The Antarctic

The discovery of the Antarctic "ozone hole" by Farman et a!. [ 12] was a watershed in atmospheric science. Prior to this discovery, the idea that human activity could affect stratospheric O, was theoretical and the effect was expected to be relatively small. In the years after the discovery, it was firmly established that chemical compounds emitted by human activities at the Earth's surface could cause spectacular changes of stratospheric O,. In this chapter, we will discuss the scientific evidence linking the emission of chlorofiuorocarbons (CFCs) at the surface with winter and springtime ozone loss in the high latitudes of both hemispheres.

7.1.1 Observations

The term "Antarctic ozone hole" refers to the annual decrease of column O, at high southern latitudes in late winter and early spring. Its formation follows a similar pattern every year. Starting in mid-August, corresponding to the return of sunlight to the polar region, a region of low column O, develops, centered approximately over the South Pole. In subsequent weeks, the area and depth of the low column 0; region increases, reaching a maximum in early to mid-October. Thereafter, column O, increases, leading to a reduction in the size of the low-O, region. By the beginning of December, column O, is approximately back to normal, and the ozone hole is gone until the next year.

Figure 7.1 shows the area of the Antarctic ozone hole as a function of time. In 1980, the area of the ozone hole was small; in fact, it is generally agreed that the Antarctic ozone hole did not exist prior to the late 1970s. By the mid-1990s the ozone hole covered ~25 million km2, or about 10% of the area of the southern hemisphere. The uncertainty in the data in Figure 7.1 is quite small; year-to-year fluctuations in the area are caused by year-to-year meteorological variability.

One can also measure the "depth" of the ozone hole—how low the column O, gets within this region of low O,. Figure 7.2 shows a time series of October-average Antarctic column O,. Prior to the mid- to late 1970s, high-latitude column ozone was

0 0

Post a comment