Pollution of Earth’s atmosphere by ozone-depleting substances, such as chlorine or bromine from halons and chlorofluorocarbons (CFCs), decreased over the past years, new research shows. Ozone is, however, not only influenced by these substances, but acts as a greenhouse gas itself: Its impact on climate near the ground is highest in the tropopause region, the transition zone between stratosphere and troposphere at 7 to 17 kilometers altitude.