Volcano clouds timing of ozone recovery

Citing 20Q&As, 2022 - Q13

While I was reviewing a paper last week, I found myself referring back to one of the iconic figures from the Twenty Questions and Answers document that I was helping with for the last year or so. The now-completed document - that was led by my colleague Ross Salawitch - forms part of the just-published Scientific Assessment of Ozone Depletion.

From that compilation, Question 13 asks:

“Do changes in the Sun and volcanic eruptions affect the ozone layer?”

The short answer is Yes. The small effects due to changes in solar output are well understood and predictable. But not so for volcanoes. They complicate attribution of the effects of the Montreal Protocol because it just happens that there was a huge volcanic eruption of Mount Pinatubo in the Philippines in 1991. Aerosols injected into the stratosphere lingered for years after the event, and reactions on the surfaces of those aerosols lead to significant ozone depletion

The truly excellent figure to help answer the question is reproduced below. It spans the period from 1960, before ozone depletion began, up to 2030. The 5 panels summarize just about everything you need to know about these things.

The top panel shows how the amount of chlorine changes over time in the stratosphere, with peak loadings occurring around 1998 before the start of a slow recovery thanks to the Montreal Protocol, which is gradually leading to a recovery in atmospheric ozone. By the way, the y-axis label EESC, stands for “Equivalent Effective Stratospheric Chlorine”, which includes a small contribution from bromine. It’s concentration in the stratosphere peaked a few years later than the peak in its production rates.

The second panels show changes in solar output, dominated by an 11-year cycle in variability; and the third panel shows the changes in aerosol optical depth due to volcanic eruptions that occurred sporadically over the period, the largest by far being the eruption of Mount Pinatubo in 1991.

The bottom panel shows the observed changes in ozone over the same period. The overall pattern is that ozone decreased in the period when chlorine increased - up to about 1998, and decreased thereafter. But, there’s a lot of short term variability too that clouds the pattern. Most of the short-term changes are related to changes in solar output and volcanic effects. If you look carefully at the lowermost purple curve, you can see the effects of those two other complications.

Firstly, during periods of high solar output, ozone levels are elevated by a few percent compared with periods of low solar output. Counter-intuitively, this means that during periods of high solar output, the amount of UV transmitted to Earth’s surface is slightly lower than when solar output is close to its minimum.

Secondly, ozone tends to be lower during periods when aerosol optical depths are elevated following those volcanic eruptions. Its most noticeable following the eruption of Mount Pinatubo. As far as UV radiation transmitted to Earth’s surface is concerned, there are two competing effects: the amount transmitted is reduced by the aerosols extinctions, but is increased by the lower ozone. The paper I was reviewing (read it and my review here) was trying to - among other things - determine the exact dates when the turnaround in ozone occurred as a function of latitude. But, as you can see in that lowest panel, it’s complicated by aerosols eruption of Mt Pinatubo persisting until close to the turnaround time in atmospheric chlorine.

Murphy’s Law again.