Stratospheric Ozone Depletion and Recovery

This chapter provides an overview of the depletion of Earth’s ozone layer due to human activity and the eventual recovery due to legislation that banned ozone-depleting substances (ODSs) such as chlorofluorocarbons (CFCs) and bromine-bearing halon gases. The importance of ozone in protecting life on Earth is introduced, followed by details on how the release of CFCs and halons led to significant stratospheric ozone losses, as first observed in the mid-1980s. The relevant chemistry in the stratosphere is presented, with particular focus on the processes responsible for severe depletion of polar ozone and modest depletion of midlatitude ozone. The Montreal Protocol and subsequent amendments are shown to have been critical in limiting the loss of ozone, particularly over heavily populated regions. The ozone layer is expected to eventually recover as the abundance of ODSs in the atmosphere declines to pre-industrial levels. While full recovery of the ozone layer is still many decades away due to the long atmospheric lifetimes of CFCs and halons, initial signs of recovery for upper stratospheric ozone are described. Finally, the chapter concludes by showing that future anthropogenic emissions of greenhouse gases actually contribute the largest source of uncertainty for projections of the thickness of Earth’s ozone layer by end of century.