Ozone Layer

V

Discovery of the Ozone Hole

Since the mid-1980s the ozone layer over Antarctica has thinned dramatically during spring. This “Antarctic ozone hole” was first discovered in the late 1970s by a research group working for the British Antarctic Survey. (Satellite data would have revealed the ozone hole sooner, had not the data processing algorithm been set to reject such low values as mistakes in the satellite instrument.) The amount of ozone over Antarctica becomes very low in October, although the exact size and depth of the ozone hole varies from year to year due to the natural variability of the atmosphere. In fact, there isn’t really a “hole” in the ozone layer; ozone remains in the atmosphere both above and below the layer in which the chemical destruction of ozone occurs.

VI

Formation of the Ozone Hole

A combination of factors is needed to produce the large loss of ozone over Antarctica. A crucial first step is that the stratosphere over Antarctica becomes isolated by strong westerly circumpolar winds of up to 100 m/s (up to about 200 knots) during the polar night. The temperature drop is such that a special type of cloud, known as a polar stratospheric cloud (PSC), can form at temperatures below about -80° C (-112° F). Very fast chemical reactions occur on the surface of these clouds, converting inactive forms of chlorine to molecular chlorine (Cl₂). When sunlight returns in September, catalytic cycles involving chlorine atoms become active and destroy the ozone.

The amount of chlorine in the atmosphere dramatically increased through the use and release of chemicals known as chlorofluorocarbons, or CFCs (compounds of fluorine). First developed in 1930 by General Motors Research Laboratories as a safe replacement for refrigerants in use at the time, their chemical inertness also made them valuable in other areas of industry. Once released into the atmosphere, they were transported into the upper atmosphere where they were broken down by the much higher levels of ultraviolet. This is the only way in which CFCs released into the atmosphere can be destroyed. Almost all of the chlorine in the atmosphere is due to human activity.

No similar ozone hole has yet been seen in the Arctic because the meteorological conditions in spring are very different from those in the southern hemisphere and much warmer. However, there are chlorine molecules in the Arctic stratosphere, and on the occasions that temperatures do decrease enough to favour ozone depletion, chemical ozone destruction can also take place in the Arctic. According to the 1998 report on ozone depletion of the World Meteorological Organization, ozone had been particularly low over the Arctic during late winter and spring in six out of the previous nine years.

The most obvious danger from a reduction in the amount of ozone in the atmosphere is the increase in the amount of ultraviolet radiation reaching the surface, particularly the more dangerous UV-B. However, this must be considered in context. Springtime ultraviolet levels in Antarctica are still less than typical values in low latitudes such as Florida. The real danger is to local biological life. One concern is for the phytoplankton living in the surface water around Antarctica. These small organisms form a part of the important food chain. Other issues concerning the loss of ozone include induced changes in climate, discussed later.

VII

Efforts to Reduce Ozone Depletion

International efforts to attempt to limit the production and release of CFCs began once the role of CFCs in ozone destruction was established. In 1987 the United Nations Montreal Protocol was agreed and came into effect in January 1989. The countries that signed up to the protocol aim to phase out the use of CFCs globally. The main CFCs ceased to be produced by the signatories in 1995, and the European Union ceased using them in 1998, except for a very small amount in limited and essential uses such as medical sprays. Although the Montreal Protocol has been successful, it should be noted that without the subsequent amendments (London, 1990; Copenhagen 1992; and Vienna, 1995), recovery of the ozone hole would have been impossible.

The hydro-chlorofluorocarbons (HCFCs) were developed to replace CFCs. These gases can still damage ozone if they reach the stratosphere, but they are less likely to since their extra hydrogen atom allows them to be destroyed in the lower layers of the atmosphere. These gases are also controlled under the Montreal Protocol and were phased out after 2004. The gases that replaced both the CFCs and HCFCs are hydro-fluorocarbons (HFCs), which do not contain any chlorine atoms and so have no ozone depleting effect. Unfortunately, many of them are powerful greenhouse gases and could contribute to global warming if emitted in large quantities.

Since the CFCs have atmospheric lifetimes of about 50 to 100 years, and take 5 to 10 years to reach the upper atmosphere where they are broken down, the atmosphere reacts slowly to the cuts made in emissions of these gases. Stratospheric ozone should begin to increase as the amount of chlorine and bromine decreases. However, ozone is affected by changes in other gases, such as methane, temperature changes due to climate change, and also indirectly by particles from volcanic eruptions.

Compounds containing bromine, such as methyl bromide (mainly of natural origin) and the brominated CFCs (halons: used mainly as fire retardants), are also ozone-depleting chemicals. While the total amount of chlorine in the lower atmosphere peaked in 1994, and is now slowly declining, the total amount of bromine is still increasing. An assessment by the World Meteorological Organization in 1998 estimated that global and Antarctic ozone levels would return to pre-1980 levels by 2050, and in 2003 evidence suggested that the rate at which ozone is disappearing had indeed slowed down markedly, although estimates as to when ozone can return to a proper balance have now been revised to the latter half of the 21st century. However, many factors influence ozone, and future levels are not completely predictable.

VIII

Ozone and Climate Change

Although the ozone hole itself is a separate issue from the greenhouse effect, changes in the amount of ozone in the atmosphere do have an effect on climate change. Reductions in stratospheric ozone cause the lower stratosphere to cool (roughly about 0.6° C per decade from 1979 to 1994).