An Old Favourite

Four panels say it all

The old chestnut

One of my favourite pictures to summarise the ozone problem and its solution is this one, despite the obvious limitation of a lack of numbers on the axis scales of the four graphs. The colourful plots are designed to be understood by non-scientists like politicians (a big ask), executives (too busy?), and lawyers (who cares?), so they succinctly encapsulate the problem with a minimum of jargon. The only exception is the very first word - ODS - an acronym for Ozone Depleting Substances (like the CFCs I mentioned in an earlier post).

It was published in the Executive Summary of the 2006 Scientific Assessments of ozone Depletion, so the time labelled “now” meant about then (but still roughly applies today), and the full range of the x-axis is from about 1950 to 2100.

The plot could be a basis for a whole lecture on the subject, but I’ll try to summarise it in a few sentences below.

Panel (a) shows the changing production rate of CFCs in dark blue. The light blue shows the replacement chemicals - that are less harmful to ozone - and are still in their phase-out stage.

Panel (b) shows the changing concentrations of chlorine from those CFCs (but also including the contribution from bromine which is more effective than chlorine at depleting ozone). The initial concentration is about 0.6 ppb, and the maximum, shortly after the turn of the century, was about 3 ppb. Concentrations have gradually reduced since.

On those first two panels, the y-axis minimum is zero. But the range is more restricted for the next two panels.

Panel (c) chows the change in ozone since global data (shown in black) became available (satellite data became available only in the late 1970s). But it’s a bit misleading without the scale because the maximum reduction in global ozone is small: less than 5%, around the same time as the chlorine peaked. The red shading shows the calculated changes including the range of uncertainty.

Panel (d) shows the calculated changes (in dark yellow) of sun-burning UV due to those changes in ozone. The wider range in light yellow includes the further uncertainties due to possible interactions with climate change. Again, it’s a bit misleading, as the y-axis range is truncated. The maximum increase in UV is less than ten percent, so the range of the y-axis is approximately 0.95 to 1.05 (rather than the 0 to ~2 which one might have assumed).

Decreases in ozone (and therefore increases UV) are much larger in the Antarctic spring, and the apparently much larger changes shown in the lower two panels (assuming the y-axis goes down to zero) would be more realistically indicative of those in Antarctica during the month of October.

And a (partial) successor to the old chestnut …

Shown below is our current thinking about ozone changes (this time with scales on the axes). The plots show measured ozone changes since the 1970s (in red), and the “current” ozone projections (in blue) for the entire world (at top) and for the spring in the Antarctica (at bottom). The message remains consistent with that in panel (c) above which was drawn 12 years earlier. The main difference is a reduction in the spread of uncertainty in the model projections. We’re on track for a full recovery, but it will take decades to get there.

From Q20 in WMO/UNEP’s “Twenty Questions and Answers” about ozone. The anomalously low global ozone amounts in the early 1990s were due to efficient ozone-depleting reactions on the surfaces of aerosols injected into the stratosphere after the volcanic eruption of Mt Pinatubo.

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