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Ozone layer destruction

Ozone layer, UV penetration, effects on plants and animals, action

Alice Thompson

Ozone layer
The production and destruction of ozone

The natural production of ozone
The natural destruction of ozone

Concern about ozone

UV penetration and its effects
UV effects on plants
UV effects on humans
UV effects on other species
UV and aquatic animals

The Montreal protocol
The Australian response
What individuals can do

 

Ozone layer

The term ‘ozone’ is derived from the Greek word ozein, which means ‘to smell’. It was first used in the middle of the nineteenth century to describe the gas responsible for the pungent smell in the air which is particularly noticeable after thunderstorms. It was in the late 1800s that scientists suggested that ozone may form a layer in the upper atmosphere, forming what has come to be called the ozone layer.

It is now known that up to 90 per cent of all atmospheric ozone is in the stratosphere, which is the zone of the atmosphere between, on average, 15 and 25 km above the surface of the Earth. Here it is mostly in concentrations below 10 parts per million by volume (ppmv). This stratospheric ozone absorbs significant amounts of incoming ultraviolet radiation from the sun, protecting living organisms on the Earth’s surface from its harmful effects.

Some ozone is also present in small amounts in the lowest zone of the atmosphere, the troposphere. There has been a significant increase in tropospheric ozone since the preindustrial times as a result of human activities, and because it acts as a greenhouse gas it contributes to global warming. Ozone in the lower troposhere is a component of photochemical smog in urban areas, where it may be toxic to humans and other organisms, including some plants.

 

The production and destruction of ozone

The ozone layer in the stratosphere is produced through a process known as ‘photodissociation’ in which oxygen (O2) filtering up from the top of the troposphere is transformed, under the influence of sunlight, to ozone (O3) (see Box 1). Although ozone is produced (and destroyed) at all altitudes, the process of dissociation of oxygen is greatest above the equator and the tropics, simply because that is where solar radiation is strongest. However, as a result of atmospheric circulation patterns, the highest concentrations of ozone are above the North and South Poles.

There are also processes that destroy ozone, preventing the overall loss of oxygen and build up of ozone in the atmosphere over time. Under the influence of ultraviolet radiation some ozone molecules photodissociate and break apart into ‘ordinary’ oxygen (O2) and a single oxygen atom (O) (Box 2).

Ozone is thus constantly being produced and destroyed in the stratosphere in a dynamic balance. The total amount of the gas in the stratosphere has remained more or less constant over a very long period of time, and it is estimated that photodissociation creates about one gigatonne of ozone annually, and that around one gigatonne is destroyed every year through natural processes.

Human activities are now interfering with the balance of these reactions by increasing the rate of ozone destruction, while the rate of stratospheric production remains the same. The result is a progressive and significant decline in the concentration of stratospheric ozone.


Box 1

The natural production of ozone

Ozone is formed when a molecule of ‘ordinary’ oxygen (O2) breaks up into two free oxygen atoms under the influence of ultraviolet radiation (with wavelengths below 242 nanometres), a process known as photodissociation:

O2 + UV radiation → O + O

Solitary atoms of oxygen are rare because they are highly reactive chemically, and tend quickly to recombine with other molecules.

Most oxygen atoms that result from photodissociation rapidly recombine to form new oxygen (O2) molecules, establishing an equilibrium between the breaking up of oxygen molecules and the recombining of atoms into oxygen molecules. Occasionally, a free oxygen atom produced by photodissociation combines with an oxygen molecule (O2) to form ozone (O3), but this requires the involvement of another molecule – a mediator or ‘third body’ molecule – to take up the energy released in the reaction. This molecule is usually nitrogen (N2), but many other molecules can play this role as a catalyst for this reaction, so it is convenient to label it M:

O2 + O + M → O3 + M

While the main outcome of this process is the production of ozone, these reactions provide enough energy to the molecule M to cause it to move faster, thus warming the surrounding atmosphere.

