Ozone (O₃) is a gas composed of three oxygen atoms that occurs in the Earth’s upper atmosphere and absorbs radiation from the sun. Ozone can also be man-made and be found at ground-level, which has a number of adverse health effects for humans (see tropospheric ozone).
Due to the usage of Chlorofluorocarbons (CFCs), the ozone layer was greatly depleted. In 1987, nations around the world came together and created the Montreal Protocol, a treaty that phased out the usage of CFCs and other ozone depleting chemicals. The treaty serves as a paragon for international climate collaboration as the ozone layer has seen great improvement since its signing.