Stratospheric ozone

2011

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

Box 3.5 Ozone and UV radiation

Ultraviolet (UV) radiation levels at ground level are principally an inverse function of the amounts of ozone in the upper atmosphere, the concentration of aerosols and water vapour in the atmosphere, and the extent of cloud cover. Long-term trends in UV radiation levels are measured at Lauder in New Zealand’s South Island and are modelled at a number of sites in Australia. UV is expressed as an erythemal UV index—a measure that describes the strength of the skin-burning component of UV radiation.

Figure A shows total column ozone levels over Melbourne and erythemal index values for Lauder and three Australian capital cities. The top panel shows January mean total ozone values, measured by the Bureau of Meteorology’s Dobson network at locations around greater Melbourne from 1979 to 2011. The green line shows a five-year running mean. Although year-to-year variability is evident—as is a clear signal of the 11-year solar cycle (which peaked around 1980, 1991 and 2002)—the underlying negative trend in ozone ceased in the mid-1990s. The long-term ozone behaviour closely follows the concentration of ODSs measured in the global atmosphere, which peaked in the mid-1990s (see Figure 3.19)

The middle panel shows summer-time peak UV index values in 1990–2010, as measured by UV spectroradiometer at Lauder by New Zealand’s National Institute of Water and Atmospheric Research. Although UV radiation values are affected by factors other than just ozone, the underlying trend quite closely follows the expected inverse relation to the ozone timeseries, with highest UV index values in the late 1990s when ozone was lowest.

The bottom panel shows modelled clear-sky UV index values for three Australian cities (Sydney, Adelaide and Melbourne), illustrating the effect of location on the amount of UV radiation received. The values were calculated using summer satellite measurements of ozone and meteorological fields from the Bureau of Meteorology forecast model, as input to the UV radiation code.

Some differences in the detail of panels 1 and 3 are evident. However, the overall pattern of rising UV through the 1980s and 1990s, followed by a stabilisation, corresponds to the decline and subsequent stabilisation of ozone during the same period. The differences are primarily due to the use of satellite-measured ozone values rather than ground-based values, a slightly different averaging period (all of summer in panel 3 compared with just January in panel 1) and some missing periods of satellite data in the late 1990s, when ozone values were low.

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.

(2011). Ambient air quality: Stratospheric ozone. In: Australia state of the environment 2011, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/ambient-air-quality/topic/stratospheric-ozone-3, DOI 10.4226/94/58b65c70bc372