Page 10 - climate-change2013
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Impact of climate change on

                     marine and coastal ecosystems

                     Global changes in climate                      of ocean acidification is not the total amount of CO  en-
                                                                    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
                     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
                     Furthermore, increased uptake of CO emissions by
                     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
                     carbonate (CaCO ):
                     CO  + H O    H CO     HCO + H     CO  + 2H +
                       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

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