Ozone depletion

Something you don't understand? Look it up on the Environment and Energy Wiki Encyclopaedia (beta version)

The Environment and Energy Portal is now operational

What is ozone-depletion?
How bad is ozone depletion?
The effects of ozone-depletion
Conclusion
Further reading

What is ozone-depletion?

In the upper troposphere and lower stratosphere, from about 12 to 50 km altitude, much of the oxygen in the air is converted to ozone by the action of radiation from the sun. This is the so-called ozone layer. This is a continuous process of both the production and disintegration of ozone molecules, in a natural equilibrium. The total quantity of ozone at any one time is very small, but it does serve to stop intense ultraviolet radiation (this is the energy used to convert the oxygen to ozone) from reaching the earth's surface. Without the ozone layer, life on earth, as we know it, would be impossible.

This was realised in the 1920s by Dobson, who devised instruments for measuring the equivalent thickness of the layer, the first monitoring station being set up at Davos, Switzerland in 1924. In 1970, Paul Crutzen hypothesised that exhaust gases from stratospheric flying may upset the natural balance of the ozone layer. In 1974, Mario Molina and Sherwood Rowland added that certain chlorine-containing molecules, such as in chlorofluorocarbons, could also have a similar effect. (All three of these scientists shared the 1995 Nobel Prize for Chemistry for this work.) Many scientists were alarmed at this possibility, although others were opposed to the theories. In the late 1970s, a scientist, Joseph Farman, in the British Antarctic Survey at Halley Bay, observed that the ozone layer became thin in the Antarctic spring. This was not confirmed by Nimbus 7 satellite observations. Not believing his Dobson instruments, he obtained a new one and observed the same phenomenon in succeeding years. Consultation with NASA revealed that the Nimbus 7 instruments were calibrated to ignore abnormally low ozone levels as faulty readings. By sifting through the archives, it was found that the Nimbus 7 records, over several years, had shown ignored low levels over the Antarctic from September to October. Thus the so-called "ozone hole" was discovered. Farman published his results in 1985 and this was the first scientific evidence that the ozone layer was thinning to dangerous levels, vindicating the theoretical work of the other scientists.

This situation was very alarming, even though there was no formal scientific proof, at that time, that the ozone depletion theories were explaining the observations. The groundwork for international studies were laid at an ad hoc conference in Vienna, 1985, and this culminated in the Montreal Protocol being signed in September 1987, with a programme to phase down the emissions of what were believed to be ozone-depleting gases and to support intense scientific studies as to the causes and effects of the depletion. Results were quick to arrive. In the following year, the formal scientific proof that CFCs and similar gases were responsible was obtained. It was also shown that the situation was not only worse than had been imagined, predictions showed that the Protocol phase-down programme would not stop the depletion or even significantly slow it down. Later amendments introduced drastic measures to totally ban most of the gases in a very short time scale.

Nowadays, the mechanisms of ozone-depletion are very well known and the main culprits are halons and CFCs, plus a few other substances, used mainly for:

These uses represent over 90 per cent of the man-made ozone-depletion.

How bad is ozone depletion?

Very bad, and getting worse every year! The problem is that most of the gases and vapours that cause ozone-depletion take a long time (10 - 15 years) to homogenise throughout the atmosphere, but they can last up to 100 years for CFCs and 300 years for halons (to remove roughly half of them). Fortunately, one substance that was used very massively as a solvent, 1,1,1-trichloroethane, has an Atmospheric Residency Time of only 12 years, so that, as it was banned in most major using countries in 1996, the levels of this compound in the air are already measurably improving.

It should be noted that the "ozone hole" is not really a hole but simply an important thinning of the ozone levels over the Antarctic in each southern spring. The cause is quite complex but is simply driven by a weather situation, in conjunction with the man-made chemicals in the stratosphere. On average, it has become worse each year since it was first discovered. 2003 was the worst yet, with 2005 and 2001 a very close second. In 2002, a peculiar event happened, in that there were two "holes", each of slightly less individual importance. The 2004 "hole" was similar to the 2003 one, very slightly less in depletion but covering a larger area. Even with the application of the Montreal Protocol, it will become significantly worse before it gets better, probably peaking around 2040 - 2050 and then starting to improve. It is expected to heal itself by about the end of this century, provided the Protocol is followed to the letter.

This is a map of the "hole" as it was measured in September 2003, just after the equinox. To explain it, the contours are in Dobson Units (DU), which is defined as the column thickness of ozone in 1/100ths of a millimetre, with the ozone converted to NTP (760 mm Hg pressure, 293.15K or 20°C temperature). The global average thickness is 380 - 400 DU or 3.8 - 4 mm, which shows how little ozone there is, at the best of times, to protect us. It can be seen that, with the exception of a patch centred to the south of Australia, the whole of the area of this map has some considerable thinning, down to under 100 DU at the Pole itself.

The British Antarctic Survey issued the following statement at about the time of this map:

"Temperatures in the ozone layer are low enough that Polar Stratospheric Clouds (PSCs), the key precursors to significant ozone depletion, have formed widely over the continent. Ozone levels dropped quickly during August and September with the return of sunlight and depletion exceeds 55% in places. The ozone hole grew rapidly and peaked in size at around 28 million square kilometres (roughly twice the size of Antarctica) in mid September. It was larger than previously for the time of year in August and early September, but the maximum size only equalled the all time record. Ozone sonde flights from Rothera show substantial depletion between 12 and 23 kilometres, with effectively 100% depletion at times at some altitudes."

Although the "ozone hole" has captured the public's imagination, it is not this that is causing concern; it is depletion over inhabited areas. In Argentina, Chile, New Zealand and Australia, the situation is very preoccupying, with peak depletion levels likely to cause damage to inhabitants, marine life, livestock and plant life. However, there is 10 - 15 per cent depletion over most of Europe, Asia (N. of the Himalayas) and North America, as well. This is already causing problems, such as a significant increase of skin cancer (melanomas).

The effects of ozone-depletion

Of course, the depletion itself is harmless; it is the increased levels of ultraviolet light that causes the problems. These include, in humans, skin cancer, cataracts and reduced immune responses. Livestock in affected areas suffer similar problems plus a reduced growth rate. Some plants, notably soya beans, produce smaller crops. Possibly the worst long-term effect is to reduce the fertility of krill, which is the basic bottom of the food chain for marine life on which the viability of life in W. South America is highly dependent. It is not impossible that this, combined with over-fishing, will cause severe famine in coastal areas, especially of Chile and Peru.

Conclusion

With care, the effects of ozone-depletion will not be too bad. On the other hand, a mechanism for the disposal and safe destruction of ODSs and equipment containing ODSs does not reliably exist in many countries and should be implemented as soon as possible, with qualified and conscientious personnel for the maintenance.