September 2017 marked the 30th Anniversary of the Montreal Protocol, often dubbed the world’s most successful environmental agreement. The treaty, signed on September 16, 1987, is slowly but surely reversing the damage caused to the ozone layer by industrial gases such as the chlorofluorocarbons (CFCs).
Each year, during the southern spring, a hole appears in the ozone layer above Antarctica. This is due to the extremely cold temperatures in the winter stratosphere (above 10 km altitude) that allow byproducts of ozone depleting substances, such as the CFCs and related gases, to be converted into forms that destroy ozone when the sunlight returns to the Antarctic region in spring.
As ozone-destroying gases are phased out under the Montreal Protocol, the annual ozone hole is generally getting smaller – a rare success story for international environmentalism.
The Antarctic ozone hole has continued to appear each spring, as it has since the late 1970s. This is expected, as levels of the ozone-destroying halocarbon gases controlled by the Montreal Protocol are still relatively high. The figure below shows that concentrations of these human-made substances over Antarctica have fallen by 14% since their peak in about 2000. The Atmospheric Composition and Chemistry Group undertake measurements of all of the ODSs covered by the Montreal Protocol, either in situ at Cape Grim as part of the AGAGE network or in our laboratories in Aspendale.
Figure 1: Past and predicted levels of controlled gases in the Antarctic atmosphere, quoted as equivalent effective stratospheric chlorine (EESC) levels, a measure of their contribution to stratospheric ozone depletion. The data in the above figure are based entirely on measurements made at Cape Grim, from the Cape Grim Air Archive (link to CGAA page) and on Antarctic firn air from Law Dome. The ozone depleting gases used in the calculation of EESC above are methyl chloride (CH3Cl), methyl Bromide (CH3Br), chlorofluorocarbons (CFCs), methyl chloroform (CH3CCl3) and carbon tetrachloride (CCl4), hydrochlorofluorocarbons (HCFCs), and halons. Further information on these chemicals can be found here. EESC is calculated from Cape Grim data – both in situ and from the Cape Grim Air Archive – and from Antarctic firn air, measured as part of the Advanced Global Atmospheric Gases Experiment (AGAGE).
It typically takes a few decades for these gases to cycle between the lower atmosphere and the stratosphere, and then ultimately to disappear. The most recent official assessment, released in 2014, predicted that it will take almost 60 years for the levels of ozone depleting gases in the Antarctic atmosphere to reduce to the levels seen in 1980, and hence for the Antarctic ozone hole to recover.
Signs of Antarctic ozone hole recovery
Monitoring the ozone hole’s gradual recovery is made more complicated by variations in atmospheric temperatures and winds, and the amount of microscopic particles called aerosols in the stratosphere. In any given year these can make the ozone hole bigger or smaller than we might expect purely on the basis of halocarbon concentrations.
Studies have shown that the size of the ozone hole each September has shrunk overall since the turn of the century, and that some of this shrinking trend is consistent with reductions in ozone-depleting substances. However, careful analysis is needed to account for a variety of natural factors that could confound our detection of ozone recovery.
Below is a figure showing the Antarctic ozone hole annual peak area, in millions of kilometres squared, averaged over 15 days, based on satellite data dating back to 1979. This is one of the metrics used to assess the annual Antarctic ozone hole. The different coloured symbols indicate different satellite sensors that have been used over time. The orange line is obtained from a linear regression to Antarctic EESC (shown in figure 1 above) plotted against time and shows that in general the size of the Antarctic ozone hole has been decreasing roughly in line with the decline in ozone depleting substances. As noted above, careful analysis is required to take into consideration the year-to-year meteorological conditions and stratospheric aerosol loading when assessing ozone hole recovery.
Figure 2: Maximum ozone hole area using a 15-day moving average during the ozone hole season, based on TOMS (green), OMI (purple) and OMPS (red) satellite data. The orange line is obtained from a linear regression to Antarctic EESC. The error bars represent the range of the ozone hole size in the 15-day average window. TOMS, OMI & OMPS: the Total Ozone Mapping Spectrometer, Ozone Monitoring Instrument, and Ozone Mapping and Profiler Suite, are satellite borne instruments that measure the amount of back-scattered solar UV radiation absorbed by ozone in the atmosphere; the amount of UV absorbed is proportional to the amount of ozone present in the atmosphere. Information on the various satellite platforms and ozone instruments can be found here.
Annual tracking of the Antarctic ozone hole by the ACC Group
Each Austral spring, members of the ACC group track the progress of the Antarctic ozone hole using satellite data and meteorological analyses from NASA and other sources. Weekly assessment reports on the Antarctic ozone hole are produced for the Australian Government Department of the Environment and Energy (DoEE), along with an annual summary report that compares the current ozone hole to historical metrics. The reports can be accessed from the DoEE page here.
Below is an animation showing the October 1-15 averaged total ozone column images from 1979 onwards, for all available years of satellite data. The physical size of the Antarctic ozone hole, and the ozone hole ‘depth’ (how much ozone is lost), can be seen to increase from 1979 to reach peak values during the mid-1990s to mid-2000s, before reducing slightly from mid-2000s onwards, but also exhibiting large variability from year to year due to meteorological conditions. The extent of the Antarctic ozone hole is indicated by the red 220 DU contour line. The top 5 Antarctic ozone holes in terms of 15-may moving average area, occurred in 2000, 2006, 2015, 2003 and 1998 respectively, and reached areas of 26.8 to 28.7 million km2. To put this in perspective, the size of Australia is approximately 7.7 million km2.