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The Antarctic ozone hole
Since the 1970s, the level of ozone over Antarctica has decreased significantly each spring, creating the phenomenon commonly referred to as the 'Antarctic ozone hole'. The ozone hole persists for 3-4 months and then dissipates due to meteorological processes. During the spring of 2006, the ozone hole was arguably the largest on record, with average ozone levels over Antarctica amongst the lowest ever observed. The ozone hole is directly caused by halon gases such as chlorofluorocarbons (CFCs) that have been released into the atmosphere by industrial activity. International regulations have resulted in a drop in levels of these gases in the lower atmosphere, and there are signs of improvement in the stratosphere as well. These trends will ultimately repair the ozone hole, but according to recent research, full recovery of ozone levels is not likely before 2068.
Ozone balloon release
| Scientists investigating the 2006 ozone hole have used a range of instruments to measure ozone concentrations, including satellites, balloons, and ground-based spectrometers and gas samplers. Measurements showed that the area of the hole was greatest on 24 September 2006, with an area of approximately 29 million square kilometres, more than twice the area of Antarctica. This value is equal to the previous record set on 29 September 2000. Another indicative measurement is the total mass of ozone that has been destroyed. The 2006 total is about 2 million tonnes greater than observed in 2000.
The 2006 record was due to the 'polar vortex' being unusually large and stable. The polar vortex is a region in the stratosphere over Antarctica where air undergoes little mixing with the rest of the atmosphere, allowing temperatures to drop and stay low during the polar night. The low temperatures cause Polar Stratospheric Clouds (PSCs) to form. These clouds chemically alter the normally inert CFCs to forms that destroy ozone in the presence of sunlight. The potential for ozone destruction is related to the amount of PSC formation, and the size and stability of the polar vortex.
At mid-latitudes, atmospheric waves of planetary scale ('planetary waves') are a common feature during the winter in the lower stratosphere. These waves can distort and erode the polar vortex, mixing in air from lower latitudes, and thus diluting the potential for ozone depletion. In 2006, planetary wave activity was not particularly high, and this allowed the vortex to remain strong and cold. Indeed record low temperatures were observed within the vortex.
Work in the Ice, Ocean, Atmosphere and Climate program is aimed at helping to assess trends in temperatures and ozone concentrations in the stratosphere, as well as understanding the chemical processes related to PSCs. Climate modelling and observations indicate that the lower atmosphere is warming as a direct consequence of enhanced greenhouse gas emission. Increasing CO2 warms the lower atmosphere and cools the upper atmosphere. Since the late 1970s, a general cooling trend of approximately 1 degree Celsius per decade has been clearly identified in the stratosphere. This trend has been punctuated by short lived warmings due to large volcanic events such as Mt Pinatubo in 1992. About half of the cooling trend is directly attributable to increased greenhouse gas concentrations, while the remainder is related to ozone loss.
Since the early 1990s, the size of the ozone hole has been controlled primarily by the size and strength of the polar vortex. CFC concentrations in the stratosphere have been relatively stable, and are now showing signs of decline due to the action of the Montreal Protocol. However, a decrease in the ozone hole size is not expected to be noticeable until about 2018 because of the delaying effect of temperature trends in the stratosphere, and the time required for ozone depleting chemicals to drop sufficiently in concentration. At present, the variability in the size of the ozone hole from year to year is dictated primarily by meteorological conditions influencing the polar vortex. Aside from the ozone hole over Antarctica, there are smaller ozone loss episodes in the Arctic during the spring in some years, and a general reduction of global ozone outside the tropics of approximately 2 per cent.
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