BOX 1.1 EXAMPLES OF RECENT ABRUPT CHANGE IN THE EARTH SYSTEM
Stratospheric Ozone Depletion
During the early 1970s, concerns arose in the scientific community that inputs of nitrogen oxides (known as “NOx”) from a proposed fleet of supersonic aircraft flying in the stratosphere and of industrially produced halocarbon gases containing chlorine and bromine (CFCs or chlorofluorocarbons and chlorofluorobromocarbons) had the potential to deplete the amount of ozone in the stratosphere. Halogen oxide radicals were predicted to form from the degradation of halocarbons in the stratosphere. Intensive study of the stratosphere, extending more than a decade, confirmed the rising concentrations of CFCs and halons in the atmosphere, and of halogen oxide radicals in the stratosphere. International negotiations led to the signing of the Montreal Protocol in 1987, requiring a 50 percent reduction in CFCs and a 100 percent reduction in halon production by 2000 by the developed countries.
However, two years prior to the treaty, scientists learned that the column amount of ozone over Antarctica in the austral spring had been declining since the late 1960s, and it had been reduced by almost a factor of two by the mid-1980s (Farman et al., 1985); See Figure A. The continuous record of column ozone abundances measured at Halley Bay, Antarctica, showed
FIGURE A Total column ozone in Antarctica, at the Halley Bay station of the British Antarctic Survey (black) and averaged over the whole polar region of Antarctica (blue, from satellite data). (Adapted from WMO/ UNEP  plus data from the British Antarctic Survey [http://www.antarctica.ac.uk/met/jds/ozone/data/ZOZ5699.DAT, downloaded 26 April 2013].)