In addition to photolytic destruction by infrared light (wavelengths of 1140 nanometers or less), stratospheric ozone is destroyed by interaction with a variety of chemical substances in the stratosphere. Some of these chemicals, such as the chlorine free radical--liberated by the photolysis of chlorofluorocarbons, are not normally produced there in nature at such high concentrations. This leads to the conclusion that human activities, and not natural chemical cycles, are responsible for the current rate of stratospheric ozone destruction. Ozone destruction is most noticeable (alarmingly!) over the South Pole, where seasonal plummeting in stratospheric ozone concentrations have been measured since the 1970s and are apparently still increasing (1999). That said, estimates based on declining CFC use (because of the Montreal Protocol) suggest that the damage to the global ozone layer will max out soon in the 21^st century and that that ozone damage will decrease within the next 75 to 100 years.