Ozone layer

The ozone layer in Earth's atmosphere was damaged in the course of global warming. In part, this was a plot by the Zygons to transform the planet's atmosphere to make it more amenable to Zygon life. The Zygons encouraged the introduction of modified CFCs Earth's atmosphere, by investing in industrial projects, and releasing any toxic products to the humans which would be harmful to the ozone layer. (AUDIO: The Zygon Who Fell to Earth)

In December 1992, the women of Cheldon Bonniface blamed the ozone for the unusual weather. (PROSE: Timewyrm: Revelation)

Prior to 1997, the Bandrils attempted to destroy the Earth's ozone layer but were thwarted by UNIT in what the Brigadier would later cite as an example of an incident which was resolved without the assistance of the Doctor. (PROSE: The Dying Days)