Saving our Skins: Epilogue
Saving our Skins: Epilogue
Atmospheric Reflections from a Lauder Stargazer
The game breaker.
Updated September 6, 2021
It’s time now to finish my story of Saving our Skins, so this fading stargazer can gracefully dim into irrelevance. On September 3, 2019 - just short of the 30-year anniversary of the Montreal Protocol coming into force - our clincher paper demonstrating its success in curbing UV increases throughout the globe was published in Nature’s Scientific Communications. A simplified version of the final figure from that paper is reproduced below. Since it’s essentially the bottom line in the book, please bear with me as I try to explain what it all means.
The figure compares measured and calculated trends in summertime UVI since 1996 at all the world’s sites where there are long-term records of the highest quality UV data. The number of years of data for each site are shown alongside the site names at the top. Sites where the time series spans 19 years, or more are denoted by bold text and solid symbols. At some sites (e.g., Lauder) measurements began before 1996 but that year was chosen as the reference to get the longest time series possible without complications from effects of the Pinatubo eruption. The blue circles with error bars show measured trends, including their 2-sigma (i.e., the ninetyfive percent confidence level) uncertainties.
At any site where the upper limit of the error bar just touches the (dotted) zero line we are confident at the 95 percent level that the UV has really been decreasing. Or, to put it another way, there’s only 1 chance in 20 that it’s not decreasing. If there’s a gap between the top of the error bar and the dotted line, our confidence is higher. And the similarity in trends between those southern sites increases that confidence even further. You can see that the error bars are smaller at sites with more years of data. Among the best are those from Lauder, Melbourne, Alice Springs, Mauna Loa, and Boulder, which use our own NIWA UV spectrometers.
The other coloured symbols show the UV trends calculated from
measured ozone changes (black diamonds),
model calculations with full compliance to the Montreal Protocol (green squares), and
model calculations for the “World Avoided” if there had been no Montreal Protocol (purple squares).
In the world avoided, there would have been large increases in UVI since 1996, with the largest increases at high southern latitudes. The increase there of 40 percent per decade would correspond to a near doubling of UVI in the last 24 years. In contrast, for the other calculation methods the derived trends since 1996 are close to zero at all sites.
As far as the UV measurements are concerned (blue circles), Austria’s Hoher Sonnblick is the only site showing significant increases since 1996.[1] The model calculations there show that the increase is not due to ozone, so there must be some other cause (e.g., a decrease in aerosols or clouds). Or it may be a measurement problem at that site, which experiences extremely challenging weather. Co-located data from the Austrian UV Monitoring Network showed results more consistent with the other sites. All of those show near zero, or negative trends - in agreement with those calculated.
The bottom line is that for Lauder and all sites south there’s a clear pattern of reductions in UV since 1996 (i.e., a negative trend in measured UVI), with the recovery more obvious at the higher latitudes where ozone depletion was most marked. The important point is that for some of these sites, the decrease in UVI since 1996 is now statistically significant. We physicists attach a lot of importance to that. The pattern of results is similar if the start year for the analysis is changed by a year or two.
In spring – when Antarctic ozone depletion is most severe - the patterns are similar, though the trends are larger. But despite those larger trends, the recovery is not yet statistically significant because of the larger year-to-year variability which causes the error bars from the measurements to encompass the zero line.
There’s just a tiny note of caution that must be added here. I said earlier that ozone holes don’t occur in the Arctic, but there have been two notable events there in recent years. The first was in the spring of 2011, when ozone fell below the 220 DU threshold that’s usually taken to define an ozone hole. The second was in the spring of 2020 when ozone fell to a record low of 205 DU, which is well below that threshold. But these ozone amounts are still more than twice the minima that routinely occur in Antarctica. I personally find it rather odd that the threshold for an ozone hole is that high, as amounts much lower than that - close to 200 DU - occasionally occur even in the tropics. For example, in the early winter of 1994 ozone over Hawaii dipped below 200 DU. Even at Lauder it occasionally drops below 220 DU in the autumn. Perhaps 200 DU would be a better threshold for an ozone hole. Either way, events like this may recur during spring in the next few years (if winds are suitable) while atmospheric chlorine remains elevated and the stratosphere continues to cool in response to climate change. And, because of the proximity to population centres, there is a potential health risk. But fortunately, as chlorine levels continue to decline, these events will be increasingly unlikely.
In any case, it’s the summers we need to be concerned about. That’s when our risk of UV damage is greatest. Because of our work we can now say that UV has now begun to decrease in the summer at high southern latitudes where the problem was worst. Being able to finally say that, was a very satisfying moment for me. A vindication of 4 decades of work. (Shortly after completing this chapter, the first evidence of ozone hole recovery affecting climate was published in Nature).
That long-delayed gratification reminds me of an observation I made shortly after taking up my job at Lauder. Before that, I’d been a high school teacher for three years, and then a university lecturer for the next three. There you have immediate feedback when you’ve done a decent job (or not). In the classroom the kids stop (or start) metaphorically climbing over desks and throwing chairs. In the lecture hall, the students’ eyes light up (or glaze over). Instant gratification (or dejection) in each case.
Feedback is far more measured in the research environment. Most valued for me is feedback from my peers when I publish a paper – usually after months of work, or from the audience after oral presentations. Well down my list is the meaningless feedback from annual performance appraisals by supervisors (see the plot of my salary changes in chapter 20). As for this book, it’s feedback enough to know you’ve got this far through it.
I think I should finish by recognising the huge efforts from all the scientists, politicians, lawyers, policymakers, manufacturers, and activists who’ve contributed to the success of the Montreal Protocol. They’ve all helped to Save our collective Skins. A job well done! They and their successors will need all of our support in solving the Climate Change crisis, and the more immediate crisis from the Covid-19 pandemic.
As my hero Bill McKibben reported in the New Yorker, “Time is of the essence. Both crises require urgent action and the consequences of delayed action are frightening”. I’m sure all you loyal readers will be trying your best to help on both of those battle fronts. Thank you and thank you for that.
Kia kaha! Tēnā koutou katoa
[1]. There is an apparent positive trend at tropical Saint Denis, but because the time series available there is short, the uncertainty in determining the trend is large. In this case, the error bars encompass zero, so the trend is not statistically significant.
[2]. Shortly after posting this chapter, the first evidence of ozone hole recovery affecting climate was published in Nature.
We’re just about done now. That’s all for “Saving our Skins”. But that’s part of a wider picture. For those who are interested in that, I’ve provided a couple of addenda. The first fleshes out the Comings and Goings at Lauder, which includes staff changes, scientific outputs, measurement programmes, climate statistics, and a listing of staff personnel along with their start and finish dates (where known). The second is a little more of my own Back Story…..
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If you want to read the whole story from the beginning, click below