THE OZONE HOLE

South Pole in 1989 as viewed from a satellite. This is a computer reconstruction in which the colors represent dif­ferent concentrations of ozone (03), a form of oxygen gas (O2). As you can easily see, over Antarctica there is an "ozone hole" that is about the size of the United States, within which the ozone concentration is much less than elsewhere. This ozone hole was first reported in 1985 by British environmental scientists. Reviewing the available satellite data, we now know the zone of ozone thinning appeared for the first time in 1975. The hole is not a permanent feature, but rather one that becomes evident each year for a few months at the onset of the Antarctic spring. Every September from 1975 onward, the ozone "hole" has reappeared-and each year the layer of ozone is thinner, and the hole is larger, sometimes reaching southern new Zealand, Australia, and southern South America. In 1985 the minimum ozone concentration in the hole was 30% lower than 5 years earlier.
The major cause of the ozone depletion was suggested in the early 1970s, but gen­erally accepted slowly. Chlorofluorocarbons (CFCs) are chemicals that have been manufactured in large amounts since they were invented in the 1920s, and are largely used in cooling systems, fire extinguishers, and styroforam containers. The CFCs were percolating up through the atmosphere and reducing 03 molecules to O2. Al­though other factors have also been implicated in ozone depletion, the role of these CFCs is so predominant that worldwide agreements to phase out their production by the year 2000 have been signed. Production of CFCs and other ozone-destroying chemicals is banned in the United States after 1995. Nonetheless, large amounts of CFCs that were manufactured earlier, and which are still being manufactured, are moving slowly upward through the atmosphere, so that the problem will grow worse before the ozone layer that protects all life is stabilized once again.

The thinning of the ozone layer in the stratosphere, 25 to 40 kilometers above the surface of the earth, is a matter of serious concern. This layer protects key biological molecules, especially proteins and nucleic acids, from the harmful ultraviolet rays that bombard the earth continuously in huge amounts that originate from the sun. Life on land, in fact, may have become possible only when the oxygen layer was sufficiently thick that it generated enough ozone, in chemical equilibrium with the oxygen, that the surface of the earth was sufficiently shielded from these destructive rays. This fac­tor may account for the billions of years in which all life was aquatic.
Ultraviolet radiation is a serious human health concern. Thus every 1% drop in the atmospheric ozone content is estimated to lead to a 6% increase in the incidence of skin cancers. At middle latitudes, the drop of approximately 3% that has occurred worldwide, therefore is estimated to have led to an increase of perhaps as much as 20% in skin cancers, which are one of the more lethal diseases afflicting humans. Hu­mans are relatively resistant to increased ultraviolet radiation, but other organisms, such as the highly susceptible photosynthetic plankton species that are so important to global productivity, are apparently much more highly susceptible.