 

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Box 2

The natural destruction of ozone

There are also processes that destroy ozone, preventing the overall loss of oxygen and build up of ozone in the atmosphere over time. Under the influence of ultraviolet radiation some ozone molecules photodissociate and break apart into ‘ordinary’ oxygen (O2) and a single oxygen atom (O):

O3 + UV radiation → O2 + O

Most of the single atoms of oxygen produced in this reaction recombine with molecular oxygen (O2) to form ozone again. However, some of them combine with ozone molecules to form two oxygen molecules:

O3 + O → O2 + O2

However, the most significant loss of ozone is through catalytic cycles in which, in the presence of a mediator or ‘third body’ molecule, ozone is converted to molecular oxygen.

 

The catalytic substance could be a free radical of the nitrogen, hydrogen, chlorine or bromine families. For example:

NO + O3 → NO2 + O2
and
NO2 + O → NO + O2

With the net effect:

O3 + O → O2 + O2

 

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Concern about ozone

Concern that human activities may alter the concentrations of ozone in the stratosphere was first raised in the late 1960s, following proposals for fleets of Supersonic Transport Aircraft (SST) that would fly higher than current aircraft, releasing hydrogen and nitrogen compounds directly into the stratosphere at rates. Initially, the main impact was envisaged as climate change due to the effect of vapour trails on the penetration of ultraviolet (UV) radiation, but it was soon realised that emissions from SSTs had the potential to destroy ozone and so reduce its concentration in the stratosphere.

A report was ordered by the US Congress in 1971 to investigate the scientific evidence for the likely impact of the proposed SST fleet on the environment. This three year research program concluded that a fleet of 500 Boeing SSTs would cause a 16 per cent reduction in stratospheric ozone in the northern hemisphere and a loss of around 8 per cent in the southern hemisphere. But by the time the final report was released, programs to develop SST fleets had already been scrapped due largely to economic considerations.

Chlorofluorocarbons - Chlorofluorocarbons (CFCs) were the next compounds to figure prominently in the history of concern about the ozone layer. They are a class of synthetic chemicals invented in the late 1920s that have multiple industrial and commercial applications. They are non-toxic, non-corrosive and chemically relatively inert. By the 1970s they were widely used for refrigerant, solvent and other purposes, although over 75 per cent of the CFC emissions came from spray cans in which they were used as propellants.

In 1974 Mario Molina and Sherwood Rowland suggested the possibility that CFCs might be an important source of chlorine that could serve as a catalyst to destroy ozone in the stratosphere. Their work sent shock waves through scientific and political communities and gave rise to a highly contentious debate, especially since the hypothesis was based on very few hard data and only limited laboratory observations.

The debate stimulated further scientific research, and in 1979 NASA published a report that highlighted the significance of the CFC-ozone link and predicted an eventual loss of 16.5 per cent of stratospheric ozone. NASA suggested a degree of urgency about the CFC issue. However, disagreements within the scientific and political communities initially impeded the development of effective national policies and international negotiations addressing these concerns.

There are certain other substances entering the atmosphere as a result of human activities that also contribute to the breakdown of ozone in the stratosphere. Methyl bromide and the halons are particularly important. Methyl bromide is used as a biocide for the control of insect pests in the soil and in grain products, and the main source of the halons is fire extinguishers.

The ozone hole - It was in the early 1980s that the first conclusive evidence for the loss of stratospheric ozone was produced. A group of scientists, members of the British Antarctic Survey group, had been measuring ozone levels above Halley Bay for almost 25 years. During the spring of 1981, they noted significant decreases in ozone levels over the Bay, although these levels recovered a few months later. The same phenomenon occurred the following year, but to a greater extent. Unsure of the significance of these data, the group did not publish their findings at the time. They believed their equipment might have been faulty. However, new equipment was installed and measurements the following year from the new apparatus still showed low springtime ozone levels. These results were finally published in 1985, describing a loss of almost half the ozone over Antarctica during the spring months, with full recovery several months later.

The ozone hole over Antarctica has continued to occur seasonally every year since its discovery, but the early 1990s saw particularly pronounced losses of ozone. Record low levels were measured in 1992-1993, when the total column of ozone over the South Pole was reduced by around 60 per cent. These unusually low levels are partly attributable to the eruption of Mt Pinatubo in 1991, which resulted in the release of sulphate aerosols which increased the destruction of ozone by chlorine and bromine. Although the size of the ozone hole plateaued from the mid-1990s, as international agreements reducing emissions of ozone depleting substances came into effect (see below), the hole then covered around 26 million square kilometres – an area nearly three times as large as Australia.

