Though the stereotype of scientific progress is that it’s driven by critical new evidence, Molina’s breakthrough, like many efforts in science, was a bit different. He didn’t perform any experiments or gather any new data. Instead, he took on the tough job of pulling together a lot of existing facts and hypotheses about chemical reactions, atmospheric processes, and CFC levels, to show that if all the individual facts and hypotheses were accurate, the result would be a serious environmental threat. It was the sum of this scientific evidence that provided him with nearly all the information he and Rowland used to form their hypothesis about how CFCs could affect atmospheric ozone. All they had needed to do was to use known chemical theory to calculate an estimate for the long-term effects of CFCs on ozone.

Molina and Rowland’s over-arching hypothesis (that releasing CFCs into the atmosphere would cause significant ozone depletion) was based on many supporting hypotheses (sometimes called auxiliary hypotheses or assumptions). For example, one of the sub-hypotheses contained within the Rowland-Molina hypothesis was how fast chlorine reacts with ozone. These sub-hypotheses were backed up by their own lines of evidence, but also came with their own uncertainties. If a key sub-hypothesis turned out to be false, it could mean that Molina and Rowland’s over-arching hypothesis about ozone depletion was also false. In fact, some scientists were skeptical of the importance of ozone depletion due to CFCs, not because they doubted Molina and Rowland’s work, but because they were skeptical of some of the sub-hypotheses. For example, at the time the paper was published, there was not yet any experimental evidence to support the idea that CFCs would release a chlorine atom when exposed to solar radiation. It did not take long before this hypothesis was checked in a laboratory experiment and confirmed. However, some of the other hypotheses were not so easy to test, and a lot of hard work would be needed to persuade skeptics.