When did ozone's decline end for you?
... and what's happened since. A slightly flawed analysis, but still useful ...
As I’ve mentioned before, the Montreal Protocol on Protection of the Ozone Layer is the most successful Environmental Agreement of all time. It’s already averted the disastrously high UV levels that would have occurred without action. At the same time it has mitigated a significant part of global warming due to increases in greenhouse gases.
But how is ozone itself faring these days? A just-published paper - written colleagues at NASA - shows the turnaround date for ozone as a function of latitude. It doesn’t just look at the ozone data itself. It also takes into account the known perturbations in ozone due to other factors. Factors like seasonal cycles, the 11-year cycle of variability in UV output from the sun that affects ozone chemistry, and other dynamical effects that are known to affect ozone (e.g., the QBO, or ENSO).
Unfortunately though, the analysis doesn’t remove the effects of volcanic eruptions. It’s well known that they can have a big effect. For example, the eruption of Mount Pinatubo in 1991 led to significant ozone depletion in the northern hemisphere. Because of that omission, the turnaround date reported is not necessarily due to the Montreal Protocol alone. The paper isn’t wrong, but I find it frustrating because it doesn’t answer the much more relevant question: “What is the turnaround date for ozone attributable to our efforts to solve the problem”. Hopefully they’ll repeat the analysis soon to include removal of those volcanic effects.
The paper starts with the very instructive colour-coded contour plot below, which summarises all of their available satellite-derived ozone data in one nice 3-D picture. Putting all the data together to produce this is no mean feat. It includes measurements from several different satellite-based instruments, each with its own foibles and drifts in calibration. Ironing out those differences involves a lot of hard work. But the resulting figure is a gem.
The dark blue tongues on the left since the early 1980s show the emergence of the springtime ozone hole in Antarctica, and the green areas in the middle show the stable low ozone amounts in the tropics. The highest ozone amounts, shown in red, are seen at mid-latitudes (and high northern latitudes), where there are strong seasonal variations with a maximum in spring.
It shows that ozone amounts are much lower at southern mid-latitudes than at corresponding northern latitudes where those seasonal changes are much larger. In particular, the springtime maximum in ozone is much smaller in the south than in the north. Those lower ozone amounts in the southern hemisphere contribute to the higher UV levels in the south, but seasonal difference in Sun-Earth separation, and regional differences in air pollution are also important.
Outside the Antarctic region, long term changes in ozone have been small, so determining the turnaround date can be challenging. Annually averaged ozone amounts and their ranges are shown below as a function latitude.
Although the turnaround date they calculate is not the one I’m most interested in, I’ll summarise the results of the rest of the paper for those are interested. Feel free to nod off at this point otherwise. 😊
….
Their deduced dates of the ozone turnaround are shown in the plot below. In the southern hemisphere, where volcanic influences are small, the turnaround dates are consistent with the reduced amounts of chlorine in the atmosphere, as controlled by the provisions of the Montreal Protocol. But in the northern hemisphere, the deduced turnaround dates are several years earlier due to ozone losses caused by the eruption of Mount Pinatubo.
The bottom line of the paper is the estimation of ozone trends as a function of latitude before and after these turnaround dates in the 1990s, as shown below. In the earlier period (black line), there was a clear downward trend, which was larger in the in the southern hemisphere, and especially in Antarctica. In the later period (red line) ozone appears to be increasing through most of the southern hemisphere, especially at high latitudes, but the trends are not yet statistically significant. The corresponding blue and purple lines show results of a different trend analysis of the same data set, showing very similar patterns.
Those trends in the above plot look very similar to those deduced for the period before and after the date that chlorine reached a maximum in the ozone layer - around 1998.
I’d be really interested to see how these last two figures would look if volcanic effects were taken into account. I suspect I’m not alone there among my peers.
Pretty diagram