Starting in the late 1970's, an ozone hole was discovered over Antarctica.

These observations provided a correlation with the prediction from the hypothesis that CFC in the atmosphere can deplete the ozone layer.

Other hypotheses may also explain the depletion of the ozone layer:

• the ozone in the stratosphere was mixing with ozone in lower layers.
• transient solar activities (sunspots)

These other hypotheses can be ruled out by measurements of CFC concentrations in the atmosphere and also concentrations of chlorine that is a product of the breakdown of CFC.