Hole in the Ozone Layer

Julius Sumner Miller Fellow, The Science Foundation for Physics, School of Physics, The University of Sydney
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Every springtime, we hear that chlorofluorocarbons (CFCs) have punched another "hole in the ozone layer". It sounds unbelievable.

Ozone is a gas - how can a gas have a "hole"? Most CFCs are used in the northern hemisphere, so why does the "ozone hole" happen in the southern hemisphere?

Let's get a few things straight. The "ozone hole" does not cover the whole planet. It happens only once a year, for a few months - from August to November.

Ozone some 10 - 50km above ground level absorbs ultraviolet light. This 40 kilometre thick layer is the famous protective "ozone layer".

CFCs destroy ozone, and make the ozone layer "thinner". So the infamous "hole in the ozone layer" is actually the ozone "blanket" getting "thinner".

Normally CFCs are very stable and unreactive. But in the upper atmosphere, the very high levels of ultraviolet light split the CFCs. When a CFC molecule splits, a highly energetic atom of chlorine escapes. this atom of chlorine will ultimately destroy about 10,000 ozone molecules.

You need three things to destroy ozone molecules - CFCs, low temperature and sunshine.

These conditions happen over both North and South poles. But the South Pole has something the North Pole does not have - whirling winds.

Every August, down at ground level, the Roaring Forties build up into a gale that sweeps clockwise around Antarctica. But in the upper atmosphere where the ozone is, an even faster wind whips around at hundreds if kilometres per hour. This wind is so strong, that the air inside the Antarctic whirlpool is cut off from the rest of the planet.

In August/September, the sun peeps over the horizon and the ozone-destroying reactions begin. The ozone-depleted air is trapped inside the whirlpool, and so the "ozone hole" is very noticeable. Around October/November, the whirlpool of wind begins to break down. Only then do the bubbles of ozone-thin air began to move towards the Equator, while at the same time, ozone-rich air sweeps in from the Equator.

Things are different at the North Pole. It's closely surrounded by land. These land masses break up the winds, so the whirlwind doesn't get as strong. Recently, there has been an ozone hole over the North Pole - but it is quite small.

For the past 20 years, the ozone hole has got bigger. Forty companies make CFCs. In 1994 CFC pollution reached is maximum, and has dropped by 1% each year. But the ozone hole should still reappear every spring until around 2050.

By strange coincidence, the weight of ozone on our planet is roughly equal to the weight of human flesh on the planet, Is nature trying to tell us that if the planet gets less ozone, it will also get less human flesh? And where will the lost flesh come from?

© Karl S. Kruszelnicki
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