Atmosphere Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Dr Peter Manins, Environmental Consulting and Research Unit, CSIRO Atmospheric Research, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06746 9
Stratospheric Ozone (continued)
Antarctic ozone hole [A Indicator 2.4]
Global stratospheric ozone data have been obtained from several satellite-borne UV and infrared spectrophotometers. The longest (1979-present) satellite-derived total ozone record has been obtained using total ozone mapping spectrometer (TOMS) UV instruments carried on three satellites (Nimbus-7, Meteor-3 and Earth Probe).
The largest ozone depletions have occurred over Antarctica, particularly in spring (Sept.-Oct.). As with ozone losses at mid-latitudes, the cause of Antarctic ozone depletion from the 1980s has been the steady accumulation of stratospheric chlorine and bromine compounds that can catalytically destroy ozone. These catalytic processes are particularly effective over Antarctica in spring because of the presence of ice nuclei (polar stratospheric clouds) and weak Antarctic sunlight which provide a very efficient mechanism for halogen-catalysed ozone depletion.
Southern Hemisphere TOMS ozone data for 1979 to 2000 (Figure 73) show a progressive ozone loss at mid-latitudes and polar latitudes, with particularly severe losses (more than 60%) over Antarctica, the so-called Antarctic ozone 'hole'. The hole is defined as the region where total ozone levels are less than 220 DU. The ozone losses over south-east Australia from 1979 to 2000 are about 10% (i.e. about 4% per decade), similar to the summer ozone losses. The ridge of high ozone (55S-60S) between Australia and the ozone hole has declined by about 15 to 20% over the same period.
Figure 73: Total ozone levels (DU) over the Southern Hemisphere for 1-15 October in 1979, 1984, 1989, 1994, 1999 and 2000.
The 220 DU contour is shown as a red line.
Source: NASA GSFC; USA; CSIRO
The Antarctic ozone hole was first observed in the Dobson spectrophotometer ozone record from Halley Bay, Antarctica (66S, Farman et al. 1985, Figure 74). The October mean ozone level over Halley Bay is one of the most cited indicators of ozone variability over Antarctica.
Figure 74: October mean total ozone amount (DU) recorded by the Dobson spectrophotometer located at Halley Bay (66S), Antarctica.
The solid line (red) is a polynomial best fit (order 4) to the 1956-2000 data, not including the 1992-1995 data, which are affected by the Mount Pinatubo eruption.
Source: Farman et al. (1985); J. Shanklin, British Antarctic Survey (2000)
A slow decline in ozone occurred from the mid-1950s (320 DU) to the mid-1970s (290 DU), followed by a rapid decline to the mid-1990s (130 DU) (i.e. an overall ozone loss of about 60%). Ozone loss appears to have stabilised during the 1990s. The lowest October mean ozone observed at Halley Bay was 112 DU in 1993. In the early 1990s ozone levels over Antarctica (and mid-latitudes) were affected by a high loading of stratospheric aerosol resulting from the 1991 eruption of Mount Pinatubo in the Philippines. By 1996, the volcanic aerosol had been removed from the stratosphere and Halley Bay ozone levels appear to have stabilised at about 140 DU.
Although ozone levels over Antarctica appear to have recovered from the effects of the Mount Pinatubo eruption, there is no evidence of long-term ozone recovery. From 2000 to 2010, while stratospheric chlorine remains at near maximum levels, stratospheric ozone at mid-latitudes and polar regions is susceptible to further volcanic eruptions that deposit significant aerosol material in the stratosphere.
The area of the Antarctic ozone hole is another parameter that is used to assess its year-by-year variability. The area grew rapidly during the 1980s (at about 2 million km2 /year) (Figure 75) and less rapidly during the 1990s (about 0.5 million km2 /year). The data suggest that the area of the Antarctic ozone hole is near or at maximum values
(20-24 million km2).
Figure 75: The area of the Antarctic ozone hole (million km 2), defined by the area contained within the 220 DU contour for 1-15 October, from 1980 to 2000.
The solid line (red) is a polynomial best fit (order 4) to the 1981-2000 data. The dashed line (black) indicates the area of the Antarctic continent.
Source: NASA; CSIRO
The largest ozone hole observed during 1-15 October to date was about 24 million km2 in 1998. The hole in 2000 was the largest observed at any time and it occurred in the first two weeks of September (average area about 27-28 million km2). The previous largest hole was 26-27 million km2 for one week in mid-September 1998. The 2000 hole was bigger in peak area, but not as deep or long-lasting as typical ozone holes of the late 1990s. Year-to-year fluctuations in the size, depth and duration of the ozone hole are expected and are related to meteorological factors such as stratospheric temperature and wind strength and are not due to variations in the amount of ozone-depleting substances in the atmosphere.
Implications
Ozone loss over Antarctica appears to have stabilised during the 1990s. Although ozone levels appear to have recovered from the effects of the Mount Pinatubo eruption, there is no evidence to date of long-term ozone recovery. From 2000 to 2010, while stratospheric chlorine remains at near-maximum levels, stratospheric ozone at mid-latitudes and polar regions is susceptible to further volcanic eruptions that deposit significant aerosol material in the stratosphere.