Atmosphere

Theme commentary
Tom Beer, Michael Borgas, Willem Bouma, Paul Fraser, Paul Holper and Simon Torok
CSIRO Atmospheric Research
prepared for the 2006 Australian State of the Environment Committee, 2006

Major issue: stratospheric ozone

Ozone is an important constituent of the upper atmosphere because it absorbs most of the harmful ultraviolet B (UV-B) radiation that emanates from the sun, preventing it from reaching the Earth’s surface. UV-B causes several human health problems (skin cancer, cataracts, suppression of the immune system) and damage to agricultural crops, livestock, and industrial and domestic materials.

The Antarctic ozone hole, first reported in 1985, is a massive seasonal loss of ozone (up to 2.5 trillion kilograms each year) that occurs in the stratosphere above Antarctica from August to December. It is caused by chlorine and bromine, released in summer by sunlight acting on ozone depleting substance, such as chlorofluorocarbons (CFCs), solvents, fire-fighting chemicals and agricultural fumigants that have made their way from their point of release in cities and on farms into the upper atmosphere above Antarctica.

These chlorine and bromine releases in the stratosphere are enhanced in the presence of ice particles (polar stratospheric clouds) that form in winter at temperatures below –78°C. The ice particles endure through spring until early summer in a very cold, otherwise near particle-free, region of the stratosphere called the Antarctic polar vortex.

The Antarctic ozone hole  results, on average, in a 60 per cent loss of total ozone in a vertical column above Antarctica in spring, and it is the major cause of summertime ozone losses at mid-latitudes in the southern hemisphere, including southern Australia (Ajtic et al. 2004).

The chlorine and bromine levels in the stratosphere (upper atmosphere) can be estimated from tropospheric (lower atmosphere) measurements of ozone depleting substances such as those made at Cape Grim, and expressed as ‘effective equivalent stratospheric chlorine’ (EESC) or, more simply, ‘total stratospheric chlorine’. This is a measure of the atmosphere’s ability to drive ozone through chlorine and bromine catalysed chemical destruction of ozone.

Skin cancer rates  for Australian males and females showed a steady increase from the early 1980s to the late 1990s. From 1997 to 2001, skin cancer rates seem to have stabilised. This stabilisation is not related to the stabilisation of ozone and UV levels that occurred about the same time (late 1990s) because there is significant lag (about 50 years) between the time of maximum ozone depletion (about 2000) and the resultant maximum in the excess cases of skin cancer (about 2050) that are likely to be caused by the increase in UV due to ozone depletion (Slaper et al. 1996; De Gruijl et al. 2003).

Ozone-depleting substances

Chlorine- and bromine-containing chemicals (for example CFCs and halons) are released at the Earth’s surface from a number of industrial, domestic and agricultural processes. They remain in the lower atmosphere for one to five years before eventually reaching the stratosphere (15 kilometres above the Earth), where they are broken down by solar ultraviolet radiation, producing reactive chlorine and bromine species that catalytically destroy ozone.

The major chlorine-containing chemicals that participate in this process are the CFCs, the solvents carbon tetrachloride (CCl4) and methyl chloroform (CH3CCl3), the naturally-occurring methyl chloride (CH3Cl, also known as chloromethane) and the interim CFC replacements, hydrochlorofluorocarbons (HCFCs). The major bromine containing chemicals involved in ozone destruction are methyl bromide (CH3Br) and the halons.

The data show the levels of all the important ozone depleting substances  found in the clean, background or ‘baseline’ air at Cape Grim (Tasmania), expressed as ‘total stratospheric chlorine’. The Cape Grim measurements closely approximate the background levels in the entire extra-tropical southern hemisphere.

The stratospheric chlorine observations are compared with scenarios to 2020 that were developed under the auspices of the Montreal Protocol. These scenarios assume global compliance with the protocol, with the consumption of ozone depleting substances being largely phased out in the developed world by the mid-1990s (except CH3Br, which was phased out by 2005), and by 2010–2015 in the developing world. To date, the scenarios and observed levels of ozone depleting substances agree well (Figure 11). CFCs and chlorinated solvents are the largest contributors to stratospheric chlorine, accounting for about 40 per cent and 25 per cent of ‘equivalent effective stratospheric chlorine’ respectively since 1970. All ozone depleting substances except HCFCs are expected to decline after 2005; HCFCs are predicted to decline after about 2010.

Total stratospheric chlorine peaked in the late-1990s and it is now in decline by about one per cent per year. This means that ozone depletion should have peaked in the late 1990s and it should now be slowly recovering, assuming all other factors that influence stratospheric ozone (solar radiation, volcanic activity) have not changed. It is unlikely that the current satellite and ground-based network for detecting ozone changes is capable of resolving the implied ozone increase from background ozone variability.