The 1999 Antarctic ozone event was the second largest and strongest to that time, when low ozone values occurred over an area greater than 10 million square kilometres for 98 days.

It has been estimated that, despite increased control of emissions of ozone-depleting substances, a substantial hole in the ozone over Antarctica will continue to appear each spring for several more decades, as atmospheric concentrations of chlorine and bromine slowly return to the levels of the 1970s.

Other ozone losses - Recent measures of stratospheric ozone have suggested that the ozone layer over the Arctic is also depleted at certain times of the year, but not as dramatically as over Antarctica.

In addition to the ozone hole above Antarctica, ground-based and satellite measurements have revealed a general thinning of stratospheric ozone over much of the globe during the past few decades. On average, the concentration of stratospheric ozone has decreased by around 4-5% per decade in both the northern and southern hemispheres. Cumulative losses of around 10% during winter and spring, and 5% during summer and autumn, have been noted over locations in Europe, North America and Australia.

The future of the ozone layer is very uncertain. Soon after the turn of the millennium it was thought that global ozone loss had peaked in the late 1990s, and was responding to the reductions in ozone-depleting substances resulting from international compliance with the Montreal Protocol and its Amendments and Adjustments (see below). This led to predictions that the ozone layer would be back to normal by the middle of this century. However, in 2005 the hole over the Antarctic was the biggest ever (29.5 million square kilometres), and recently significant loss of stratospheric ozone has been recorded in the northern hemisphere.

The situation is indeed extremely complicated. For example, while it seems clear that there has been a substantial decline in the concentrations of the ozone-destroying pollutants, there has also been a significant increase in the formation of ice clouds in the stratosphere in winter time in the northern hemisphere, which make it easier for the chemical reactions that destroy ozone to take place. These clouds may be the result of global warming due to the enhanced greenhouse effect.

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UV penetration and its effects on living systems

Evidence for the loss of stratospheric ozone has generated a great deal of concern throughout the international scientific community, because any thinning of the ozone layer will result in increases in UV radiation at ground level. It has been estimated that for every 1% loss of stratospheric ozone, there is a 2% increase in the amount of UV radiation allowed through the atmosphere.

Increases in levels of clear sky UV radiation were detected in both hemispheres over the period 1979-1993, a trend consistent with the gradual thinning of the ozone layer over this period. These increases in radiation were greatest in the southern hemisphere. A NASA study found an average increase in damaging UV radiation of around 5% per decade, with maximums of nearly 10% in South Chile and Argentina.

UV effects on plants

In field and laboratory experiments it has been found that increasing the exposure of plants to UV-B (wavelength 320 nm - 290 nm) disrupts photosynthesis, reduces growth rates and increases susceptibility to disease. These studies have also suggested that increased exposure to UV-B may alter the functioning of plants through decreasing leaf expansion and stem elongation, altering flowering time and changing chemical composition.

In non-agricultural ecosystems, like grasslands and forests, increased UV-B radiation is likely to have a number of detrimental effects, including interference with the timing of flowering and pollination, and changes in the competitive balance of species. Trees are especially vulnerable to exposure to UV-B when they have been previously weakened through disease, predation, drought or acid rain.

The impacts of increases in UV radiation on agricultural yields and forest growth are not yet clear, because of deficiencies in data collection and uncertainty about the influences of climates and micro-climates and of air pollutants. But experiments with several hundred plant species and crop strains indicate that as many as two thirds are sensitive to UV radiation. The most sensitive types include peas, beans, melons, cabbages and mustard, although growth was also adversely affected in maize, rye and sunflower seedlings.

There are uncertainties surrounding the combined impacts of increased UV exposure and global warming on plant systems.

UV effects on humans

Increased UV levels as a result of the thinning of the ozone layer would have major impacts on humans.

One estimate suggests that a 1% decrease in stratospheric ozone is likely to lead to a 2-5% increase in incidence of skin cancers.

