Ask The Scientist

Frequently Asked Questions

What is ozone, and why is it important?

Ozone is simply a molecule consisting of 3 oxygen atoms, which reacts strongly with other molecules. Ozone is created in the stratosphere when high energy uv radiation causes on O₂ molecule to split. The free oxygen atoms collide and react with other O₂ molecules to form O₃.

Production is highest where the solar uv is the greatest eg near the tropics, but once created, the ozone is then circulated towards the poles by the atmosphere. The amount of ozone in the stratosphere can vary with location, season and even day to day climatic conditions.

The process of ozone creation is what makes the O₃ in the atmosphere very effective at shielding the Earth from harmful uv radiation, which can cause many biological problems, such as skin cancer. However, due to its high reactivity, the uv found in the tropospher at ground level can aslo be dangerous as a toxic pollutant which is harmful to plants and lung tissue, and is a major cause of smog.

When was the ozone hole discovered?
The discovery of the annual depletion of ozone above the Antarctic was first announced in a paper by Joe Farman, Brian Gardiner and Jonathan Shanklin which appeared in Nature in May 1985. Later, NASA scientists re-analyzed their satellite data and found that the whole of the Antarctic was affected.

Why does the ozone hole form over Antarctica ? 

The answer is essentially 'because of the weather in the ozone layer'. In order for rapid ozone destruction to happen, clouds (known as PSCs, Stratospheric Clouds Mother of Pearl or Nacreous Clouds) have to form in the ozone layer. In these clouds surface chemistry takes place. This converts chlorine or bromine (from CFCs and other ozone depleting chemicals) into an active form, so that when there is sunlight, ozone is rapidly destroyed. Without the clouds, there is little or no ozone destruction. Only during the Antarctic winter does the atmosphere get cold enough for these clouds to form widely through the centre of the ozone layer. Elsewhere the atmosphere is just too warm and no clouds form. The northern and southern hemispheres have different 'weather' in the ozone layer, and the net result is that the temperature of the Arctic ozone layer during winter is normally some ten degrees warmer than that of the Antarctic. This means that such clouds are rare, but sometimes the 'weather' is colder than normal and they do form. Under these circumstances significant ozone depletion can take place over the Arctic, but it is usually for a much shorter period of time and covers a smaller area than in the Antarctic.

How long has the Antarctic ozone layer been studied?
Ozone was first measured from British Antarctic stations during the International Geophysical Year of 1957/58. It was originally studied because of its influence on the temperature structure of the atmosphere, and also as a tracer for the circulation of stratospheric air. In the 1970s, ozone became the focus of attention as a possible indicator of long-term changes in the atmosphere. Scientists realised that ozone might be affected by the increasing concentration in the atmosphere of man-made gases such as nitric oxide and CFCs. In 1995, Paul Crutzen, Mario Molina and Sherwood Rowland received the Nobel prize for this pioneering work.

Does the ozone hole affect the rest of the world?
At the moment, catastrophic ozone depletion is only seen in the Antarctic during the spring, but surrounding areas experience lowered ozone levels as the ozone hole decays at the end of the spring. As the ozone hole rotates, it may extend over populated areas for a short while when it is very elongated. For example it covered the tip of South America and the Falkland Islands for over a week in October 1994 (Fig. 12).

Limited ozone depletion can occur above the Arctic, but at present it is confined to parts of the region and only lasts for a few days at a time. If CFC releases had continued at the high rates of the mid 1980s, a continental sized ozone hole might have appeared over the Arctic. Elsewhere in the northern hemisphere, stratospheric ozone amounts over temperate latitudes have fallen by 5 to 10% during the winter.

How does the ozone hole damage living things?
All living cells, whether microbes, plants or animals, contain a complex molecule called DNA which carries the genetic code. This is the set of instructions which describes the structure and biochemistry of an organism. Unfortunately, DNA readily absorbs high-energy UV-B radiation and becomes damaged so that the instructions cannot be read properly. If the amount of UV-B entering the cell increases (as during the ozone hole), the risk of damage also increases and may result in malfunction or death of the organism. Some Antarctic organisms such as algae, lichens and mosses also contain a pigment called chlorophyll. This absorbs visible light as the energy source of photosynthesis for making organic compounds. Chlorophyll also absorbs UV-B light so that the system becomes bleached and non-functional. Even enzymes and other proteins are damaged by this high-energy radiation. Living organisms therefore have to protect themselves from UV-B. Humans can cover their skin with artificial sunscreens, but natural protection systems have also evolved. Many microbes, plants and other animals synthesize protective pigments. Our skin cells synthesize brown melanin to protect against sunburn (which is caused by UV-B radiation), and so do Antarctic lichens on rocks near the edge of the polar ice-cap. A variety of suncreen pigments are produced by Antarctic organisms on land, in freshwater and in the sea. That is why exposed, snow-free rocks are often covered with bright orange and yellow lichens. Some lichens and microbes even live inside translucent rocks to shelter from high radiation levels and desiccating winds!

