Ocean Acidification

In addition to its climate impacts, CO2 released by human activities can influence ecosystem dynamics in aquatic systems by altering water chemistry—in particular, the reaction of CO2 with water to form carbonic acid (H2CO3), which lowers (acidifies) ocean pH. Roughly one-third of all CO2 released by human activities since preindustrial times has been absorbed by the sea (Doney et al., 2009; Sabine and Feely, 2005; Sabine et al., 2004; Takahashi et al., 2006); consequently, ocean pH has decreased by approximately 0.1 units since preindustrial times. While this might not seem like a large change, it actually represents a 25 percent increase in acidity, because pH is measured on a logarithmic scale. By the end of this century, the oceans are projected to acidify by an

time (million years before present)

FIGURE 9.6 Estimates of ocean pH over the past 23 million years (white diamonds) and for contemporary times (gray diamonds). Projections are made for the future using IPCC projections of atmospheric concentrations of CO2. The projected changes in pH are extremely large and rapid, considering the relative stability of oceanic pH in the past. SOURCE: Blackford and Gilbert (2007).

time (million years before present)

FIGURE 9.6 Estimates of ocean pH over the past 23 million years (white diamonds) and for contemporary times (gray diamonds). Projections are made for the future using IPCC projections of atmospheric concentrations of CO2. The projected changes in pH are extremely large and rapid, considering the relative stability of oceanic pH in the past. SOURCE: Blackford and Gilbert (2007).

additional 0.3 to 0.4 units (Orr et al., 2005) under the highest IPCC emissions scenario (Figure 9.6).

Because pH interacts with temperature to determine saturation levels for various related chemical species, cold-water ocean areas are projected to become undersatu-rated with calcium carbonate (CaCO3)—a key building block for the shells of many marine organisms—as early as 2050 (Orr et al., 2005). A broad array of marine species produce CaCO3 skeletons during at least part of their life cycle, so ocean acidification threatens nearly all ocean ecosystems by altering calcification rates while simultaneously increasing the rate of CaCO3 dissolution (Yates and Halley, 2006). Physiological studies suggest wide variations in the ability of organisms to cope with such changes (Doney et al., 2009). Acidification is especially challenging for coral reefs, which are defined by the CaCO3 skeletons of corals. Acidification, in tandem with elevated temperatures and other human stresses, decreased calcification rates on the Great Barrier Reef by 21 percent between 1988 and 2003 (Cooper et al., 2008). Numerous controlled experiments under elevated pH now complement these field observations (e.g., Doney et al., 2009). Projections of future ocean chemistry and climate change indicate that, by the time atmospheric CO2 content doubles over its preindustrial value, there will be virtually no place left in the ocean that can sustain coral reef growth (Cao and Caldeira, 2008; Silverman et al., 2009). Ocean acidification could also have dramatic consequences for polar food webs since several prominent species at the base of the food web may be unable to form shells—including species that salmon and other iconic species depend on for survival. Overall, ocean acidification has the potential to alter marine ecosystems catastrophically, but the details and consequences of these impacts are only beginning to be understood (see also NRC, 2010f).

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