The faintest of traces

Why rare greenhouse gases matter (9 minute read)

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Writing, or reading, about environmental problems can be like a long, slow trudge up a steep hill. For all the effort, it can seem as if you are getting nowhere. Above you, the hill seems to rise forever – sometimes you think you can see the top, but when you get there, there’s just more uphill slope. That’s when it pays to stop, turn around, and look at where you’ve already been. I love the feeling when I turn around and see my progress. It puts things into perspective and makes all the effort seem worthwhile.

So, before I look at my next climate change topic, I want to acknowledge an environmental problem which, although it hasn’t been completely solved, has seen a remarkable change within my lifetime – the ozone hole.

The story of the ozone hole starts with one of the technologies which transformed the world – refrigeration. Commercial refrigeration was developed in the second half of the 19th century, using ammonia. It works the same way that sweat cools your body, only instead of evaporating water from the skin surface, early fridges used ammonia evaporating as it passed through tubes. (If you want a full explanation of the process, the clearest that I’ve found is linked here. The explanation is quite long and spread over several pages.)

But ammonia is corrosive and toxic, so if it leaked, it was dangerous. Some of the alternatives, such as methyl chloride, were equally dangerous, sometimes fatally so.

When gases known as chlorofluorocarbons or freons were developed in the 1930s, they were a real breakthrough. They weren’t corrosive, they weren’t toxic and they weren’t flammable, which meant that fridges became much safer. Soon, other uses were found for chlorofluorocarbons too. They were used as propellants for aerosol cans, as solvents, for creating foams and for a range of other industrial uses. Before long, they were everywhere.

And that was the problem. Chlorofluorocarbons are comparatively stable chemicals – that’s one reason why they proved so useful. There are no natural processes that break them down or remove them from the atmosphere at the earth’s surface. Although they are heavier than air, chlorofluorocarbons stay in the atmosphere, carried around the world by wind and air currents. Some of them are carried all the way to the stratosphere, the layer of earth’s atmosphere that is 10-50 kilometres above the earth’s surface.

Part of the stratosphere, the layer 20-25 kilometres above the earth’s surface, is called the ozone layer. It contains a large amount of a form of oxygen made up of three oxygen atoms, called ozone. The ozone layer is important for life on earth, because it absorbs much of the ultraviolet light which reaches the atmosphere, in particular the higher energy forms known as UVB and UVC. Ultraviolet light is harmful to living things. It causes burns and eye damage, and it also damages DNA, which can lead to cancer. Since it doesn’t have the energy to be carried far into our bodies, most of the damage it does is to the skin.

But ultraviolet light doesn’t only affect living things - it also breaks down chlorofluorocarbons. Once they reach the stratosphere these chemicals aren’t stable as they are in the lower atmosphere. Instead, they break down, releasing chlorine atoms as a result. And then these chlorine atoms react with ozone. (Another chemical, known as bromine, can also be involved – more on that later.)

It’s not a simple chemical reaction between the ozone and the chlorine, which would use up the chlorine in the process. If it were, it wouldn’t be so bad. Instead, the chlorine acts as a catalyst for the breakdown of ozone. That means chlorine helps the chemical reaction happen without getting used up itself. A single chlorine atom can break down more than 100,000 ozone molecules. (There’s an excellent animation of the process here.)

In the early 1970s, scientists discovered that chlorofluorocarbons were capable of destroying ozone, but it took another decade for the problem to be taken seriously. That was when scientists from the British Antarctic Survey discovered that the amount of ozone over Antarctica was declining. At first, they didn’t notice the decline, because there was a large backlog in the data. Then, they thought the problem was a faulty instrument, but when they replaced it, the result was the same. Then, they assumed it was some sort of anomaly in the area where the measurement had been made, Halley Bay, on the Weddell Sea to the east of the Antarctic Peninsula.

It wasn’t. Consultation with NASA revealed that the area of depleted ozone, which became known as the ozone hole, was roughly the size of the USA. It would have been noticed earlier, but NASA’s scientists and computers had the same problem as the British scientists. There was too much data, and the anomalous ozone measurements had been discarded as probably due to faulty equipment.

