The stratosphere is the layer of the atmosphere that begins at an altitude of around 10 kilometres above Earth’s surface and extends to approximately 50 kilometres. It is situated between the troposphere (near Earth’s surface) and the mesosphere.97 Stratospheric ozone limits the amount of harmful ultraviolet B (UVB) light (UVB wavelengths are 280–315 nanometres) passing through to lower layers of the atmosphere. The ozone layer, therefore, has a vital role in protecting life on Earth, as increased levels of UVB may result in damage to a range of biological systems, including human health. In humans, UVB—although necessary for the production of vitamin B—causes nonmelanoma skin cancer and is a significant factor in the development of malignant melanoma. In addition, it is associated with the development of cataracts.98-100 (However, it should be noted that, whereas ozone in the stratosphere is protective of human health, ozone near the ground, where it can be breathed in, is a pollutant and harmful to health. Section 3.1.2 further discusses ozone as a pollutant.)
Photosynthesis in many species of plants is impaired by UVB radiation, and overexposure can reduce yield and quality in some crop species, including varieties of rice, winter wheat, soybeans, corn and cotton. UVB radiation may also change the susceptibility of plants to insect and pathogen attack. In aquatic systems, photosynthesis in phytoplankton is more sensitive to UVB than in terrestrial plants, and short-term exposure to increased UVB levels can reduce productivity in such systems.101-103
The ozone layer was threatened by human-produced ozone depleting substances (ODSs), principally chlorofluorocarbons (CFCs) and halons, which were widely used in refrigerators, air conditioners, fire extinguishers and electronic equipment, as solvents for cleaning (including dry cleaning) and as agricultural fumigants. These substances are stable and long lived in the lower atmosphere, but slowly drift up to the stratosphere, where they are subject to breakdown through the action of UV radiation. This releases highly reactive molecules (chlorine and bromine) that react with ozone molecules and break them apart.
Since peaking in the mid-1990s, levels of stratospheric chlorine and bromine from CFCs and other ODSs have declined. The latest World Meteorological Organization (WMO) Scientific assessment of ozone depletion104 concludes that:
… the atmospheric abundances of nearly all major ODSs that were initially controlled [under the Montreal Protocol] are declining [Figure 3.19]. Nevertheless, ozone depletion will continue for many more decades because several key ODSs last a long time in the atmosphere after emissions end.
This has important implications for climate, since all ODSs (except methyl bromide) are powerful GHGs, and the gradual recovery of the ozone layer is expected to interact with climate change through a complex series of linkages. These relationships may, for example, reduce the capacity of the oceans to absorb carbon dioxide and delay the recovery of stratospheric ozone.105