When bad news is good news
And vice versa. Like a good joke, it's all in the ... timing.
The plot below is one I re-drew for a chapter in ‘Saving our Skins’, using the data from the 1985 paper that alerted the world to the what became known as the Antarctic Ozone Hole. It shows the October mean ozone values from Halley Bay, Antarctica over the period from 1957 to 1984. You can see from - from that steep decline since the mid-1970s - why it caused alarm bells to ring, and precipitated immediate international actions, culminating in the Montreal Protocol on Protection of the Ozone Layer.
At the time of that 1985 publication, the ozone data was limited to ground-based measurements. Data from satellites had been ‘available’ since the late 1970s but were mainly used to explore the geographic and seasonal variability in ozone. They were not yet considered reliable enough to detect long-term changes like these.
Ozone measurements - both from the ground and from satellite - have continued to this day, as shown in an update of the 1985 figure, from a recent publication. I learnt of the paper only through a Google alert in early May. I don’t know whether it’s been peer-reviewed. In places it presents rather a sceptical view of our mainstream understanding. Some are a little unfair. Others, though speculative, are worth further consideration.
The success of the international actions to protect the ozone layer - the Montreal Protocol - is clearly demonstrated in their updated plot (replicated below). It shows that, because of those actions, the overall decline in ozone continued at that site only until the early 1990s. Since then, while there have been occasional years with very low ozone amounts, the overall picture is that ozone has been gradually recovering.
Interestingly, it also shows how different things might have been if the paper had been published a year later. As you can see in the updated figure, that precipitate decline to 1984 was interrupted with much larger ozone values in 1985 and again in 1987. With the inclusion of those later data, I suspect there would have been less urgency, and we’d be in a much more parlous state (as we are currently with Climate Change).
One of our own papers, published in 1999, benefitted from a similar timing serendipity. It showed significant increases in peak summertime UV over Lauder New Zealand in response to decreasing ozone amounts over the previous decade. The importance of that work was demonstrated by it becoming my most-cited paper (with about 450 citations, to date). But, in the years that followed its publication, further increases in UV failed to materialise, as I showed here. That was lucky, for me in more ways than one 😊.
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I’d like to finish with a few more words about the new paper …
It goes on to say that the recovery of ozone is rather slower than expected, so its not only a chlorine issue. The slower recovery is for reasons that aren’t yet fully understood but probably include interactions with climate change. It’s not simple though. In a warming climate, temperatures high in the stratosphere decrease, and at those cooler temperatures, chemical reactions that destroy ozone should be slower, which would lead to a faster recovery instead of the observed slower recovery. But the current composition of the atmosphere (and also the biosphere) is very different from that in 1980 when the problem first became apparent. The slower-than-expected ozone recovery may involve changing interactions between competing chemical processes or induced dynamical changes rather than simple chemical changes. As the authors says, natural processes like volcanic eruptions, or changes in solar activity also play a part.
I think we all agree that chlorine remains the main driver of changes in ozone. But I disagree with the author’s assertion that there has been ‘no’ recovery in ozone. There has, as reported here, (as shown in the plot below). The first signs of recovery were becoming apparent several years earlier.
In fact, when uncertainties in both the measurements and models are included in the trend analyses, any ‘inconsistency’ disappears. I think we agree that the reversion to low-chlorine values is accompanied by other changes that complicate the issue, and that more work is needed to better understand what causes some of these changes and precisely how they influence the rate of ozone recovery.