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The hole in the ozone layer

I thought the ozone hole was goneThe ozone hole hasn't gone away; it is still up there over the North and South Poles, growing and shrinking in size depending on the time of year and the air temperature kilometres above Earth's surface.

Okay, so why are we talking about it again?The ozone hole – actually not a hole at all but a thinning of the ozone up there – is back in the news because scientists are predicting a record loss of ozone after very cold temperatures in the stratosphere kilometres above the Arctic. The World Meteorological Organisation said that the ozone had thinned by 40 per cent, well above the previous record of 30 per cent loss in a single year.

Why should I care? It doesn't affect me, right?Ozone plays an essential role in protecting us – humans, animals and plants – from the ultraviolet light from the sun. Without it the DNA in our skin would get chopped to bits, causing cancers, and we would lose our vision to cataracts. Ozone blocks up to 99 per cent of the incoming ultraviolet light, but that 1 per cent is enough to give us a bad burn if we sit on the beach too long.

So what is ozone anyway?Ozone is a form of oxygen. Usually two atoms of oxygen will join in a clump to form a molecule, but if three oxygen atoms clump together we call it ozone. It takes energy to do this, and 15km up in the stratosphere there is plenty of free energy, in the form of ultraviolet light, to make ozone. Ozone is amazing stuff, really, because it takes so little of it to protect us. If you could squeeze all the ozone in the stratosphere and compact it into a single layer at sea level it would be only a few millimetres thick.

How did the hole get there?It all comes down to human activity. Over many decades we released different kinds of gases that contained things like chlorine and bromine. One of the most important groups was the chlorofluorocarbons, or CFCs, which we used in refrigerators and as the propellant in billions of aerosol cans. Over the years these chemicals floated upwards to collect high in the atmosphere where the ozone is. In the Arctic and Antarctic winters, when temperatures in the stratosphere get to minus 78 degrees, the CFCs begin dismantling the ozone, breaking it up into normal oxygen. In certain conditions the chemicals can do this faster than the ultraviolet light can make more ozone.

But what's so special about this year?Conditions were just right for a big loss of ozone over the Arctic, with low temperatures and lots of ice clouds. The resultant hole has spread out all the way down to Scandinavia, and this causes a smaller amount of thinning closer to Ireland.

Does that mean we are in danger now?Not really. There is still ozone up there; it is just thinner. It means, though, that we need to be more careful this spring and early summer to protect against ultraviolet rays. Wear a hat to shade the face and eyes or wear sunglasses in bright sun. Cover up against the sun or use sunblock to limit UV exposure to your skin.

Can we do anything about the ozone hole? Can't we fill it up somehow?We already are. In 1989 the Montreal Protocol came into force, under which countries agreed to stop releasing chemicals that harm the ozone layer. And it is beginning to work. Scientists estimate that the Arctic winter-spring hole will stop forming by 2040 at the latest.

The Antarctic hole will be a thing of the past after 2060, they believe. The fact that recent ozone holes are still setting records, however, shows that it takes a great deal of international agreement and many decades to have any effect, a frightening thought after a week’s haggling and little progress at the climate-change talks this week in Bangkok.