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Ozone Levels Still Decreasing Away from Poles


While ozone levels in the upper atmosphere near the poles have been recovering, new research has found that the bottom part of the ozone layer at more populated latitudes is not recovering.

The worst ozone-depleting substances are chlorofluorocarbons (CFCs) and other volatile chemicals containing chlorine or bromine. Emissions of these substances have been drastically reduced by international agreement, under the Montreal Protocol, to ban or restrict their production and consumption. As a result (we’d like to believe), the decline in the stratospheric ozone concentration has been arrested and there are some signs of recovery.

However, given the steep decline in emissions, we might have expected a better result for the ozone layer. This analysis of research results, published by an international group of scientists (Atmospheric Chemistry and Physics,, help us to understand why we haven’t.

Measurements of ozone concentration are made for a notional column of air stretching up from about 10 km above Earth’s surface to about 50 km. For the column as a whole, there has not been much change in ozone concentration during the period 1998–2016. When results for “slices” of the air in that column were examined, a more complex picture emerged. In the upper stratosphere (32–48 km) there has been a steady increase in ozone concentration, which is what was expected from the actions taken under the Montreal Protocol.

However, in a lower slice of the air column(13–24 km) there has been a continuous decline. The reasons for this decline remain unknown. Unlike the positive change at higher levels, this negative is not predicted by models of atmospheric chemistry.

The result of their analysis is nicely summed up by the authors: “We find that the negative ozone trend within the lower stratosphere between 1998 and 2016 is the main reason why a statistically significant recovery in total column ozone has remained elusive”. In colloquial language: if you add a positive change and a negative change, you get zero change.

Professor Ian Rae is an expert on chemicals in the environment at the University of Melbourne’s School of Chemistry.

Although the authors suggest otherwise, this paper could be a challenge to the effectiveness of the Montreal Protocol. The Montreal Protocol banned CFCs and was the world’s first universal agreement to cooperate on behalf of global human health. Estimates say it’s already saved many more than 280 million lives.

What this new paper is saying is that the hole in the ozone layer, predicted to be completely repaired by around 2060, has a whole section that’s not repairing itself. The section in question is about 20 km above Earth, between the tops of clouds and the height where aeroplanes cruise, and extends from just outside the Arctic Circle to the start of Antarctica. Even though the polar regions and the higher stratospheric levels of ozone are repairing themselves, this lower, middle section is going in the opposite direction – the amount of ozone is still falling just like it was before the Montreal Protocol.

A million things could be causing this, some natural, some not. But this paper tries with all its might to get around a host of past problems to see the real trend, and the ozone is definitely falling in that region, even after seasonal, time series and measurement adjustments. After eliminating the obvious, they’re still left with some disturbing possibilities: did we under­estimate the anthropogenic effect, the volcanic effect, or is there some missing chemistry?

What they propose are three explanations related to climate change: firstly, an expanded troposphere; secondly, an accelerating Brewer–Dobson circulation; or thirdly, a disproportionate acceleration of it closer to the tropopause. In other words, the ozone is being transported out of this section faster.

If so, it’s a worry because it means the actual repair might be due to more rapid accumulation in the higher stratosphere, rendering the Montreal Protocol targets perhaps too lax from the beginning; that is, unless the higher stratosphere is actually doing the lion’s share of protecting us from UV radiation.

This paper suggests climate change is interfering with the ozone system as well, creating a scenario where the Montreal Protocol, although still necessary, might not yet be sufficient for repair by 2060.

Dr Paul Read is at the Melbourne Sustainable Society Institute at the University of Melbourne.

As stated in Conclusion (iv) of the paper, “there is no significant change in total ozone column density between 1998 and 2016”. Therefore, there will have been no corresponding increase in harmful UV radiation at ground level, and people need not be concerned. The authors are engaged simply with explaining a change in the distribution of ozone in the atmosphere, which may well be due to transport mechanisms only. This is not to say that continued studies are unnecessary.

Emeritus Professor Brenton Lewis is from the Research School of Physics and Engineering at The Australian National University.