Global production of ODSs continues to decline (Figure 3.30). However, due to the long atmospheric lifetimes of a number of important ODSs, they will continue to impact levels of stratospheric ozone for many decades. In addition, future recovery of the ozone layer will be influenced by emissions of GHGs that are not controlled under the Montreal Protocol—notably carbon dioxide, methane and nitrous oxide—through their effects on temperature, wind and chemistry.104
As new countries ratify the Montreal Protocol, the number of countries reporting national production increases. Therefore, the number of countries is different in 1990 and 2009.
Carbon dioxide has an indirect influence on stratospheric ozone through its effect on temperature, which affects the rates of chemical reactions that control the abundance of ozone. Increasing levels of carbon dioxide have been observed to cause cooling of the mid-to-upper levels of the stratosphere (via radiation to space), leading to a decrease in the rate of ozone loss in these parts of the atmosphere and an increase in the rate in the lower stratosphere. Increases in methane’s abundance in the troposphere will lead to more methane reaching the stratosphere. There, it interacts with compounds that contain active chlorine (which is able to destroy ozone) to produce inactive hydrogen chloride, which does not destroy ozone. Methane levels also influence stratospheric water vapour, which affects both ozone and climate. The net effects of carbon dioxide and methane are expected to be positive for the recovery of stratospheric ozone levels.104 However, this is not the case with nitrous oxide, which is produced by a variety of natural and human-related sources (notably agricultural processes).
As well as being a potent GHG, nitrous oxide from human sources is currently the single most important ODS and can be expected to remain so throughout this century.156 This reflects the fact that, although the ozone depleting potential of nitrous oxide is only about one-sixtieth that of CFC-11, human emissions are large and increasing. Even in 1987, when CFC emissions were at or near their peak, annual ozone depletion potential–weighted emissions of nitrous oxide were some 17% of the combined emissions of CFC-11, CFC-12 and CFC-112.156 In 2009, Australia’s emissions of nitrous oxide were 26.7 MtCO2-e, which is approximately 0.7% of the world’s human-sourced emissions.38-39