Does the Greenhouse effect cause the ozone hole?
The Greenhouse Effect (producing global warming) and ozone depletion are two separate problems, however there are links between them. Warming at the earth's surface is caused by certain gases in the atmosphere which can trap energy from the sun. An increase in the amount of these gases produces an increase in the surface temperature. The largest increase is in carbon dioxide from burning coal, oil, gas and forests, but other gases such as methane (from cattle and rice fields) play a part. A link with ozone depletion is that CFCs are gases which also contribute to greenhouse warming.

A further link is that although the Greenhouse Effect warms the surface, it allows the higher atmosphere, where ozone is present, to cool. This means that more stratospheric clouds may form and so make the ozone hole worse.

Even if the problem of ozone depletion is solved, global warming will still remain. It will cause a rise in sea-level and change the regions where crops can be grown. The issue will be harder to tackle than ozone depletion, but is one which concerns everyone on our planet.

What is the Montreal Protocol?
The Montreal Protocol is an international agreement which was drawn up in September 1987. It originally aimed to half the use of CFCs by 1999. However, reviews of the protocol held in 1990 in London and 1992 in Copenhagen imposed more stringent controls, so that all production of CFCs, CCl4 and halons should cease by the year 2000. Many countries have even agreed to stop using CFCs before this deadline. Production of other ozone depleting gases is to stop in the early years of the 21st century. Unfortunately the ozone hole will not immediately disappear as CFCs are such stable gases that they will remain in the atmosphere for decades after release.

Independent reviews by panels of scientist (eg the UK Stratospheric Ozone Review Group reports) present conclusive evidence that CFCs are still increasing in the atmosphere and that chlorine from them is also increasing and is responsible for ozone depletion. Thanks to the provisions of the Montreal Protocol and its subsequent amendments the level of ozone depleting gases in the atmosphere will start dropping by the end of the 1990s.

Where were CFCs used?
CFCs were used in a wide variety of products. Thanks to public pressure the use of CFCs by the aerosol industry declined rapidly. The other major uses were "foam blowing" for upholstery padding, freezer linings, fast-food cartons, cavity-wall insulation, and as the fluid in refrigeration and air-conditioning systems. CFCs were also used as solvents in industrial and electronic cleaning processes. Halons (bromo-fluoro-carbons) were extensively used in fire extinguishing systems. CFCs were introduced because they were generally odourless, non-toxic, stable, non-flammable and compressible substances. It was their high stability which allowed them to get into the stratosphere where they were broken down to release active chlorine. The two simplest CFCs are CFC11 and 12 which have the chemical formulae CFCl3 and CF2Cl2.

Is the ozone hole recovering ?

 Some reports in the media suggest that the ozone layer over Antarctica is now recovering. This message is a little confused. Recent measurements at surface monitoring stations show that the loading of ozone destroying chemicals at the surface has been dropping since about 1994 and is now about 6% down on that peak. The stratosphere lags behind the surface by several years and the loading of ozone depleting chemicals in the ozone layer is at or near the peak. Satellite measurements show that the rate of decline in ozone amount in the upper stratosphere is slowing, however the total ozone amount is still declining. The small size of the 2002 ozone hole was nothing to do with any reduction in ozone depleting chemicals and it will be a decade or more before we can unambiguously say that the ozone hole is recovering. This assumes that the decline in ozone depleting chemicals continues and that there are no other perturbations to the ozone layer, such as might be caused by a massive volcanic eruption or Tunguska like event. It will be the middle of this century or beyond before the ozone hole ceases to appear over Antarctica. What we saw in 2002 is just one extreme in the natural range of variation in the polar stratosphere and is the equivalent of an extreme in 'stratospheric weather'. By contrast the 'weather' in 2003 moved to the opposite extreme and we saw one of the largest ozone holes on record.

How can we mend the ozone hole?
The only way to med the ozone hole is to stop releasing CFCs and other ozone depleting gases into the atmosphere. The restrictions of the Montreal Protocol and its extensions are helping to do this.