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Planet Earth's nine lives: Our planet's limits

The ozone hole

This is something of a good news story for a change. You probably know something about it already. Scientists discovered there was a problem with ozone and all the world's countries agreed to fix the problem. And the treaty they all agreed to seems to be working quite well.

But what is ozone? It's a gas molecule made of three oxygen atoms (O3) and is not like the oxygen we all need to live. The oxygen we breathe is a molecule formed by just two atoms (O2) and makes up about 21 percent of the atmosphere. Almost all the rest of the air is made of nitrogen gas. The ozone form of oxygen makes up only a tiny amountInformationOzone in the airOzone makes around 0.000004% overall up of the atmosphere and up to 8 parts per million molecules of all gases in the atmosphere at between an altitude of 30 and 35 kilometres. This is the action zone where the radiation-absorbing reactions take place. There are two odd things about ozone:

  • most life - including people - couldn't exist without it
  • it's poisonous! So there's 'good' and 'bad' ozone. I'll explain because this sounds silly but it is both true and very important.

Almost all ozone exists high in the stratosphereInformationWhat is the stratosphere?The stratosphere is part of Earth's atmosphere. It begins around 10 kilometres (6 miles) above us and the air is very thin. Almost no living creature can survive - unless in the pressurized cabin of an aircraft! The part of the atmosphere we live in is the densest part. It's called the troposphere and is where almost all weather happens (rain, hurricanes, snow and so on). It is formed continually as powerful ultraviolet (UV) radiation from the sun splits some ordinary oxygen (O2) molecules into oxygen atoms (O) which react with each other to make ozone (O3)InformationOxygen and ozoneO+O2=O3 Simple maths!. It's a very good thing that this happens because the ozone acts as a special shield from dangerous UV radiation from the sun. Without the ozone shield to absorb most of the UV radiation, there would be nothing to protect us living things from it. The sun would be like a giant death ray gun which never stopped. So that's why life can't survive without ozone. The small amount of UV radiation that does make it to the Earth's surface can cause nasty diseases in people, like skin cancer, so sunbathing at the beach is not a good idea. The bottom line is we can't survive without ozone, or we would all be burned to a crisp by the sun's UV rays.

So ozone is good and yet it's poisonous? Fortunately, 90 percent of all the ozone floats high up in the stratosphere where it's bitterly cold and the air is so thin nobody could survive anyway. Even the world's highest mountains don't reach anywhere near that high. The tiny amounts of ozone which you and I breathe in at the surface of our planet do us no harm because they are really tiny amounts. It can become a problem in the polluted air - smog - which hangs over many cities. This is because strong sunlight reacts with pollutants like car exhaust and industrial chemicals to make ozone in large enough amounts to be harmful to health, especially for people with breathing difficulties like asthma.

The ozone 'hole' Scientists began studying ozone levels in the stratosphere about 50 years ago. In the 1970s, they suddenly realised that amounts of ozone above the Antarctic - my home! - were falling rapidly, especially in the spring. A giant hole formed each spring and this let in much more UV radiation than normal. The hole became so large that people in Australia and New Zealand began to have trouble with skin cancers. Similar holes also began to form over the Arctic. The scientists discovered that the reason the ozone was disappearing was long-lived and man-made chemicals called CFCsInformationWhat are CFCs?Their chemical name is chlorofluorocarbons. When they drift up into the stratosphere, the CFC molecule breaks down in sunlight, freeing a chlorine atom. This is where the trouble begins. The chlorine released from CFCs catalyzes the break down of ozone (O3) to oxygen (O2). The chlorine atoms continue to do this many times - catalysts aren't destroyed in the reaction they catalyze - which is why even minute quantities are so damaging to the ozone layer. It turned out that these CFCs were big time ozone-eaters and people were using them in large amounts for things like refrigerators. When the CFCs escaped into the air, they floated up into the stratosphere. UV radiation from the sun knocks chlorine atoms out of the CFC molecules. The chlorine, which is deadly poisonous, quickly destroys the ozone molecules in complicated reactions. This all happens in high and very cold polar clouds during the winter. The chlorine then returns to destroy more ozone and so the telltale ozone hole forms in spring.

And the good news? Thanks to careful science, people quickly realised what was happening and took steps to stop it. There is now an agreement between all countries to quit using CFCs and other ozone-eating chemicals. This is called the Montreal Protocol. Thanks to this agreement, the worst danger seems to be over and it looks as if we won't be going over this planetary boundary. It's double good news because it also shows that humans can work together to stop bad things happening to our planet.



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