science sustainability

Planetary boundaries 1 – Ozone

earthThis is the first in a nine part series on the planetary boundaries. If you missed my introduction, it’s here. This week I’m looking at atmospheric ozone. One caveat before I start – I’m not a science writer, so while I’ve done my best to understand the science and explain it simply, I may not have succeeded. If you spot any mistakes or oversimplifications, please point them out on the comments!

I have started with the ozone boundary for a couple of reasons. Firstly, it’s a relatively simple example of how human activity can alter the planet. In talking about climate change, I still often hear the objection that it is hubris to believe that we could ever affect something as large and beyond us as planet earth. This humility is misplaced, as the ozone problem demonstrates. We developed a man-made gas that changed the chemical composition of the sky, putting human health at serious risk in the process.

The second reason to start with stratospheric ozone is that it’s a success story. It proves that we can understand planetary boundaries and organise ourselves to manage them. We began to deplete the ozone layer, but science was able to spot the problem and politicians and business rallied to change our behaviour.

But let’s remind ourselves of what the ozone layer even is. Ozone is trioxygen, three bonded oxygen atoms. It is created as ultra violet (UV) light hits oxygen in its more usual O2 form, giving us a layer of ozone in the stratosphere, 12-19 miles above the earth. This reaction absorbs the majority of the sun’s UV rays so that they never reach the surface. This is important, because UV is what causes sunburn, and extended exposure can cause skin cancer. Excess UV also harms the development of marine micro-organisms such as phytoplankton, which has repercussions up the marine food chain.

Ozone can be split apart by other atoms. This happens naturally at low levels, but most elements that would disrupt ozone cannot rise high enough into the stratosphere to do any serious damage to the ozone layer as a whole. But then in the 1920s General Motors invented a gas that could. It was intended for refrigeration and was branded as Freon, more commonly known as chlorofluorocarbons or CFCs. This predictable, inert gas had all sorts of useful industrial purposes and was widely used in various forms.

Within 50 years, enough CFCs had been released into the atmosphere for it to be found in low concentrations everywhere on earth – quite impressive really for something that had never existed in nature before. This was thought to be benign, but in 1974 Mario Molina and Sherry Rowland first suggested that CFCs could deplete the ozone layer. Because CFCs are stable at low temperatures, they could rise into the extreme temperatures of the stratosphere. Once there, UV light splits them apart and releases free chlorine atoms, which in turn break down ozone.

Molina and Rowland later won the Nobel prize for their hypothesis, because it turned out to be entirely correct. It was to remain theory for another decade, but by the mid-eighties there was a measurable thinning of the ozone layer over the poles, leaving a ‘hole’ over Antarctica every spring. If this were to continue, UV radiation levels reaching us were going to get stronger. NASA estimated that by 2065, the sun over London would be strong enough to burn you in just five minutes. Incidences of cancer were likely to soar.

That’s not going to happen now, because in 1985 the Vienna Convention for the Protection of the Ozone Layer secured international cooperation to address the problem. Two years later the Montreal Protocol saw agreement to phase out CFCs. It sounds straightforward, but as Mark Lynas describes in his book The God Species, there was intense lobbying from industry to try and thwart the ban. Interestingly, it was the US under Reagan that pushed it through, and Britain and the EU that wanted to stall and protect their chemical industries.

Business did eventually come round to supporting the ban, alternatives were developed, and good sense prevailed. CFC use fell by 90% in a decade, and the ozone layer is now stable and slowly recovering. Since CFCs can remain in the atmosphere for a century, it will be a long time before the ozone layer is entirely restored.

To maintain a safe environment, levels of ozone in the stratosphere mustn’t drop by more than 5% of their 1980 levels, according to the planetary boundaries report. But so far, we’re managing that: “The case of stratospheric ozone is a good example where concerted human effort and wise decision making seem to have enabled us to stay within a planetary boundary.”


Stratospheric ozone boundary

Safe limit: no more than 5% depletion at any given latitude, on 1980 levels.

Status: within the safe limits


  1. The Ozone layer is indeed a “planetary boundary”. Without it – and a few other protective mechanisms – the surface of Earth basically would be sterilized.

    I am reminded of another one of your posts – the one about a clear enemy, an enemy with a face. In that article you pointed out how relatively easy it was to unite against Hitler. It was relatively easy to unite against the Ozone hole, because the enemy had a long established clear name: Cancer. A simple enough cause and effect relationship was conveyed to the public, and most likely no politician wanted to be directly responsible for causing cancer. Plus they, too, are imprinted to fear cancer. The problem also was easy enough to handle, since a single isolated substance for which there were alternatives was at stake. Patents had run out as well, so industry opposition was relatively moderate. It is indeed a success story, but in the case of CO2, Methane and N2O, for example, the situation is infinitely more complicated. CO2, Methane and N2O are intricately intertwined with lifestyle, ranging from heating over traffic and consumption to every industry, agriculture, meat consumption – basically every “modern” human activity. Doing something about these gases is infinitely more difficult and there is infinitely more resistance. The Montreal Protocol is a success, but it is not easily copied.

    Oh – and I personally always was far more concerned about the effect UV radiation would have on near surface organisms in the Oceans. Killing a significant fraction of phytoplankton and corals could set off unpredictable cascades in the Geo-Biosphere. In comparison cancer here is the lesser issue. Wear a long sleeve shirt and a hat… But it sells better. Very few people have a sense of compassion for Antarctic algae. Dead micro critters just don’t feel like a clear and present danger.

    1. Good point about cancer, there was a political imperative there that has no parallel in climate change.

      I forgot to mention the marine biology link. I’ll go back and add that in, thanks for the reminder!

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