The stratospheric ozone layer began to form soon after the Great Oxygenation Event, whereby oxygen started to accumulate in the atmosphere due to photosynthesis about 2.45 b.y.a. The ozone layer did not develop fully until after 400 m.y.a., when green plants evolved and molecular oxygen mixing ratios began to approach their present levels. Absorption of 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, as well as the slow transfer of these compounds to the stratosphere, have caused a nontrivial reduction in the global stratospheric ozone layer since the 1970s. In addition, during September, October, and November of each year since 1979, such emissions have caused up to a 70 percent destruction of the ozone layer over the Antarctic. Lesser reductions have occurred over the Arctic in March, April, and May of each year. Recent international cooperation has helped replace emissions of most ozone-depleting gases with less destructive ones, reducing ozone loss. However, global warming, which warms the surface but cools the stratosphere, has delayed the recovery of the ozone layer. 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
Christian Schönbein discovered the ozone molecule in 1839 (Section 18.104.22.168). However, the ozone layer was not identified until more than 70 years later. In 1879, the French physicist Alfred Cornu (1841–1902) measured a sharp reduction in short wavelengths of UV light, relative to longer wavelengths, reaching the Earth's surface, suggesting that an atmospheric constituent might be absorbing the short wavelengths. In 1881, Walter N. Hartley explained the absorption of short UV wavelengths as due to ozone high in the atmosphere. Finally, in 1913, French physicists Charles Fabry (1867–1945) and Henri Buisson (1873–1944) discovered and began to quantify the thickness of the stratospheric ozone layer.