ABSTRACT
Atmospheric carbon dioxide concentration is expected to exceed 500 ppm sometime between 2050 and 2100, a value that signifi cantly exceeds those of at least the past 420,000 years during which most extant marine organisms evolved (Hoegh-Guldberg et al. 2007). The oceans have absorbed nearly half the fossil fuel carbon dioxide emitted into the atmosphere since pre-industrial times causing a measurable reduction in seawater pH and carbonate saturation (Riebesell et al. 2007). By the year 2100, the surface water pH will drop from a pre-industrial value of about 8.2 to about 7.8 (Feely et al. 2009). Various models indicate that aragonite undersaturation will start to occur around the year 2020 in the Arctic Ocean and 2050 in the Southern Ocean; by 2095, most of the Arctic and some parts of the Bering and Chukchi Seas will be undersaturated with respect to calcite (Feely et al. 2009). As levels of dissolved CO2 in seawater rise, growth rates of calcium-secreting organisms will be reduced due to the effects of decreased ocean CO3
2-concentrations (Feely et al. 2009). If the present rate of fossil fuel combustion continues unabated until the year 2200, a proposed model indicates that the rising atmospheric CO2 levels may drive global mean temperatures to unprecedented highs, could result in a sea surface pH drop of >0.7 units with inhibition of growth of calcifying organisms, and mayl result in an expansion of hypoxic zones with adverse effects on marine ecosystems (Raven et al. 2005; Hofmann and Schellnhuber 2009).
