Climate Change in the Arctic - The Changing Arctic Environment: The Arctic Messenger - page 297

corresponding decrease in surface ocean pH by the end of the 21st century is in the range
of 0.06 to 0.07 for RCP2.6, 0.14 to 0.15 for RCP4.5, 0.20 to 0.21 for RCP6.0, and 0.30 to
0.32 for RCP8.5.” 18
The conspiracy worsens because the free hydrogen ions can also combine with car-
bonate (CO 3 2- ) ions in the water to form bicarbonate:
H + + CO 3 2- ↔ HCO 3 −
This has major ecological implications because it removes carbonate ions from the water,
making it more difficult for some marine organisms (especially molluscs and corals) to
form the CaCO 3 they need for their shells and skeletal structures. At present, the surface
ocean is generally saturated with respect to calcium carbonate (including aragonite and cal-
cite). This means that under present surface conditions, these minerals have no tendency
to dissolve. Therefore, marine organisms do not have to work hard to take up and retain
the calcium they need. However, the solubility of calcium carbonate increases with pres-
sure and decreases with temperature. The pressure dependence dominates over the temper-
ature dependence. As a result, the deep, cold waters of the oceans are calcium carbonate
undersaturated. The depth at which the upper-saturated waters meet the deeper undersatur-
ated waters is called the saturation horizon ,below which it becomes difficult fororganisms
to find calcium carbonate. If you look into the literature, you will also find references to
something called the CaCO 3 saturation state ( Ω ). In regions where Ω arag or Ω cal is > 1.0, the
formation of shells and skeletons is favoured. Below a value of 1.0, the dissolution of pure
aragonite and unprotected calcite shells will begin to occur. Studies have shown that be-
cause of the increased movement of anthropogenic carbon dioxide into the surface waters,
the zone occupied by the calcium carbonate saturated surface waters is growing smaller. In
some regions, the aragonite saturation horizons have moved upwards to shallower depths
by50-200metres compared totheir positions before the Industrial Revolution (particularly
in upwelling zones). By 2050, this saturated surface zone may begin to completely disap-
pear in some areas.
This overall story has some finer detail. It has been estimated that by the end of the
twenty-first century, a 60% decrease in the concentration of surface ocean calcium carbon-