Feature

Ozone -- The Wider Picture

The Antarctic Ozone Hole engenders almost as much public and environmental concern as the threat of global warming. Is it time for New Zealanders to pack up and move further away from the pole?

By Christopher Dee

We've had a few years to get used to the unsettling thought that in southern climes there is a lot less ozone than there used to be. However, in spite of education campaigns and frequent media reports, there is much that is misunderstood about the phenomenon.

The basic facts have been well publicised. Ozone in the planet's stratosphere has a major role to play in preventing ultraviolet (UV) radiation from reaching the Earth's surface. Even "normal" ozone levels permit enough destructive UV-B through to cause sunburn, melanoma and other biological damage. If ozone levels above New Zealand were reduced substantially, the consequences to crops, livestock, native species and ourselves would be severe.

Since 1985, scientists have reported a substantial reduction in expected ozone levels in the lower stratosphere over Antarctica in the springtime. Satellite records have allowed them to trace this reduction back as far as 1978. Atmospheric sampling and laboratory experiments have identified a likely culprit -- raised levels of stratospheric chlorine resulting from long-term emissions of man-made gases such as chlorofluorocarbons (CFCs) and carbon tetrachloride.

The 1990 springtime hole defied a pattern noticeable until now in which even-numbered years tended to have less thinning than odd-numbered years. This year's Antarctic hole was more pronounced than ever before.

That's the simple story. It does leave a lot of questions unanswered --  how and where does ozone form? Why is it destroyed? Why is there a big hole in Antarctica and not elsewhere? Does this hole affect New Zealand? Could a new hole form over our heads?

To answer these questions, we need to catch up with the current state of research into ozone and the Antarctic hole.

Greater Complexity

The most striking thing about stratospheric ozone is how much more complex it is than generally perceived. It is not a static layer of uniform thickness covering the globe like some kind of blanket. Instead it is a tenuous band of gas being continuously created and destroyed in the same process by which it traps ultraviolet.

Most ozone is created over the equator, where massive weather systems and atmospheric circulation immediately force it out towards the poles. This means that as you move south from the equator, you are actually moving from a relatively sparse level of ozone to progressively higher concentrations until these peak at around 55S, just south of the Auckland Islands.

At a given latitude, ozone levels can vary significantly on a seasonal, daily and even hourly basis. The major driving force is the level of sunlight -- the creation of ozone from oxygen molecules is driven by energy obtained from ultraviolet light. This means that ozone levels tend to follow a regular seasonal pattern. Further, atmospheric circulation or local weather conditions can lead to a notable short-term loss of ozone in a region. For example, springtime ozone levels in the Antarctic have always been lower than at other times of year -- observations starting in 1956 show this quite clearly.

Consequently, measurements intended to detect ozone depletion have to be made over a long period of time and must also allow for natural seasonal variations. This makes it harder to separate out a trend from normal variability, but there is no question that recent springtime ozone levels in Antarctica have been steadily decreasing from the long-term average established since the late 1950s.

Chlorine and Ozone

Ozone is subject to attack by other chemicals in the atmosphere, most notably chlorine. A single chlorine atom acts as a catalyst in the conversion of ozone to oxygen. The more reactive chlorine there is in the stratosphere, the more likely it is that some ozone depletion will occur.

Usually this reaction is limited by a number of factors, including the presence of nitrogen compounds which convert reactive chlorine to non-reactive forms, and the need for a "reaction surface" such as ice crystals. This means that even high chlorine levels in the stratosphere do not necessarily lead to a major reduction in ozone levels under normal conditions, although observations suggest that some depletion (5-10%) has occurred over New Zealand and elsewhere during the last ten years.

So why does a major hole form over Antarctica? Enter Polar Stratospheric Clouds (PSCs). These consist of clouds of ice crystals which form at temperatures below -80^oC. For this reason, they are essentially an Antarctic winter phenomenon, although it is thought that "freak" low years could lead to their formation in the Arctic as well. PSCs work against ozone in two ways. They remove water and nitrogen compounds from the stratosphere, both of which normally serve to limit the amount of chlorine reacting with the ozone. At the same time, they provide ideal reaction surfaces for a rapid and catastrophic destruction of ozone in the lower stratosphere.

Although the PSCs themselves form in winter, the reaction requires energy to drive it. Hence, the hole itself does not start to form until the Antarctic springtime, being detectable from the first return of sunlight in early September. A side-effect of the loss of ozone is a drop in stratospheric temperature, which might serve to enhance the formation of PSCs and hence worsen the depletion still further. Certainly the Antarctic loss is sudden and dramatic, quite unlike seasonal variations elsewhere.

The depletion is compounded by a vortex phenomenon which effectively isolates the air over Antarctica from the rest of the planet and prevents an influx of ozone from points further north. The vortex starts to break up in late October and by January Antarctic ozone levels are back to normal. This suggests that the long-term cycle of ozone depletion in springtime is almost certainly dependent on the formation of the vortex.

The key point about observations made in the past twelve years is not that springtime ozone depletion occurs at all, but that the level of depletion has increased markedly. In 1990, almost all the ozone in the Antarctic's lower stratosphere disappeared.

Chlorine and CFCs

If chlorine is the main culprit, where is it all coming from? Our atmosphere has a natural level of chlorine derived from the life-cycle of marine organisms. Researchers estimate that this natural level is 0.6 parts per billion (ppb). It is possible that this naturally-derived chlorine was behind the regular Antarctic springtime depletions noted since observations first began.

