Final answer:
The amount of chlorine in the stratosphere has decreased over the past twenty years due to the ban on chlorofluorocarbons (CFCs) by the Montreal Protocol, leading to a gradual recovery of the ozone layer.
Step-by-step explanation:
The amount of chlorine in the stratosphere over the past twenty years has been influenced by international policy changes, especially following the discovery of ozone layer depletion. The 1989 Montreal Protocol led to the ban on chlorofluorocarbons (CFCs), which are chemical compounds that release atomic chlorine into the stratosphere, where it can destroy ozone molecules. This action has resulted in a notable improvement in ozone layer recovery. Although the CFCs are stable and continue to contribute to ozone depletion, their phase-out has stopped further ozone loss, and the 'ozone hole' over the Antarctic is shrinking. Today, the concentration of stratospheric chlorine is decreasing, leading to a recovery of the ozone layer, which is critical for absorbing harmful ultraviolet radiation from the sun.