The individual contributions to the pattern of total stratospheric chlorine variations are:

  • CFCs, which peaked in 1999 and are now declining by 0.3 per cent per year
  • chlorinated solvents, which peaked in the mid-1990s and are now declining by five per cent per year
  • halons, which are still rising by two per cent per year; they should peak by 2007 according to the protocol scenarios. This prediction may be optimistic, as it depends on the future consumption of halons in the developing world, particularly in China
  • HCFCs, which have risen by about nine per cent per year over the past 30 years, with the current annual growth rate being about six per cent per year. These increases will continue under the protocol, but HCFCs are weak ozone depleters, contributing only about one per cent of stratospheric chlorine
  • methyl bromide levels, which peaked in 2002 and are now declining by six per cent per year. In the southern hemisphere atmosphere, about 70 per cent of methyl bromide is from natural sources—oceans, wetlands, some plants, and burning of vegetation. This natural methyl bromide represents the absolute limit to which levels can fall in the future, even if the protocol were strengthened to control all anthropogenic consumption of this chemical
  • the long-term past variability of methyl chloride, which is uncertain, but there is an indication that atmospheric levels reached a maximum in the early 2000s and are now in decline by about three per cent per year. It is unlikely that this decline will be maintained for many years into the future because methyl chloride levels appear to correlate with levels of global biomass burning, which in turn exhibits El Niño-driven variability.

Total chlorine levels are probably known to an absolute accuracy of ±5%, and the scenario projections known to ±20% over the next 20 years.

Stratospheric ozone and the ozone hole

  • Australian consumption of ozone depleting substances
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    The Australian and New Zealand total ozone observing network is a vital component of the global observing system. It accounts for about half of the global network in the southern hemisphere, in a region where the first signs of ozone recovery are expected to be detected (Weatherhead et al. 2000).

    From the late 1970s to the late 1990s, data from the mid-to-high latitudes (Macquarie Island, Lauder in New Zealand, and Melbourne) all show statistically significant ozone decreases  during summer, averaging about 0.4–0.5 per cent per year (eight to ten per cent over the two decades). Total ozone data from tropical latitudes (Brisbane and Darwin) do not show any significant trends.

    Some combination of a solar cycle effect and ozone recovery due to total stratospheric chlorine decline is the most likely explanation of the observed recent increase of total ozone over Australia and New Zealand. It will probably take three to five years of observations to resolve these relative contributions to ozone trends in this region.

    The year-by-year variability is assessed from the estimated area of the Antarctic ozone hole as derived from satellite ozone observations. An average of the area over the 15-day period 1–15 October is commonly used.

    The area of the Antarctic ozone hole  grew rapidly from the late 1970s to the mid-1990s. Since the mid-1990s, the area of the ozone hole has remained relatively constant at a little over 25 million square kilometres. The maximum ozone hole area ever recorded was nearly 30 million square kilometres in 2000 (Figure 12). Exceptionally small ozone holes were observed in 2002, when the hole broke up very early in the season, and in 1988. In both years the Antarctic stratosphere was exceptionally warm.

    The satellite ozone data show no evidence of a significant reduction in recent years in the area of the Antarctic ozone hole. It can be said that the Antarctic ozone hole has stopped growing since the mid- to late-1990s, presumably in response to a lack of growth in total stratospheric chlorine that occurred at the same time.

    The level of ultraviolet radiation reaching the ground is controlled largely by the amount of ozone and aerosols in the atmosphere and the degree of cloud cover. In Australia and New Zealand there are several ultraviolet radiation monitoring sites that are suitable for the determination of long-term ultraviolet radiation trends—Lauder in New Zealand, and Melbourne, Sydney and Adelaide in Australia. On cloud free days, ultraviolet radiation levels can be derived from ozone data, provided the level of aerosols that interact with ultraviolet radiation are known and allowed for in the calculations. Ultraviolet radiation levels are often expressed as an erythemal UV index, or more simply UV index, which is a measure of the amount of ultraviolet radiation in the wavelength region that causes skin reddening (that is, sunburn).

    At Lauder, UV index values, as determined by direct measurements of UV and ozone-derived calculated UV  levels , increased by 0.5 per cent per year from 1978 to 1998, which corresponds to the period of observed ozone losses. Since 1998, at Lauder and the southern Australian sites, there has been a one per cent per year decline in the UV index. The long-term increase in UV index measured at Lauder up to 1998 is consistent with ozone changes, but the rapid decline of approximately six per cent in the index since 1998 (in both southern Australia and New Zealand) is larger than calculated based on ozone increases (Figure 13).

