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

The ozone hole

The impact of ODSs on the stratospheric ozone layer has been observed at all latitudes, except in the tropics (i.e. 20°N and 20°S), where ozone depletion is negligible.¹⁰⁴ However, by far the most pronounced ozone losses are associated with the Antarctic ozone hole, which occurs each year over Antarctica between August and December. The ozone hole reaches its maximum extent in spring, when 60% of the total ozone in the vertical air column is lost. The depleted ozone layer then breaks up and disperses over the areas surrounding Antarctica during the summer and autumn months (see also Chapter 7: Antarctic environment). The break-up of the ozone hole during summer is the cause of reductions in stratospheric ozone in the Southern Hemisphere, as parcels of ozone-depleted polar air move north and mix with mid-latitude air.^107-109 There is also increased evidence that the Antarctic ozone hole affects the Southern Hemisphere’s climate, acting as the driver of changes in pressure, surface winds and rainfall at mid-to-high latitudes during summer.¹⁰⁴

Images available from Environment Canada illustrate the break-up of the ozone hole and the dispersal of the depleted ozone layer across Tasmania and southern Australia (Figure 3.20).

Following a period of rapid growth from the late 1970s to the mid-1990s, the area of the ozone hole has remained relatively stable over the past 15 or so years (Figure 3.21), with October mean column ozone levels within the polar stratospheric vortex approximately 40% of 1980 values.¹⁰⁴ The ozone holes of 2000 and 2006 were the most severe on record; the 2006 hole was the deepest and the 2000 hole the largest (in area). However, the hole can fluctuate markedly from year to year, with 2010 being one of the smallest on record in the past two decades.^111-112

The relative stability of the ozone hole reflects the fact that there have been only moderate decreases in stratospheric chlorine and bromine in the past few years. Since around 1997, ODS levels have been nearly constant, and the depth and magnitude of the ozone hole have been controlled by variations in temperature and climate dynamics. Although summer ozone levels over Antarctica have yet to show any statistically significant increasing trend, recent simulations of the effect of reductions in ODSs that are projected to continue to flow from controls under the Montreal Protocol indicate a return to pre-1980 benchmark values late this century.^104,113 Modelling results suggest that this recovery may be accelerated by climate change in the form of stratospheric cooling, linked to increases in GHGs.¹⁰⁴

Ozone hole impacts

As noted above, the most recent WMO Scientific assessment of ozone depletion¹⁰⁴ comments on the importance of the ozone hole as a driver of changes in Southern Hemisphere seasonal surface winds at mid-to-high latitudes. However, the influence of the hole extends to the whole of the hemisphere.¹¹⁴ Modelling by Son et al.,¹¹⁵ which incorporates stratospheric chemical interactions and takes into account the likely influence of recovering ozone levels, indicates that the anticipated recovery of the hole may result in a reversal of the current acceleration of these seasonal surface winds (summer tropospheric westerlies) on the poleward side. The authors concluded:

… our analyses suggest that stratospheric processes, and ozone recovery in particular, may be able to affect SH [Southern Hemisphere] climate in major ways and thus should be included in predictions of SH climate in the 21st century.

In addition to its influence on climate, the ozone hole has been of concern in relation to UVB effects on health. The progressively more rigorous controls established under the Montreal Protocol during the 1990s are expected to lead to the avoidance of a significant increase in cases of skin cancer that would otherwise have been associated with large reductions in global stratospheric ozone (Figure 3.22). This is of particular importance in Australia, where high levels of UVB radiation combine with outdoor lifestyles to produce one of the highest incidence rates of skin cancer in the world.^116-117

From the early 1980s to the early 2000s, Australian skin cancer rates for both sexes showed a generally increasing trend, after which rates appear to have stabilised (Figure 3.23). Most recent data for melanoma show a decline in both male (7.1%) and female (10.7%) rates from 2005 to 2007.^122-123 However, the period involved is too short to tell whether the reduction indicates a genuine decline or is the result of fluctuations in the data.