|Chemical coupling with the atmosphere
Of the greenhouse gases, carbon dioxide is perhaps the most important because of its links to human activities. Since the Industrial Revolution, atmospheric carbon dioxide has risen by 30 percent, while average global temperatures have climbed about 0.5°C. On average, carbon dioxide resides in the atmosphere about 100 years before it settles into the ocean, or is taken out of the atmosphere by plants. The oceanic removal of carbon dioxide from the atmosphere has a cooling affect on global temperatures.
Over geological time, most of the world's carbon (more than 90 percent) has settled into the ocean. There are many physical and biological processes that result in chemical exchanges between the ocean and the atmosphere, and between the upper ocean and the deep ocean. Carbonate chemistry regulates much of the transfer of carbon dioxide from air to sea; but biological processes, such as photosynthesis which turns carbon dioxide into organic material, also play an important role. Over time, organic carbon settles into the deep ocean—a process referred to as the "biological pump." The upper ocean has lower concentrations of total carbon than the deep ocean as a result of this pump. But if the ocean were completely mixed from top to bottom, as could happen if its "thermohaline" (heat and salt) circulation system was disrupted, much of this carbon could be churned up toward the surface. The ocean could become a source, rather than a sink, of carbon dioxide—a phenomenon that would have a catastrophic impact on global temperatures.
Worldwide, winds transport about 1010 kilograms of dust on any given day—this is roughly equivalent to the mass of three supertanker ships. Windblown dust from soils and desert sands are rich in iron that, when it settles into the ocean, serves as "fertilizer" for phytoplankton. Global climate models suggest that as temperatures rise, the interiors of continents will become hotter and drier, which would result in greater amounts of dust being blown out to sea. In turn, more iron dust settling in the ocean would greatly enhance phytoplankton productivity, thus slowing the rate of carbon dioxide increase in the atmosphere. In summary, in continental dust, nature may have a negative feedback mechanism that it uses to delay or even reverse global warming trends.
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