    Figure 13: Total ozone in a vertical column and the values of the erythemal UV index

     Total ozone in a vertical column and the values of the erythemal UV index

    Source: Graphed by CSIRO Marine and Atmospheric Research on the basis of data obtained from A Downey, Bureau of Meteorology, and R McKenzie, NIWA

    Other factors, such as an increased aerosol loading, could be involved because increased stratospheric aerosol can reduce ultraviolet radiation levels at the Earth’s surface. The variability of the aerosol loading of the stratosphere is, in part, dependent on the time since a significant volcanic eruption. Currently the stratospheric aerosol is not affected by volcanic aerosol, because the last two decades have been unusually devoid of volcanic activity. There are other sources of upper-tropospheric and stratospheric aerosol, for example tropical biomass burning, which was prevalent during the Indonesian fires of 1998. Thus the 15 per cent drop in UV index measured at Melbourne and Lauder in the late 1990s could be influenced by biomass burning aerosol loading of the atmosphere.

    Despite the uncertainty, ozone levels over southern Australia and New Zealand show signs of a slight increase since 2000 (as is consistent with the decrease in total stratospheric chlorine) and overall the UV  index  has decreased over the same period.

    Australian consumption of ozone depleting substances

    Table 1 (DEH 2003, p. 248), provides a summary of the control measures under the Montreal Protocol to phase out ozone-depleting substances.

    Australia has a very good record of phasing out ozone depleting substances. In 1986 (the Montreal Protocol base year) Australian consumption of ozone depleting substances was close to 20 000 tonnes (ozone depletion potential (ODP)-weighted), which was dominated by consumption of CFCs.

    After Australian consumption of CFCs was phased out in 1995, ozone depleting substances consumption dropped  to below 1000 ODP tonnes, and in 2004 was about 300 tonnes. During the late 1980s to early 1990s, Australian consumption of ozone depleting substances was phased out at more than double the rate required unde the Montreal Protocol, and levels are still significantly lower than protocol requirements. The remaining 300 tonnes is made up of methyl bromide (to be phased out in 2005), HCFCs (to be phased out to 65 per cent of current levels by 2010), and essential (medical) CFC use.

    Table 1: Summary of the Montreal Protocol control measures
    Ozone-depleting substances Developed countries Developing countries
    CFCs Phased out end of 1995a Total phase-out by 2010
    Halons Phased out end of 1993 Total phase-out by 2010
    Carbon tetrachloride Phased out end of 1995a Total phase-out by 2010
    Methyl chloroform Phased out end of 1995a Total phase-out by 2015
    HCFCs Freeze from beginning of 1996b Freeze in 2016 at 2015 base level
      35% reduction by 2004  
      65% reduction by 2010  
      90% reduction by 2015  
      Total phase-out by 2020c Total phase-out by 2040
    Hydrobromofluorocarbons Phased out end of 1995 Phased out end of 1995
    Methyl bromide Freeze in 1995 at 1991 base leveld Freeze in 2002 at average 1995–1998
      25% reduction by 1999 base level
      50% reduction by 2001 20% reduction by 2005e
      70% reduction by 2003  
      Total phase-out by 2005f Total phase-out by 2015
    Bromochloromethane Phased out by 2002 Phased out by 2002

    a An exception is made for a very small number of internationally agreed essential uses that are considered critical to human health and/or laboratory and analytical procedures.
    b This is based on 1989 HCFCs consumption with an extra allowance – Ozone Depletion Potential (ODP)-weighted – equal to 2.8% of 1989 CFC consumption.
    c Up to 0.5% of base level consumption can be used until 2030 for servicing existing equipment.
    d All reductions include an exemption for pre-shipment and quarantine uses.
    e This will be reviewed in 2003 to decide on interim further reductions beyond 2005.
    f The protocol allows for 'critical use exemptions' to be granted from the methyl bromide phase-out where end users meet certain strict criteria, and where they can demonstrate that viable alternatives will not be available to them by 2005.

    In 2004 Australia imported  around 160 ODP tonnes of HCFCs and around 124 ODP tonnes of methyl bromide for non-quarantine and pre-shipment uses.

    Australian imports of HCFC will fall by 30 ODP tonnes every two years until imports are essentially phased out in 2015, five years ahead of the Montreal Protocol requirement. Since 2005, Australian non-quarantine and pre-shipment methyl bromide imports have been restricted to critical use exemptions that are approved by the international community. Australian use for this purpose will continue to move towards total phase out as alternatives are identified for the remaining uses where there are currently no technically or economically feasible alternatives available. Australia had eight exemptions approved in 2005, whereas only two critical use exemption applications were received in 2005 for the 2007 calendar year.

    Discussion and conclusion: stratospheric ozone

    The extra five years of measurement that have accumulated since Australia State of the Environment 2001 was published confirm that the accumulation of total chlorine from ozone-depleting gases in the stratosphere slowed during the early 1990s and levels are now declining slowly. Although there is no evidence of a significant reduction in the Antarctic ozone hole in recent years, it stopped growing in the mid- to late 1990s.

    Ozone data and the erythemal UV index data exhibit marked variability, but there is evidence that, since 2000, the amount of total ozone in the stratosphere over Australia and New Zealand has started to increase. There has been a consequent decrease in the erythemal UV index (a measure of skin cancer potential) over the southern part of Australia.