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 CFOs 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)