Knowledge and implications of global change in the oceans for biology, ecology, and ecosystem services - Ocean Sustainability in the 21st Century - page 130

4.2.3 Ocean acidification and deoxygenation
Over the past 250 years, atmospheric carbon dioxide (CO 2 ) levels have increased by nearly
40%, from pre-industrial levels of approximately 280 parts per million volume (ppmv) to
nearly 384 ppmv in 2007 (IPCC, 2007 ; Khatiwala et al., 2009 ) . This rate of increase is
at least one order of magnitude faster than has occurred for millions of years (Doney and
Schimel, 2007 ) ,andthecurrentconcentrationishigherthanexperiencedonEarthforatleast
the past 800,000 years (Lüthi et al., 2008 ). Atmospheric CO 2 levels are predicted to con-
tinue to increase at least for the next century and probably longer, and unless emissions are
substantially reduced, may well reach levels exceeding 1000 ppmv by 2100 (Royal Society,
2005 ) . Oceans play a fundamental role in the exchange of CO 2 with the atmosphere; since
pre-industrial times, the oceans have absorbed about a half of the CO 2 emissions produced
from burning fossil fuels and cement manufacture. This demonstrates the integral role that
oceans play within the natural processes of cycling carbon on a global scale (Royal Soci-
ety, 2005 ). Carbon dioxide is a weak acid and the continued uptake of anthropogenic CO 2
triggers changes in ocean carbonate chemistry and pH, referred to as ocean acidification
(Caldeira and Wickett, 2003 ). At present, the mean pH of ocean surface waters is already
0.1 units (equal to 30%) lower compared with pre-industrial times and a decrease of 0.4
units (equal to 120%) is projected by the year 2100 in response to a business-as-usual emis-
sions pathway (Caldeira and Wickett, 2003 ) . This change in pH drives profound changes
in carbonate chemistry and is likely to affect the structure and functioning of marine eco-
systems (Fabry et al., 2008 ) . Ocean acidification may interfere with shell formation in the
broad suite of benthic calcifiers (e.g. molluscs, echinoderms, crustaceans, bryozoans, ser-
pulid polychaetes, foraminifera, sponges, and corals) and weaken existing skeletons, partic-
ularly if ocean pH falls below 7.5 (Gazeau et al., 2007 ; Kuffner et al., 2008 ; Wood et al.,
2008 ) . Ocean acidification also has the potential to alter ocean biogeochemical dynamics
for OC and nutrients via several more indirect pathways. Increased carbonate dissolution in
the water column could decrease the contribution of CaCO 3 to the ballasting of OC to the
deep-sea sediments (Passow, 2004 ) , causing more OC to remineralize in shallow waters and