FIGURE 8.10 Ice-core measurements of trapped atmospheric gases from Law Dome Station, Antarctica, which indicate that global carbon dioxide concentrations (in parts per million) during the past millennium have been increasing geometrically since the mid-19th century concurrent with the ongoing expansion of our civilization (Fig. II). Modified from Ethridge et al. (1998).

global changes not only in the past but for the present and future. Perhaps the most compelling example of Antarctica as an human impact indicator is the ''ozone hole'' that forms in the stratosphere (Fig. 1.1b) above Antarctica each spring. In 2000, the area of the ozone hole (where ozone concentrations are depleted by more than 50%) extended across nearly 30 million square kilometers—which is larger than the entire continent of North America.

The ozone layer in the stratosphere (Fig. 1.1a)—which is distinct from ozone in the troposphere that contributes to ground-level smog and air pollution— absorbs incoming high-energy ultraviolet radiation from the Sun. The concern is that decreased stratospheric ozone concentrations will allow more ultraviolet radiation, especially ultraviolet-B (240 to 320 nanometers; Fig. 8.7), to reach the Earth's surface and dramatically increase genetic mutations among living organisms.

Since the early 20th century, millions of tons of chlorofluorocarbons from aerosols, solvents, and various coolant systems have been introduced into the atmosphere. Over time, these anthropogenic chemicals mixed upward into the overlying stratosphere where they began persisting (Table 8.1). Equation (8.2) shows the stratospheric chemistry of ozone depletion. In the presence of sunlight, ultraviolet radiation causes the constant regeneration of ozone from diatomic oxygen molecules and individual oxygen atoms [Eqs. (8.2a-8.2c)]. With chlorofluorocar-bons in the stratosphere, however, these oxygen atoms will bind with chlorine

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