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Ozone depletion
page 13 of 13
Looking back over the scientific investigation of ozone depletion, we can see how the Molina-Rowland hypothesis evolved with new data. When chlorine nitrate, a chemical that ties up chlorine in an ozone-safe form, was added to the hypothesis, the expected amount of ozone loss decreased. Then, when polar clouds were considered, the expected ozone loss increased. Through all these minor revisions, the core of the hypothesis, the idea that CFCs lead to ozone depletion, never changed. Taken out of their scientific context, these fluctuations in expectations about ozone loss might have seemed to indicate that scientists were confused or disagreeing with each other, but on closer inspection, we saw that the fluctuations were a normal part of the scientific process as scientists came to a more complete understanding of an immensely complex system, the atmosphere.

Many paths of inquiry strengthened the hypothesis that CFCs were causing ozone depletion.
Many paths of inquiry strengthened the hypothesis that CFCs were causing ozone depletion.

Total stratospheric chlorine from CFCs and other sources
Now the hairspray you see on drugstore shelves is CFC-free, as are the refrigerators in appliance stores, and the air conditioners in new cars and homes. Modifying regulatory policies to bring about these changes was a hard-won battle for politicians and citizens concerned about the environment — and for Molina, Rowland, and many other scientists. Not only did they collect scientific evidence; they also invested a considerable amount of time and energy in conveying their results and the implications to lawmakers and the public. Molina and Rowland had to persist in these efforts for more than a decade before the ban they had been calling for all along was finally achieved! With the CFC-ban in place, atmospheric levels of chlorine are beginning to decline and the ozone layer is, we hope, on its way to recovery. If our current scientific understanding of the situation is correct, the ozone hole will start to shrink significantly by around 2018, with full recovery not expected until around 2070.

While Molina and Rowland are given much of the credit — their work has earned them many awards and honors, including the most prestigious award in chemistry, the Nobel Prize — successfully averting an environmental disaster required more than just their efforts. It was the accumulated knowledge of the scientific community that allowed Molina and Rowland to meld a bunch of seemingly disjointed pieces of information into a new hypothesis about the effects of CFCs in the atmosphere. The dire results predicted by their ideas — the depletion of the ozone layer and prospect of increased health and environmental problems — led many scientists to become involved in testing and fine-tuning the hypothesis, with chemists, atmospheric scientists, and mathematical modelers all contributing pieces of the puzzle. These diverse perspectives played a crucial role in the scientific process, allowing science to build a much more complete understanding of the phenomenon than if Molina and Rowland had worked alone. As with many scientific triumphs, this success belongs to no one individual, but to the scientific community as a whole — and to the broader community, which took it upon itself to act on pressing scientific findings.


Popular and historical accounts:

  • Roan, S. 1989. Ozone Crisis: The 15-year Evolution of a Sudden Global Emergency. New York, NY: John Wiley & Sons, Inc.
  • Dotto, L., and H. Schiff. 1978. The Ozone War. Garden City, NY: Doubleday & Company, Inc.
Some scientific papers:
  • Molina, M.J., and F.S. Rowland. 1974. Stratospheric sink for chlorofluoromethane: chlorine atom-catalyzed destruction of ozone. Nature 249:810-812.
  • Solomon, S. 1999. Stratospheric ozone depletion: review of concepts and history. Review of Geophysics 37:375-316.
key points
Use this story to introduce your high school students to the Science Flowchart with this activity.

Data for total stratospheric chlorine from CSIRO Atmospheric Research and Cape Grim Baseline Air Pollution Station, The Australian Antarctic Division and Australian Bureau of Meteorology

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