As it turned out, the ozone problem was far worse than Rowland and Molina could have imagined. The first warning signs of a bigger crisis did not appear until the late 1970s, but the studies that uncovered these findings had their roots in research dating back nearly a century.

In the 1880s, W.N. Hartley discovered that a broad band of ultraviolet light reaches Earth almost unimpeded. This band, known as UV-A, has wavelengths just slightly shorter than ordinary visible light. The ozone layer partly absorbs another ultraviolet band, known as UV-B, before it can reach Earth. During the 1920s, G.M.B. Dobson managed to measure the ratio of UV-A to UV-B in incoming sunlight. By doing so, he determined for the first time the total amount of ozone in the atmosphere.

Dobson had hoped his study would lead to a new method of predicting the weather. Instead, he became interested in the seasonal variations in ozone concentrations. An instrument that he developed, the Dobson spectrometer, has become the standard for monitoring ozone from the ground.

When chlorine and ozone react, they form the free radical chlorine oxide, which in turn becomes part of a chain reaction. As a result of that chain reaction, a single chlorine atom can remove as many as 100,000 molecules of ozone.

The rapid development of new scientific tools after World War II—many of them based on wartime instrumentation—led to a flowering of studies in earth science. In 1957–1958, this led to a worldwide scientific effort known as the International Geophysical Year (IGY). IGY sparked an international outpouring of research on the oceans, the atmosphere, and unexplored land areas of the planet.

Monitoring ozone levels in the south polar region, researchers found them to be consistently about 35 percent higher in late spring than in winter. Annual monitoring showed the same seasonal pattern through the late 1970s.

But in 1978 and 1979, the British scientists found something different. In October, the beginning of spring in the southern hemisphere, the researchers detected less ozone than had been detected during the past 20 years. During the next several years, October ozone levels continued to decline.

In 1984, when the British first reported their disturbing findings, October ozone levels were about 35 percent lower than the average for the 1960s. The U.S. satellite Nimbus-7 quickly confirmed the results, and the term Antarctic ozone hole entered popular language.

By the mid 1980s, scientists had become expert in measuring the concentration of chlorine-containing compounds in the stratosphere. Some monitored the compounds from the ground; others used balloons or aircraft. In 1986 and 1987, these scientists, including Susan Solomon and James Anderson, established that the unprecedented ozone loss over Antarctica involved atomic chlorine and chlorine oxide radicals.

At the same time, measurements in the lower atmosphere established that CFC levels had increased steadily and dramatically since the first recordings taken by Lovelock in 1970. The conclusion was clear: The prime sources of the ozone-devouring chlorine atoms over Antarctica were the CFCs and two other pollutants, the industrial solvents carbon tetrachloride and methylchloroform.

A satellite operated by the National Aeronautics and Space Administration appears to have removed any possible doubt about the role of CFCs. Data collected over the past three years by the Upper Atmosphere Research Satellite revealed these compounds in the stratosphere. Moreover, the satellite has traced the worldwide accumulation of stratospheric fluorine gases, a direct breakdown product of CFCs. The quantitative balance of CFCs and its products eliminates the possibility that chlorine from volcanic eruptions or other natural sources created the ozone hole.