the Atlantic ocean is the second largest of the world's oceans, covering 31.7 million sq. mi. (82 million sq. km.) or a fifth of the Earth's surface. The Atlantic's role in global climate is well-studied, and although it is not entirely understood, it is profound. Understanding how the ocean both responds to, and affects, climate change is a challenge, given the Atlantic's expanse, mass, and both short and long term variability in its physical and chemical characteristics. Oceans, including the Atlantic, absorb most of the carbon dioxide emitted by human activity, and, thus, are extremely important for their role in mitigating the effects of increasing greenhouse gases in the atmosphere.
The immediate effects on the Atlantic of increased atmospheric greenhouse gases, most importantly carbon dioxide, include changes in the flux, or movement, of carbon dioxide within water and between the ocean surface and the atmosphere, acidification of the ocean, changes in surface temperature because of exposure to a warming atmosphere, and changes in the freshwater input to the ocean resulting from melting ice and anomalous precipitation both on continents and over the ocean. Longer-term effects include rising sea level, changes in weather patterns, and, potentially, changes in the climate of the ocean, neighboring continents, and the planet at large.
carbon dioxide flux
Presently, the net movement of carbon dioxide into the Atlantic and other oceans is from atmosphere to water. The oceans are an important sink, or storage reservoir, for carbon dioxide, holding as much as 60 times the amount of carbon dioxide as the atmosphere. More than three quarters of anthropogenic (human-sourced) carbon will eventually be stored in the oceans as carbon dioxide or as various carbon-containing ions and compounds. In the Northern Hemisphere, the North Atlantic is believed to be the largest reservoir for carbon. The effectiveness of the Atlantic Ocean as a carbon sink varies naturally over time.
Part of this variability is related to a phenomenon known as the North Atlantic Oscillation (NAO). The NAO refers to periodic movement of atmospheric mass and subsequent changes in the atmospheric pressure difference between two semi-permanent pressure systems that form over the Atlantic: the Icelandic Low and the Azores (or Bermuda) High. Changes in the NAO induce changes in sea surface temperature and mixing of layers of water. A strong NAO index (a relatively large difference in pressure between the two systems), tends to create warmer water and shallower winter mixing. Carbon dioxide is more soluble in water at cooler temperatures and under pressure. Thus, the carbon dioxide concentration of ocean water increases with depth. Deeper convection, or mixing of surface waters, increases the ocean's efficiency as a sink for carbon dioxide.
Mixing also affects the nutrients available for biological activity, which has a seasonal effect on the carbon flux in the oceans. Carbon dioxide is used at the ocean surface by photosynthesizing organisms, and it is incorporated into shells of organisms in the form of carbonate, most importantly, calcium carbonate. A small portion (but still a large mass) of these organisms sinks to the ocean bottom, where much of the planet's carbon is stored. At the surface, the amount of carbon dioxide is generally at equilibrium with that in the atmosphere. Thus, as atmospheric carbon dioxide increases, so does the concentration in the surface water of the ocean. Once the ocean absorbs carbon dioxide, it combines with water to form carbonic acid and a series of acid-base reaction products, thereby lowering pH. It has been calculated that ocean pH has decreased by 0.1 units since the Industrial Revolution. This decrease in pH is expected to continue as the concentration of atmospheric carbon dioxide rises.
The effect of decreasing pH on the ocean ecosystem is being investigated, and some researchers warn that it could be a serious problem. Increased carbon dioxide may favor certain organisms and harm others. Organisms that photosynthesize need carbon dioxide, but the acidification caused by added carbon dioxide can have deleterious effects. For example, corals take calcium carbonate from water to build skeletons, and many planktonic organisms, even ones that photosynthesize, use it to build shells. The solubility of calcium carbonate increases as pH decreases. Higher solubility of carbonates impedes the building of carbonate structures. This could affect the survival and population dynamics of these organisms. Depressed populations of these organisms could have an amplifying effect on global
warming. Coccolithophores, for example, are abundant, shell-building planktonic photosynthesizers. Some of their carbon-containing shells end up in sediments. They also contribute to ocean lightening and cloud formation, both of which increase the amount of light reflected off the planet.
In addition to biological mediation of ocean carbon, the flux of carbon dioxide between the oceans and the atmosphere is affected by physical processes such as winds and circulation, and by complex chemical interactions. Layering, or stratification of oceans, affects the mixing and movements of water masses, including deep water, which differs substantially in chemistry from upper waters. For long-term storage, carbon dioxide mixed into surface water must be transported to deeper layers. Stratification in the Atlantic Ocean varies spatially by region and depth, and temporally, over short-term and long-term time scales. Near surface stratification is affected by salinity and temperature.
Stratification can be a significant barrier to mixing, and, thus, to the transport of chemicals and heat downward, and the resupply of nutrients upward. Decreases in these fluxes can negatively affect the ability of the ocean to take up carbon dioxide. Winds also influence the absorption and retention of carbon dioxide in the ocean via their effect on the ventilation of surface water. Winds, at least over some parts of the Atlantic, appear to be decreasing as a result of global warming. This lowers the rate at which carbon dioxide is taken out of the atmosphere by the oceans, thus raising the point at which atmospheric carbon dioxide will stabilize. In the southern oceans, however, winds have been speeding up, perhaps in response to warming or some other atmospheric change, such as ozone loss. The result is enhanced upwelling and ventilation of deeper, carbon-rich water.
The ability of ocean water to absorb carbon dioxide also depends upon its buffering capacity, which is determined by the carbonate and other ion concentrations in the water. The warm, tropical portion of the Atlantic, for example, has a very high buffering capacity, while cooler water is less efficient at absorbing carbon dioxide. Dissolution of carbon dioxide in water frees up hydrogen ions. These ions react with carbonate ions to form bicarbonate ions. This process serves as a buffer against change in pH; however, the buffering capacity decreases as carbonate ions are converted to bicarbonate. Thus, although the oceans will eventually take up some three quarters of human-produced atmospheric carbon, the efficiency of this process will decrease as carbon dioxide concentrations in the water increase. Further, the solubility of gases in water decreases as temperature increases.
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