For Australia, which already has one of the highest rates of skin cancer in the world, the changes that have taken place in UV radiation are predicted to lead to a 3-4.5% increase in non-melanoma skin cancer (compared to the projected global increase of 2-2.5%). Estimates for malignant melanoma are less certain, although they are projected to increase over the next few decades. They have increased worldwide in white populations by 3-7% since the early 1960s

Basal cell carcinoma (BCC), is predicted to increase by 13-15% in the northern hemisphere and by up to 20-30% in the southern hemisphere. The increase in the less common squamous cell carcinoma (SCC) is likely to be double that of BCC.

Our eyes are constantly absorbing ultraviolet radiation  Excessive UV exposure can cause serious damage to ocular cells, leading to melanomas of the eye or blindness. Exposure to UV-A (wavelength 400 nm - 320 nm) has also been associated with the formation of cataracts. It has been estimated that for every 1% depletion of stratospheric ozone, cataract incidence would increase by 0.6-0.8%. Overall, a 10% reduction in stratospheric ozone could result in an additional 1.75 million cases of cataract a year world wide.

Research in humans and other animals has demonstrated that UV radiation, even at normal levels, can lead to suppression of the body’s immune response, both locally and systemically, and hence reduce resistance to infectious disease. It is also thought that increased UV radiation may reduce the effectiveness of vaccinations.

UV and other species

The effects of increased UV radiation on other terrestrial animals are predicted to be similar to those experienced by humans, with high levels of UV radiation causing damage to the chromosomes and DNA replication. Current evidence suggests a significant increase in eye complaints of livestock, notably corneal irritations (pink eye), cataracts and eye cancer. Other predicted effects include changes in immune functioning, and even in reproduction.

 

UV and aquatic species

Much of the concern about increased ground levels of UV radiation, especially UV-B, has focused on the likely effects on phytoplankton. Commonly referred to as ‘the grass of the sea’, phytoplankton are the largest single group of photosynthetic primary producers. They are found close to the surface in all aquatic environments and they form the base of aquatic food webs. Phytoplankton make up 75% of the marine plant mass and are the main food source for most marine life, as well as being important producers of oxygen.

Many hundreds of types of phytoplankton exist, and they vary in sensitivity to UV exposure. However, most of them are vulnerable to damage by increased UV radiation because they live close to the water’s surface. Studies have shown that UV radiation can inhibit the movement and reproduction of phytoplankton, bleach their cellular pigments and impair their capacity to photosynthesise.

It has been estimated that a 10% decrease in marine phytoplankton population would result in a decrease in the oceans’ annual carbon uptake by around five gigatonnes (i.e. 5,000 million tonnes), which is about equal to annual CO2 emissions from fossil fuel use by humans.

Increased UV-B radiation is also likely to interfere with reproduction in amphibians and fish, the eggs of which are generally UV sensitive. Increased UV exposure can also damage fish, shrimps, and other marine life in the early stages of development.

Wildlife in Antarctica is already starting to show the effects of ozone depletion, with the embryos of limpets and starfish and other invertebrates failing to develop properly due to the higher levels of UV experienced during the annual springtime ozone depletion event.

The Montreal protocol

The discovery of the hole in the ozone layer over Antarctica during the mid-1980s, coupled with the growing body of scientific evidence demonstrating the role of human activities in this development, gave rise to serious concern at the international level. It was recognised that the thinning of the ozone layer represented a very real and serious threat to humans and natural systems, calling for immediate action on a global scale.

This concern led to an international treaty, the Vienna Convention for the Protection of the Ozone Layer of 1985, through which governments around the world committed themselves to the protection of the ozone layer and agreed to cooperate in scientific research aimed at improving understanding of the processes behind the ozone depletion. Further negotiations resulted in the development of the Montreal Protocol on Substances that Deplete the Ozone Layer, which was finalised in September 1987. This Protocol set mandatory targets for phasing out the production and consumption of ozone depleting substances. It was originally ratified by 57 countries, but has since been signed by over 160 countries, including Australia, all of which are committed to taking steps to actively reduce, and eventually cease, the production, use and emission of ozone depleting substances.

