Our collective eco psyche is obsessed with global warming — climate change, melting polar caps and carbon emissions have infiltrated conversations about everything from the weather to cars and travel. And rightly so.

But what ever happened to the big bad climate threat of the 1980s?

Yip, not so very long ago the hole in the ozone layer was our biggest concern — aerosols were the enemy, SPFs on sunscreens shot up from two to forty and misguided teachers convinced students everywhere that the hole in the ozone layer was right above them.

Pretty scary stuff really. And then one day, everyone seemed to stop talking about it. There were mutterings about a weird weather system called El Niño and then suddenly global warming was the eco-fight du jour. Well, to the uniformed at any rate.

As it turns out, the ozone layer and global warming are not quite as unrelated as you might think and — here's the good news — we have been considerably more successful in dealing with that little ozone problem than we have been with the potentially cataclysmic problem of an overheated planet.

But before we get ahead of ourselves — and this is largely for those of us who grew up during and after the 1980s — a rough guide to the ozone layer and the depletion thereof is perhaps necessary.

The ozone layer

Simply put, the ozone 'layer' refers to the ozone in the stratosphere (10 to 50 kilometres above the earth's surface). Although the stratosphere occupies a much bigger space than the lower atmosphere (troposphere — zero to 10 km above earth), it contains so little gas that ozone is regarded as one of the trace gases in the overall atmosphere. In fact, if all the ozone in the stratosphere were compressed to the pressure of air at sea level it would only be a few millimetres thick.

At the earth's surface, ozone is a corrosive gas, commonly known as 'smog', which can lead to severe health problems and damage to plant life. In the stratosphere, however, it absorbs between 97 and 99 percent of the sun's high frequency ultraviolet rays (UVB and UVC) which are potentially damaging to life on earth.

A reduction in the amount of ozone in the stratosphere leads to an increase in the number of harmful UV rays which reach the earth's surface. The biological consequences of increased radiation include increases in skin cancer, eye damage, plants damage and the reduction of plankton populations (and subsequently all marine life).

Understanding the 'hole'

The thickness of the ozone layer is determined by the total amount of ozone in a column above any area. As a result of atmospheric circulation patterns and solar intensity, the 'thickness' of the ozone layer varies around the world and from one season to the next.

In general, the ozone layer is thicker (or higher) towards the poles and during the spring and thinner (or lower) closer to the equator and during autumn. However, the ozone layer is also affected by the production of certain chemicals.

The ozone 'hole', which is basically the severe depletion of ozone over the Antarctic, is a result of the combination of the specific weather conditions (polar stratospheric clouds) over the region and the production of ozone-damaging chemicals. The 'hole', which is seasonal and occurs in the late winter and spring, was first observed in the early 1980s. The severity of the 'hole' is such that it generally exceeds the size of the Antarctic continent.

However, the 'hole' is not the only concern when it comes to ozone depletion. There has also been a slow but steady depletion of the ozone layer since the early 1980s and it is now an average of four percent lower around the globe than it was then.

Catalysts for destruction

In 1974 it started becoming apparent that certain man-made chemicals contribute to the destruction of the ozone layer. The most damaging of these are chlorofluorocarbons (CFCs), halons and methyl bromide.

Until the 1980s, CFCs were used in air conditioning units, refrigeration, as aerosol spray propellants and in certain cleaning processes. Halons are an effective fire extinguisher and methyl bromide was and still is used extensively in pesticides.

The problem with these particular substances is that they are un-reactive in the troposphere and are therefore not easily dissolved in rain or snow. They accumulate in the atmosphere and, over a period of up to 15 years, make their way up into the stratosphere.

Once in the stratosphere, they are converted via ultraviolet light into reactive halogen gases (atomic chlorine and bromine) which react catalytically to destroy ozone. Because they work catalytically (that is, they can be used repeatedly in a reaction without being changed), a small amount can deplete a huge volume of ozone — one chlorine atom can destroy over 100 000 ozone molecules. It can take up to 100 years before the atomic chlorine or bromine makes its way out of the stratosphere.

Montreal Protocol

When it became apparent that certain chemicals were irrevocably destroying the ozone layer, moves were made to stem the production of these chemicals. The production of ozone-depleting gases was first regulated under the Montreal Protocol in 1987. This protocol and its numerous amendments, ratified by over 191 nations, established legally binding controls over the production and consumption of ozone-depleting gases.

The 1990 London Amendment, called for a phase-out of the production of the most damaging ozone-depleting substances by 2000 (developed nations) and 2010 (developing). This process was accelerated at the 1992 Copenhagen Amendment when 1996 was established as the cut-off for developed nations.

However, the protocol does have a couple of loopholes — exemptions include the production of chemicals necessary for public health (such as CFCs in asthma inhalers) and those instances where there is no technically or economically feasible alternative (such as methyl bromide in pesticides).

Hydrochlorofluorocarbons (HCFCs), which have replaced CFCs and are used for refrigeration, blowing foams and solvents, still contribute to halogen abundance in the stratosphere and a production phase-out has been set for 2040.

As a result of the Montreal Protocol, the abundance of ozone-depleting gases in the atmosphere has begun to decrease. With the exception of the Antarctic hole, the ozone layer has not grown significantly thinner since 1998. However, it will take until the second half of the 21st century (2060-2075) before the ozone layer will return to pre-1980 levels.

Back to global warming...

So, what does all of this have to do with global warming? Well, the ozone-depleting gases (CFCs and halons), which are controlled under the Montreal Protocol, are also greenhouses gases that are considerably more potent than carbon dioxide.

In fact, it is estimated that without the reduction achieved under the protocol, the amount of heat trapped by the atmosphere due to ozone-depleting chemicals would be twice as high as it is today (equivalent to approximately 10 years of growth in carbon dioxide concentrations).

The Montreal Protocol also bodes well for global warming because it provides evidence of the fact that if there is a strong enough collective will, the necessary environmental changes can be made.

Global warming, however, does not bode well for the ozone layer. Although scientists predict that the earth will become hotter with climate change, the stratosphere is expected to cool (a small cooling has occurred since the 1970s) and if the processes involved in the production of the Antarctic hole are anything to go by, this is not necessarily a good thing.

Useful sites

For more information on the ozone hole, visit one of these sites:

US Environmental Protection Agency
National Oceanic and Atmospheric Administration (USA)
South African Air Quality Information System

AFP