The stratospheric ozone layer began to form soon after the onset of oxygen-producing photosynthesis, about 2.3 billion years ago (b.y.a.). It probably did not develop fully until at least 400 million years ago (m.y.a.), when green plants evolved and molecular oxygen mixing ratios began to approach their present levels. Absorption of ultraviolet (UV) radiation by ozone is responsible for the temperature inversion that defines the present day stratosphere. This absorption is critical for preventing UV radiation from reaching the surface of the Earth, where it can harm life. The anthropogenic emission of long-lived chlorine- and bromine-containing compounds into the air since the 1930s and the slow transfer of these compounds to the stratosphere has caused a nontrivial reduction in the global stratospheric ozone layer since the 1970s. In addition, during September, October, and November each year since the early 1980s, up to 70 percent of the ozone layer has been destroyed over the Antarctic. Lesser reductions have occurred over the Arctic in March, April, and May each year. Recent international cooperation has helped reduce emissions and slow further ozone loss. In this chapter, the natural stratospheric ozone layer, global ozone reduction, and Antarctic/Arctic ozone destruction and regeneration are discussed.

STRUCTURE OF THE PRESENT-DAY OZONE LAYER

About 90 percent of all ozone molecules in the atmosphere reside in the stratosphere; most of the remaining molecules reside in the troposphere.