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Originally Posted by http://www.whoi.edu/oceanus/viewArticle.do?id=17726 The question for policy-makers and society is “Will the ocean continue to take up anthropogenic CO2?” Our best evidence is that it will—but less effectively because of interactions between the ocean and the evolving climate.
Several factors come into play. Global warming will inevitably cause seawater temperatures to rise. Warmer water holds less dissolved gas than colder water, so the ocean will not be able to store as much anthropogenic CO2.
A warmer climate will also melt ice and increase rainfall near the poles, adding fresh water to the ocean. Fresh water is more buoyant than saltier water and “floats” on top of it, stratifying the ocean and slowing the mixing and circulation that transports anthropogenic CO2 away from the surface and into reservoirs in the deep ocean. The net effect will be even higher atmospheric CO2 concentrations and a further acceleration of global warming.
Warmer temperatures, weaker circulation, and different stratification of the ocean will have impacts on marine life and ecosystems, which in turn could affect the ocean’s ability to store carbon. How these changes may occur is not clear at this point, however, and may vary from region to region.
A more acidic ocean
The increasing amount of carbon in the ocean will cause another problem for marine life: ocean acidification. The 3-percent increase in dissolved carbon in surface water may seem small, but it is enough to significantly alter the chemistry of seawater and threaten whole groups of marine life.
The reason involves some basic chemistry. When CO2 gas dissolves in seawater, it combines with water molecules (H2O) to form carbonic acid (H2CO3). The acid releases hydrogen ions into the water. The more hydrogen ions in a solution, the more acidic it becomes. Hydrogen ions in ocean surface waters are now 25 percent higher than in the pre-industrial era, with an additional 75-percent increase projected by 2100.
A carbon-containing mineral, calcium carbonate (CaCO3), is a vital component in the ocean, used by many marine creatures to build protective shells and hard structures. Coral reefs, for example, are the accumulation of calcium carbonate skeletons secreted by small coral polyps.
Calcium carbonate shells are also used by several groups of planktonic organisms, microscopic floating plants and animals that are critical and abundant components of the marine food web. The white chalk cliffs of Dover, for example, are made out of empty shells that sank to the bottom of the sea when these organisms died.
The problem is, acidic conditions are corrosive to already formed calcium carbonate, and they also make it harder for organisms to build such hard parts in the first place.
Consequences for marine life
Will corals and shell-forming plankton be able to adapt to a high-CO2 world? We do not know for certain, but preliminary evidence from laboratory and field experiments is not encouraging.
Higher acidity has a negative impact on almost every species examined. In some experiments, you can actually watch the shells of living organisms dissolve away with time.
Especially vulnerable are small marine snails called pteropods and deep-water corals that live in high latitudes, where colder waters have already become more acidic. These species play critical roles in their ecosystems—as food or habitat for other creatures—so the impact of ocean acidification may soon extend to other marine life, including fish and marine mammals.
If you mention “climate change” to people, it often conjures up images of heat waves, melting glaciers, hurricanes, droughts, and monsoon rains—certainly not changes in the ocean, its chemistry, and tiny plankton inhabitants. But we know that future climate change will largely depend on the chemical composition of the atmosphere and the sea—and how vulnerable they are to human perturbation. Understanding how carbon cycles through the Earth system is key to unraveling vital questions about our climate.
Some policy-makers and entrepreneurs have even proposed injecting carbon dioxide into the deep ocean to sequester it from the atmosphere. Ocean carbon-monitoring projects such as the work on the NOAA ship Ronald H. Brown contribute vital data to learn about the ocean’s changing chemistry. Other methods, including experiments that use numerical modeling to form predictions and studies on how ocean acidification affects ocean life, must inform our decisions on how tightly we may want to regulate carbon emissions. |