Box 3

The Australian response

Australia’s commitments to the Montreal Protocol are met through complementary legislation and controls enacted by Commonwealth, State and Territory Governments. The Australian Strategy for Ozone Protection, written in 1989 and revised in 1994, outlines the national approach.

The Department of the Environment, Water, Heritage and the Arts is the Commonwealth agency presently responsible for the coordination of national ozone protection measures and the administration of the Ozone Protection Act 1989. This department controls the manufacture, import and export of ozone depleting substances, the issuing of licences permitting these activities and the prosecution of breaches of the Ozone Protection Act.

State and Territory Environmental Protection Agencies (EPAs) and environment departments are mainly responsible for controlling the sale and use of ozone depleting substances, but also ensure proper training and accreditation of people who service equipment containing such substances.

 

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In the years since the Montreal Protocol was first ratified, improvements in scientific understanding of the extent and mechanisms of ozone depletion and advances in ozone-benign technology have led to the strengthening of control provisions and to the acceleration of phase-out dates for several ozone depleting substances. Other ozone depleting chemicals have since been recognised and added to the list of controlled substances that the Protocol aims to reduce and eventually eliminate. These findings have been taken into account at a number of international Conventions held from 1990 onwards.

The Convention of London in 1990 resulted in the establishment of a Multilateral Fund that develops and implements cost-effective mechanisms to assist developing countries meet their Montreal Protocol commitments. A key component of this fund supports the transfer of ozone-friendly technology from developed to developing countries. Australia’s contribution to this fund involves financial contributions as well as undertaking a number of bilateral projects in developing countries.

As a consequence of these international agreements, emissions of ozone depleting substances arising from human activities are being reduced, and it is hoped that they will eventually be eliminated, allowing the recovery of the ozone layer by around 2065.

HCFCs and HFCs - Hydrochlorofluorocarbons (HCFCs) and Hydrofluorocarbons (HFCs) are currently the most popular replacements for CFC chemicals, and have shorter atmospheric lifetimes. Although the ozone depleting potential of these compounds is much lower than that of CFCs, they still have the capacity to cause some destruction of ozone because of the traces of halogens they release into the atmosphere. Increased use of these substances could result in significantly greater contributions to stratospheric ozone depletion than currently suggested. They are also potent greenhouse gases. There is currently little information on the potential toxicity of replacement chemicals, or of the safety of by-products of chemical processes used in the production of CFC replacements.

There has been a substantial drop in the use of methyl bromide as a biocide, although the United States and some other countries are still using it for certain ‘critical’ uses.

What individuals can do

Individuals can play their part in the reduction of emissions of ozone depleting substances like CFCs and halons in various ways.

Leakages of CFCs from car air conditioners should be minimised. This can be done by running the car air conditioning for 10 minutes every week to keep seals lubricated, and to reduce cracking and leakage. When these air conditioners are serviced it is important to check that the refrigerant is recycled.

CFC emissions from refrigerators can also be reduced. Old refrigerators should be taken to CFC recycling depots or to an accredited service person to remove the refrigerant prior to the disposal of the appliance. It is important that air conditioners and refrigerators are serviced by persons accredited with the CFC Registration Board.

To reduce halon emissions, BCF fire extinguishers, which contain halons, should be replaced. Fixed halon fire suppression systems in buildings must also be phased out and replaced. All of these must be taken to an authorised storer of halons.

 

Further reading

For further information on the thinning of the ozone layer see:

(1) The Australian Academy of Science’s Nova website: www.science.org.au/nova/004/004box04.htm

(2) http://www.epa.gov/Ozone/strathome.html

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Born and raised in Canberra, Alice Thompson was brought up with an appreciation of, and interest in the environment, leading her to study at the Australian National University, majoring in Geography/Human Ecology and Population Studies, and her involvement in the Nature and Society Forum (NSF). She now lives in Sydney where she currently pursues a career in Government working for the NSW Office of the Australian Bureau of Statistics (ABS). Before joining ABS Alice was employed by NSF as a Research Officer to prepare reports on important ecological issues in Australia.