In the mid-1980s, scientists began noticing that the measured
amount of ozone in Earth’s upper atmosphere had
been decreasing seasonally since the 1970s, primarily over
the polar latitudes during springtime in both the Northern
and Southern hemispheres. They referred to these depleted
regions as “ozone holes,” although they are more like depressions
than real holes in the ozone layer. Ozone depletion
appears to be caused by a seasonal buildup of atmospheric
halogens—mostly chlorine, fluorine, and bromine—such as
those found in industrial refrigerants, especially chlorofluorocarbons
(CFCs). Halogens diffuse upward into the
stratosphere, where they destroy ozone without themselves
being consumed. Such agents are called catalysts—materials
that participate in and accelerate chemical reactions but
are not themselves modified in the process. Because they
are not modified or used up, halogens may remain in Earth’s
upper atmosphere for decades or even centuries. The depletions
are greater in the Southern Hemisphere, even though
more of the chemicals originated in the north, because the
colder temperatures in the south produce a type of cloud
that provides a surface on which the ozone-destroying
chemical reactions can take place.