Test Design Lighthouse

The exchange of gases between the oceans and the atmosphere is a significant global climatic factor. The "greenhouse gas" carbon dioxide is particularly important in this respect. It accounts for only 0.035 percent of the atmosphere, but this figure is rising - a very important fact in terms of future climatic developments on this planet.

The special quality of this gas is that it is contained in seawater in three different forms: as solute carbon dioxide gas (CO₂, as hydrogen carbonate (HCO₃) and as carbonate (lime) (CO₃). When the CO₂ concentration in the surface water layers is the same as in the air above, the exchange process is in balance. Since however part of the CO₂ in the water is converted into HCO₃ and CO₃, seawater has a much greater storage capacity for carbon dioxide than the atmosphere.

The physical carbon pump

The colder seawater is, the more CO₂ is dissolved in it. The oceans in tropical and subtropical regions release carbon dioxide into the atmosphere, whereas large amounts of the greenhouse gas are in solution in the polar seas. The zones of deep-water formation, the Greenland Sea and the Arctic Ocean, in which the solute CO₂ is pulled into the depths along with the sinking surface water, are therefore important elements in the physical pump. The CO₂ is separated from the environment for several hundred years in these CO₂ sinks until this deep-ocean water returns to the surface somewhere in the world in an upwelling.

The biological carbon pump

The photosynthetic process in algae consumes carbon dioxide, thus removing solute CO₂ from the seawater. In parts of the ocean where massive proliferations of algae occur, consumption of the algae by zooplankton cannot keep pace with algae growth, whereupon the algae use up all the nutrients in the water such as nitrate and phosphate and, no longer able to proliferate, die and sink to the bottom. The carbon dioxide bound in their cells goes with them into the ocean depths in the form of organic carbon and is thus taken out of atmospheric circulation.

The lime counter-pump

The third pump, the "lime counter-pump," acts to counteract the other two pumps by releasing carbon dioxide into the atmosphere. This biochemical pump begins with the formation of calcareous shells by marine life forms, above all corals and planktonic lime-forming algae. The first impression is that lime formation would bind large amounts of carbon dioxide, but in fact the opposite is the case: Lime formation produces CO₂. This is due to the chemical reaction in which two HCO₃ are transformed into one molecule each of lime (CO₃), water and CO₂. Lime formation thus increases the CO₂ concentration in the oceans, which is then balanced with the atmosphere by release of carbon dioxide. The most recent calculations demonstrate that, for example, the coral reefs produce about four times as much lime as the lime-forming algae. Since the reefs are also located in warm, shallow seas, there is also the contributing factor that carbon dioxide is not readily soluble in warm water, so that the gas leaves the seawater even more rapidly.

Warming of the earth's atmosphere and the consequences

Thus the feedback processes at work between the carbon balance in the oceans and the world climate are quite complex. Global warming caused by the greenhouse effect means the capacity of the oceans to hold CO₂ in solution is reduced. The "physical pump" is further weakened due to warmer water and reduced salt content caused by melting glaciers and polar icecaps, in turn reducing deep-water formation in polar waters: The surface water becomes too warm and too light and is therefore not returned to the depths.

The global warming process also increases the stability of the water layering within the ocean. The amounts of nutrients from deeper layers being mixed into surface waters is reduced, thus curbing algae productivity and slowing the "biological pump." The higher water temperatures encourage development of destructive cyclones and reduced algae production may reduce fish catches.