The current level of chlorine detectable in the stratosphere is far higher -- more than 3.5ppb. The extra chlorine is thought to come from a variety of man-made sources, most notably chlorofluorocarbons (CFCs). These non-toxic, chemically inert, non-flammable gases were first introduced by Du Pont in the 1930s but did not attain widespread use until the 1950s. They have an enormous range of applications, including use in refrigeration systems, air conditioners, fire extinguishers and aerosols.

If they were more reactive, CFCs would not survive long enough to make it up to the stratosphere in any significant quantity. Because they are chemically inert, they do eventually reach that level, where they are finally broken down by UV into components which include reactive forms of chlorine. Sampling of the stratosphere over the years has shown a steady increase in CFC levels and a corresponding increase in chlorine concentrations.

While other agents do play a role, it seems quite clear that CFCs are a primary source of stratospheric chlorine. Worse, because they persist in the atmosphere for a long time, continuing rises in chlorine concentrations are likely even though CFC use is now restricted and will eventually be eliminated. The degree of persistence also means that CFCs have plenty of time to spread evenly throughout the atmosphere in both hemispheres, even though the major proportion of CFC production and use has been north of the equator.

Other Holes?

If a major depletion can occur with such clockwork regularity in Antarctica, are ozone holes going to become a problem elsewhere? Under normal conditions, the answer is no. However, there are at least two conditions which might lead to rapid ozone loss outside Antarctica.

The first is the formation of low-temperature PSCs. These are not common outside Antarctica but Dr Tom Clarkson of the Meteorological Service suggests that abnormally cold years in the northern hemisphere (such as occurred in 1967 and 1976) could lead to PSCs being formed in the Arctic. If so, the positive feedback resulting from ozone loss leading to lower temperatures leading to more ozone loss could result in the formation of a major hole.

A second threat is that of a large volcanic eruption. Volcanoes typically eject large quantities of sulphur which can readily reach the stratosphere in a major explosion. Once there, the sulphur forms sulphuric acid droplets which can take on the same role as ice crystals in the Antarctic hole -- as surfaces on which the ozone-depleting reactions can take place.

There are two factors which might serve to lessen the effects of these circumstances. Unless an isolating vortex is formed, natural atmospheric mixing would prevent a dramatic loss confined to one area and instead produce a general thinning. Also, it is possible that the winter-long removal of nitrogen compounds that occurs in the Antarctic is a necessary pre-requisite to formation of a major hole. Without this, even a volcanic eruption might have only a short-term effect.

What is clear is that, given elevated chlorine levels all over the world, the potential for radical ozone loss is higher than it was twenty years ago. As for the consequences in the affected region, they could range from lower crop yields and higher melanoma rates to devastating destruction of whole ecosystems if a hole persisted.

What About New Zealand?

In the absence of major holes forming outside Antarctica, we have a lesser but still significant problem. As mentioned earlier, ozone records kept since the 1960s suggest that New Zealand has experienced an ozone loss of 5-10% in the past ten years.

It is not yet clear whether this is directly related to the Antarctic hole. Some of the depletion is caused by the 11-year sunspot cycle which is currently at its peak, but this accounts for less than half of the loss.

The remaining depletion could result partially from the effects of compounds dispersed over New Zealand when the hole breaks up towards the end of each year. Of more significance is a comparable reaction which takes place in the upper stratosphere rather than the lower stratosphere. The same chemical agents are responsible but the reactions involved are different and the rate of depletion is far slower.

Even so, we have cause for concern. Research by Dr Richard McKenzie at DSIR Lauder in Central Otago indicates that for each 1% loss of ozone cover, about 2% more damaging UV-B will reach the Earth's surface. Other figures suggest that this might lead to an increase in non-melanoma skin cancers of anywhere between 1% and 10%, and an increase in melanoma of between 0.35% and 0.9%.

While this increase is important, New Zealand had rising melanoma rates long before ozone thinning was observed. This rise is attributed to the sun-seeking habits of the past few decades. Although New Zealand's average ozone levels are much higher than at the equator, our relatively clear air and lack of cloud cover mean that UV exposures are high by world standards.

The NZ Cancer Society's "cover up" campaign is focused on the hottest months of December to February. These partially coincide with the lowest ozone months of February and March. In the long term, covering up is far more likely to reduce ultraviolet exposure and improve health than any amount of international restrictions on CFC usage. However, the danger of a major hole forming outside the Antarctic is serious enough to warrant intensive research and far more active steps to reduce CFC usage than have been taken to date.

Ozone Research in New Zealand

New Zealand has an important role to play in ozone research because of our proximity to Antarctica and our position in the southern hemisphere, where there are relatively few monitoring stations.

Among the research programmes currently under way are ultraviolet (UV) measurement programmes, both here and in Antarctica, analysis of trace gases in the atmosphere and studies of the effects of UV on building materials and human health.

Polar weather systems are being investigated in order to find out more about the spring vortex phenomenon, polar stratospheric clouds and the role these have in promoting ozone depletion.

Antarctic mosses are being studied to try to determine past UV levels. Attempts have been made to determine how far UV can penetrate through sea ice, as this may affect the lifecycles of sea organisms such as plankton.