Atmosphere---Ocean Equilibrium

The natural transfer of carbon from surface water to the deep ocean is a slow process in most parts of the world. This factor, rather than the equilibrium between the atmosphere and the surface water, determines the rate at which carbon dioxide is taken up. It is thought that the mixing of surface and deep water can take decades, or even centuries. There are two mechanisms that pump carbon dioxide from the atmosphere to deep waters, as follows. 

Mathematical models of the ocean carbon cycle indicate that, without either of these two pumps operating, the concentration of carbon dioxide in the atmosphere would be twice as high as it is today. 

In summary, the oceans and seas have enormous potential to take up all of the carbon dioxide that will result from the future global use of fossil fuels, but the kinetics of the natural processes are slow. The practical problem is how to transfer captured carbon dioxide (or gas in the atmosphere) to the marine environment in a fashion that is not only affordable, but also acceptable to marine biologists and environmentalists generally. Clearly, much more research must be conducted in order to allay the fears of the ecological lobby. 

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Hence, the radiative equilibrium temperature is sensitive to changes in the solar constant, planetary albedo, and the radiative properties of the earth-atmosphere-ocean system. In addition, changes internal to the earth-atmosphere-ocean system may alter the climate. Table I is an incomplete list of phenomena that individually or in concert could alter climate. 

Numerous long-term observations in various latitudinal belts show a high level of correlation between temperature and C02 content. The atmosphere-ocean interaction contributes most to this dependence. Though the atmosphere and the ocean are in equilibrium with respect to C02 exchange, this equilibrium is still regularly violated. The most serious causes of this violation are... 

The connection between the equilibrium condition of C02 exchange and pH on the atmosphere-ocean border is such that when the C02 pressure in the atmosphere... 

The data given indicate that at no stage in geologic history did the partial pressure of carbon dioxide gas in the atmosphere in equilibrium with the ocean fall below 10 bar. 

Our environment has many examples of chemical equilibria. Even when systems are not in equilibrium, they are continuously changing in an effort to reach equilibrium. Equilibria control the composition of Earth s atmosphere, oceans, and lakes, as well as the composition of our body fluids. Even soot provides an example of an equilibrium. 

Anthropogenic contamination reaches the ocean bottom in the northern part of the North Atlantic because of the deep convection of the North Atlantic Deep Water. Further south in the basin the contamination reaches to depths of 2000 m. The depth at which the profiles reach half their surface maximum is between 600 and 1000 m. Note that this depth is not greatly different from the value of 800 m estimated in Fig. 11.6 for the depth of ocean equilibrium required to accommodate about half of the fossil fuel released to the atmosphere. It has been shown with global circulation models that a present-day flux of 2.2 Pg y into the ocean is required to accommodate the inventory of GO2 indicated in Fig. 11.7 (Table 11.3). 

Thus, the mean temperature of the atmosphere, which is about 20°C at sea level, falls steadily to about —55° at an altitude of 10 km and then rises to almost 0°C at 50 km before dropping steadily again to about —90° at 90 km. Concern was expressed in 1974 that interaction of ozone with man-made chlorofluorocarbons would deplete the equilibrium concentration of ozone with potentially disastrous consequences, and this was dramatically confirmed by the discovery of a seasonally recurring ozone hole above Antarctica in 1985. A less prominent ozone hole was subsequently detected above the Arctic Ocean. The detailed physical and chemical conditions required to generate these large seasonal depletions of ozone are extremely complex but the main features have now been elucidated (see p. 848). Several accounts of various aspects of the emerging story, and of the consequent international governmental actions to... 

O2, N2, and H2O are the main molecular forms coexisting in the atmosphere, but the condition of thermod5mamic equilibrium would require that HNO3 be formed from these gases and subsequently dissolve in the oceans. 

At 25"C the ratio of equilibrium pressures is 1.0088 (from the above equation). This means that pure H2 0 has a slightly greater vapor pressure than pure H2 0. The ratio of to O in the atmosphere is equal to the ratio in the ocean times 1/1.0088. 

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

In the ocean, inert gas concentrations tend to follow the temperature solubility dependence closely. This suggests that water parcels obtain their gas signatures when they are at the seasur-face close to equilibrium with the atmosphere at ambient temperature. 

If these phases all exist at equilibrium then/= 2. Sillen argued that we should fix T and [Cl ] (Cl does not enter any of the reactions and is thus conservative). If so, the composition of the aqueous and gas phases would be fixed. The implications are far reaching because these equilibria would fix PcOj of the atmosphere, the alkalinity of the ocean and thus the pH of the ocean ... 

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