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carbon cycle. Organic soil carbon estimates, rather than total soil carbon, are generally quoted. The amount of carbon in the soil is a function of historical vegetative cover and productivity, which in turn is dependent upon climatic variables.

Spillover benefits, social—Benefits from consumption or production activities that accrue to persons other than those doing the consuming or producing. Examples include the benefits of education services to those other than the students receiving them. See also, Externalities.

Spillover costs, social—Costs of consumption and production imposed on persons or economic units other than those doing the consuming or producing. See also, Externalities.

Supply—The set of quantities of a good or service per unit of time that sellers would be willing to place on the market at various alternative prices of the item, other things being equal.

Supply curve of a firm—A curve showing the quantities per unit of time a firm will place on the market at alternative price levels, other things being equal. The concept is valid for a competitive firm only, and coincides with its marginal cost curve.

Technology—The know-how and the means and methods available for combining resources to produce goods and services.

Trace gas—A minor constituent of the atmosphere. The most important trace gases contributing to the greenhouse effect are water vapor, carbon dioxide, ozone, methane, ammonia, nitric acid, nitrous oxide, ethylene, sulfur dioxide, nitric oxide, dichlorofluoromethane or Freon 12, trichlorofluoromethane or Freon 11, methyl chloride, carbon monoxide, and carbon tetrachloride.

Upwelling—The vertical motion of water in the ocean by which subsurface water of lower temperature and greater density moves toward the surface of the ocean. Upwelling occurs most commonly among the western coastlines of continents, but may occur anywhere in the ocean. Upwelling results when winds blowing nearly parallel to a continental coastline transport the light surface water away from the coast. Subsurface water of greater density and lower temperature replaces the surface water, and exerts a considerable influence on the weather of coastal regions. Carbon dioxide is transferred to the atmosphere in regions of upwelling. This is especially important in the Pacific equatorial regions, where 1 to 2 Gt C/yr may be released to the atmosphere. Upwelling also results in increased ocean productivity by transporting nutrient-rich waters to the surface layer of the ocean.

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