One of the most certain outcomes from increasing CO2 concentrations in the atmosphere is the acidification of the world's oceans. Roughly one-quarter of the CO2 currently released by human activities is absorbed in the sea. While some of the CO2 is taken up by marine organisms, most if it combines with water to form carbonic acid. The result has been a roughly 30 percent increase in ocean acidity since preindustrial times. If CO2 emissions continue to increase at present rates, ocean acidification could intensify by three to four times this amount by the end of this century. In addition, ocean acidification may reduce the ability of the ocean to take up CO2; this represents a positive feedback on global warming because it would lead to faster CO2 accumulation in the atmosphere.
Although the acidification of the sea is highly certain, the response of ocean ecosystems to changing ocean chemistry is highly uncertain. Acidification can disrupt many biological processes, including the rates at which marine animals can form shells. Coral reefs are particularly sensitive. If atmospheric CO2 levels reach twice their preindustrial values, the resulting increase in acidity could mean there will be few places in the ocean that can sustain coral growth. Polar seas could also experience major changes, since many of the species at the base of the food web may be disrupted. Hence, ocean acidification poses a major threat to ocean ecosystems, but the details are only beginning to be understood. A separate report, Ocean Acidification: A National Strategy to Meet the Challenges of a Changing Ocean (NRC, 2010f), examines ocean acidification and its potential impacts in further detail.
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