Throughout the 1970s scientists like Mario Molina, Sherwood Rowland, and others campaigned for the elimination of chlorofluorocarbons. After an initial outcry of popular support, CFCs were banned in aerosol cans, but their use in air conditioners, refrigerators, and in making plastic foams continued. No one seemed to pay much attention to the threat of ozone depletion. To be sure, Molina's theory was still in dispute. He predicted ozone levels would decrease due to CFC use, but no one had seen it happen yet. Companies that made and sold CFCs weren't ready to give up their livelihood without conclusive evidence.

Like a slap on the head from Mother Earth, that evidence seemed drop out of nowhere in 1983. A British scientist named Joe Farman had spent years working from a research base at Halley Bay, Antarctica. Using a Dobson spectrometer he noticed that for several years, the ozone levels above Antarctica had been dropping dramatically during the spring months (September through December for the Southern Hemisphere). This drop in Antarctic ozone levels became known as "the ozone hole."

Even Molina and Rowland were stunned by this news. They had predicted losses of perhaps 7% of the earth's ozone layer after many years. But right in the very present losses of as much as 50% of the earth's ozone layer were occurring. No one had predicted this.

The Hunt for Purple October

The world was stunned by this announcement from a relatively unknown scientist. Soon NASA scientists revealed that their satellites had been showing the ozone hole each spring, but the drops in ozone were so dramatic that they had assumed their satellite was malfunctioning. This hole was observed to be largest during October, and the false color image that showed the smallest ozone concentration was assigned a purple color. The annual enlargement of the ozone hole came to be known as "purple October." It took a while before scientists became convinced that the growing ozone hole was not a cyclic event. The hole really was getting larger.

The following three images show the decrease in ozone levels in the upper atmosphere over Antarctica between October 1979 and October 1993, measured in Dobson units. Higher ozone levels are shown by the color red, and lower levels by the color purple. Click here to see images for every October between 1979 and 1996.

The Smoking Gun

The world was stunned, yes, but convinced, no. A hole was there. But were CFCs to blame? Other theories were out there. One blamed the hole on normal wind patterns in the atmosphere. Another blamed it on solar flares. In her book Ozone Crisis, Sharon Roan writes that scientists had held that in order to prove Molina's theory, three things had to be observed. First, CFCs had to be actually observed in the stratosphere. Second, the breakdown of CFCs to produce free chlorine atoms in the presence of UV radiation in the stratosphere had to be documented. Third, evidence had to be found that chlorine (released during CFC breakdown) was really catalyzing ozone depletion.

CFCs had been observed in the stratosphere in 1975, and it seemed to be breaking down above at high altitudes where UV radiation was most intense. Two years later, chlorine atoms (Cl) were observed in the stratosphere using high-flying balloons to collect air samples high above the earth. The first two peices of evidence had been observed. To show that chlorine was breaking down ozone, many scientists now felt that the real smoking gun would be the presence of ClO, the chlorine oxide radical, in the stratosphere at the same time and place as ozone loss was detected. This reactive molecule is an intermediate in the ozone-destruction cycle that Molina had proposed, and its presence would make it hard to deny that CFCs were to blame for ozone loss.

There was only one way to get to the bottom of things, and that was to go to the bottom of the world.

Next: Meet Susan Solomon

Livng at the Mercy of the Sun — news story about the high risk of skin cancer in a Chilean town at the edge of the Antarctic ozone hole, from the BBC.

Nobel Poster in Chemistry 1995 — an online presentation, adapted from the Nobel Foundation's poster dealing with the prize-winning science of Paul Crutzen, Mario Molina and F. Sherwood Rowland, from the Nobel e-Museum.

Ozone level graphic: Courtesy NASA.

Reference

Roan, Sharon. Ozone Crisis: The 15-Year Evolution of a Sudden Global Emergency. New York: Wiley, 1989.