Final answer:
Chlorine is the chemical in the stratosphere that indicates ozone depletion is occurring.
Step-by-step explanation:
The chemical in the stratosphere that tells us ozone depletion is occurring is chlorine. Chlorine is one of the halogen radicals that can catalytically destroy ozone molecules. This process is known as ozone depletion and occurs when certain chemicals, such as chlorofluorocarbons (CFCs), react with ozone molecules in the stratosphere, resulting in a net removal of ozone.