Earlier start to ozone decline

But only small changes for New Zealand's latitude range

I was just revisiting a paper published last year by old NIWA colleagues and noticed that it includes an important insight that may not be appreciated by non-specialists in the field of ozone research.

Discussion of ozone trends is often limited to the period from 1979 when routine satellite observations became well established. But ozone data from ground-based measurements and balloons goes back much further.

The paper, which uses data from all sources, shows that ozone was already being depleted before the satellite-data era, and before we even realised there was a problem. It’s the clearest demonstration of that earlier onset I’ve seen so far. It also shows a clear glimmer of light at the end of the tunnel.

The first figure from the paper is reproduced below.

It shows the ozone trends (in Dobson Units per year) as a function of latitude for each month of the year, averaged over three different periods: from 1970 to 1979 in the upper panel; from1979 to 2000 in the middle panel; and from 1997 to 2020 in the lower panel.

Different shadings of red represent ozone depletion (leading to UV increases), while the blue shades represent ozone increases (leading to UV decreases). The black symbols for each month show the latitude ranges where those changes are statistically significant.

The top panel clearly shows significant negative ozone trends prior to 1979 at mid to high latitudes, especially in the southern hemisphere. The middle panel shows that the pattern persisted through the next two decades. It’s only after 1997 (bottom panel), when the levels of atmospheric chlorine started declining, that the pattern begins to reverse. That changed pattern is the signature of Montreal Protocol success.

What does it all mean for long-term UV changes in New Zealand? Not all that much, it transpires. We can thank the success of the Montreal Protocol for that. As I’ve noted before, the story would have been quite different without it. Given the normal seasonal and day-to-day variability in UV, those anecdotes about the UV seeming much stronger nowadays are not really borne out by the facts.

The latitude range spanned by New Zealand is approximately 34S to 47S. The largest changes in that range are at the higher latitudes, like Invercargill (46.4S), near the southern tip of the South Island. From a UV perspective, it’s the changes in summer that matter. That’s December/January for New Zealand. Those changes are relatively small: around 0.5 DU/year for the first period, giving a total of around 5 DU for the period from 1970 to 1979, and only slightly more for the second period, giving an overall reduction in the thirty-year period from 1970 to 2000 of around 20 DU. The average ozone amount is around 300 DU, but is slightly slightly less in the New Zealand summer, so in percentage terms that amounts to an overall ozone decrease of 5 to 10 percent, which corresponds to an increase in sunburning UV of around 10 percent. And a chunk of that change has already been recovered in the period since 1997. At Auckland’s more northerly latitude (36.9S), the changes are smaller and are not statistically significant. But, as noted above, the changes were much more substantial at latitudes poleward of New Zealand. In the Antarctic spring the ozone changes can be ten times as large.

If you suffer from the misfortune of living outside New Zealand, you can do the exercise yourself by reading the changes for your own latitude in the above plot. To calculate percentage changes you need the mean summertime ozone at your place, which is available on the “Geeks” page of the free GlobalUV smartphone app.

(I hasten to add that I don’t mean to imply that only non-Kiwis can be Geeks. Indeed I know several Kiwis - including one sitting not far from me - who could be considered so 😊).