans. The natural vari-
ations in density at different latitudes of the earth re-
From the mid-19th to the beginning of the 21st century, sult in what is known as the thermohaline circulation,
the air temperature at the earth’s surface increased by which has been described as a giant conveyor belt that
between 0.6 and 0.8°C, and this warming is expect- redistributes excess heat from equatorial and tropical
ed to accelerate during the current century if mitigat- latitudes. This mechanism is one of the main factors in-
ing measures are not put in place. The sea plays a key fluencing the earth’s climate, lowering temperatures at
role in limiting this process as more than 80% of the low latitudes and warming higher-latitude regions such
heat absorbed by the planet accumulates in the world’s as northern Europe. It has also been proposed that this
oceans (Bindoff et al., 2007). Because of the high spe- mechanism of heat regulation could change drastically
cific heat capacity of seawater, sea temperatures in- or even collapse in the future due to temperature and
crease much less than air temperatures. The thermal salinity alterations in the Polar Regions.
expansion of the oceans caused by rising temperatures
and the global increase in seawater mass caused by
the melting of continental ice are resulting in sea-level Thermohaline circulation: Ocean
rise. In parallel, changes in precipitation and evapora- circulation driven by density differences
tion rates have also been observed, altering sea salinity. caused by temperature and salinity.

CO and ocean acidification
2
Furthermore, increased uptake of CO emissions by
2
oceans is changing seawater chemistry, decreasing the
pH (increasing acidity or concentration of hydrogen ions
+
H ) and reducing the concentration of carbonate ions
(CO ) by decreasing the saturation state of calcium
2-
3
carbonate (CaCO ):
3
CO + H O H CO HCO + H CO + 2H +
-
2-
+
2 2 2 3 3 3
This consequently has the potential to significantly af-
fect shell and skeleton formation in many marine organ-
isms, including some commercial shellfish species.
Global measurements indicate that average seawater
acidity has increased by 30% since the beginning of the
industrial revolution (equal to an acidification of 0.1 pH
units), and the oceans are predicted to become pro-
gressively more acidic as they continue to absorb more
carbon dioxide (Denman et al., 2011). The major cause

Red star (Echinaster sepositus) in Posidonia oceanica seagrass bed.
Photo: J. C. Calvin, OCEANA

8 COLLECTION