Evidence for Ozone Depletion

In 1974, after millions of tons of CFCs had been manufactured and sold, chemists F. Sherwood Rowland and Mario Molina of the University of California began to wonder where all these CFCs ended up. Rowland and Molina theorised that ultraviolet (UV) rays from the Sun would break up CFCs in the stratosphere, and that the free chlorine atoms would then enter into a chain reaction, destroying ozone. Many people, however, remained unconvinced of the danger until the mid-1980s, when a severe springtime depletion of ozone was first monitored by the British Antarctic Survey above Antarctica. The depletion above the South Pole was so severe that the British geophysicist, Joe Farman, who first measured it assumed his spectrophotometer must be broken and sent the device back to England to be repaired. Once the depletion was verified, it came to be known throughout the world through a series of NASA satellite photos as the Antarctic Ozone Hole.

Laboratory studies backed by satellite and ground-based measurements, show that free chlorine reacts very rapidly with ozone. They also show that the chlorine oxide formed in that reaction undergoes further processes that regenerate the original chlorine, allowing the sequence to be repeated up to 100,000 times. This process is known as a "chain reaction". Similar reactions also take place between bromine and ozone. Observations of the Antarctic ozone hole have given a convincing and unmistakable demonstration of these processes.

Based on data collected since the 1950s, scientists have determined that ozone levels were relatively stable until the late 1970s. Severe depletion over the Antarctic has been occurring since 1979 and a general downturn in global ozone levels has been observed since the early 1980s. Global ozone levels declined by about 3% between the late 1970s and early 1990s. This rate of decline is about three times faster than that recorded in the early 1970s. In addition to Antarctica, ozone depletion now affects almost all of North America, Europe, Russia, Australia, New Zealand, and a sizeable part of South America. Short-term losses of ozone can be much greater than the long-term average. In Canada, ozone depletion is usually greatest in the late winter and early spring, associated with the development of an Arctic ozone hole.