The reason the hole first appeared over Antarctica related to the specific atmospheric conditions which develop in the Antarctic winter. The exceptionally cold temperatures lead to the formation of polar stratospheric clouds. It is in these clouds that the breakdown of ozone, facilitated by chlorine, occurs. Polar stratospheric clouds also occur in the Arctic, but they are less stable, so there is less time for them to break down the ozone. But ozone can be, and is, depleted over the Arctic too, and there has even been an area of depleted ozone found over Tibet.

Ross Island, Antarctica (image credit: Melanie Newfield)

Here is where the story of the ozone hole diverges from the story of climate change. Once the growing hole in the ozone layer was recognised, action was swift. In 1987, just two years after the publication of the landmark paper documenting the ozone hole, an agreement known as the Montreal Protocol was signed to limit the use of chemicals which could damage the ozone layer. The agreement included phasing out the chlorofluorocarbons, but, crucially for New Zealand, there were some exceptions in the agreement.

Although I’ve mostly talked about chlorofluorocarbons and chlorine in this article, there is another molecule which behaves the same way if it ends up in the stratosphere – bromine. It’s a close relative of chlorine and occurs in similar kinds of molecules. One important bromine compound which damages ozone is methyl bromide, which is used in fumigation. Although there were alternatives for some purposes, the Montreal Protocol recognised critical uses, where there were no viable alternatives. In particular, there was no alternative to methyl bromide as a quarantine treatment – that’s where imported or exported goods need to be fumigated to prevent the spread of pests.

Mostly, though, there were alternative chemicals which could be used instead of chlorofluorocarbons and similar bromine compounds. Most important were hydrofluorocarbons and hydrochlorofluorocarbons (I’m sorry, that is just about the largest word I’ve ever used in an article, but it doesn’t have an easier name, just an acronym HCFC). These two groups of chemicals have similar properties to chlorofluorocarbons, but don’t affect the ozone layer. The availability of similar chemicals is one crucial reason that the global community was able to act so swiftly to solve the problem with the ozone layer.

Although it no longer gets much attention, the ozone hole hasn’t gone away. There is still chlorine and bromine in the stratosphere, breaking down ozone when conditions are suitable, especially Antarctic winters. But the ozone hole is no longer growing, and by the middle of this century, it’s expected to be gone. And, as a consequence of of the phase-out of chlorofluorocarbons, the United Nations estimates that we may have avoided up to two million cases of skin cancer worldwide.

But the story doesn’t finish here. Because chlorofluorocarbons, as well as the alternative chemicals we are now using, are also greenhouse gases.

As I mentioned when I wrote about nitrous oxide two weeks ago, the amounts of chlorofluorocarbons and related chemicals in the atmosphere are vanishingly small – measured in the parts per trillion. That means there are only a few hundred of any one of them, for every trillion other molecules in the air. But the problem is, there are quite a few different gases out there, and they are extremely powerful greenhouse gases, much, much worse than carbon dioxide.

How much worse? Some are hundreds of times worse than carbon dioxide. Some are thousands of times worse. And some are tens of thousands of times worse.

But are they as much of a problem as fossil fuels? Or, for that matter, cattle and sheep? The short answer is no – their total contribution to global warming, so far, is about 2%. But there are worrying signs. One crucial point about these gases is that they are all different. One, known as HFC-23, is around 10,000 times worse than carbon dioxide, and emissions of it are higher than at any point in history.

The ozone hole was a very different sort of problem from climate change. The chemicals damaging the ozone layer were important to us, but in most cases there were alternatives we could use without changing much of what we were doing. But our dependence on fossil fuels, and on other activities which contribute to climate change, is much deeper and more pervasive. We won’t solve the problem by changing just one thing. Everything needs to change. And that’s overwhelming.

But the success of the Montreal Protocol may still make a difference to climate change. In 2016, countries set ambitious targets to reduce usage of these most damaging of greenhouse gases, in what is known as the Kigali Amendment. Developed countries began reducing their use in 2019 and will have reduced them by 85% in 2036. Developing countries will reach the same reduction by 2047.

If successful, the Kigali amendment will allow us to avoid a 0.5°C global temperature increase by 2100. On the scale of what we are facing – up to 5°C by 2100 – it’s not a huge difference. But it’s certainly enough to be worth doing.

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