the ocean and climate change
  Chemical coupling with the atmosphere

Chemical coupling with the atmosphere
The second way that ocean and atmosphere are linked is chemical, as the ocean is both a source and sink of greenhouse gases. Much of the heat that escapes the ocean is in the form of evaporated water, the most significant greenhouse gas by far. Yet, water vapor also contributes to the formation of clouds, which shade the surface and have a net cooling effect. In the long run, scientists don't know which of these oceanic influences (shading from increased cloud formation or heat retention from higher levels of water vapor) will exert the larger influence on global temperatures.

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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The ocean and atmosphere continually exchange particles and gases in a kind of ongoing "dialogue" that influences regional and global climate. As incoming sunlight warms the ocean's surface, water evaporates into a gas that rises up into the atmosphere. Wind blowing across the face of the ocean produces a fine mist of sea spray in the lower atmosphere. As the ocean spray evaporates in the sun's warmth, it leaves behind its salt content as tiny "aerosol" particles suspended in the air. Later, when the surrounding air cools, or when the surrounding air can hold no more evaporated water, condensation occurs and water droplets begin to aggregate onto aerosol particles, forming clouds. Windblown dust from deserts, dry landscapes and pollution sources often settles into the ocean. This dust is rich in iron and other nutrients that, much like fertilizer, enhance phytoplankton growth. As phytoplankton photosynthesize, they "inhale" carbon dioxide from the atmosphere. When phytoplankton die, their microscopic bodies settle to the bottom. Over geologic time, vast amounts of carbon have been "pumped" into ocean's depths. But cold, upwelling currents carry some of this carbon back up toward the surface where it may again be used as a source of nutrition for phytoplankton, or even escape back into the atmosphere again in the form of carbon dioxide. (Illustration by David Herring)

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