Time to Heal

Ozone is on the repair, but it won't happen overnight ...

I agree with US President-elect Joe Biden that it’s a “time to heal”, but I just wish the ignorant incumbent hadn’t done so much to fan the fires of destruction. His actions (and inaction) over the last four years have insured that our atmosphere’s wounds will fester longer.

As I reported in my book, the Montreal Protocol on Protection of the Ozone Layer is working. But don’t expect a complete fix overnight. That will take decades. And still longer if we keep emitting greenhouse gases at the current rate.

The chemicals that cause ozone depletion are now controlled substances, and their atmospheric concentrations are on the decline (as expected). But they have very long lifetimes in the atmosphere, so it will be decades before their concentrations reduce to those in the 1970s before the emergence of the Antarctic ozone “hole”. Consequently, Antarctic ozone holes are expected to recur each spring for decades to come, and to only gradually diminish in severity until a full recovery in the final quarter of the century.

You can keep track of developments at NASA’s Ozone Watch pages. The plots below from there clearly show that the problem is improving, but only gradually. After their rapid growth through the 1980s and 1990s, the size of the springtime ozone holes levelled out at their most severe around the turn of the century. They’ve been gradually diminishing in severity since, in line with the decreasing amounts of chlorine. The average area of the ozone hole has been decreasing and the average ozone minimum has been increasing.

But there are also large year-to year differences. For example this year’s hole was relatively large (and persistent), while the previous year’s was the smallest in decades. That persistence is an important factor as far as UV is concerned because high UV levels require low ozone amounts as well high sun elevation angles.

There are a couple more flies in the ointment ….

In addition to requiring high concentrations of chlorine (or bromine) in the atmosphere for the formation of the ozone hole, temperatures must also be low enough for ice crystals to form in the ozone layer. That used to happen only in Antarctica’s polar night. But in recent years it has sometimes been cold enough even in the Arctic. And in Antarctica, the volume of air that gets cold enough is increasing.

Why is that? Well, the most obvious effect of climate change due to increasing greenhouse gases like carbon dioxide, is a warming at the surface. But the trapping of energy lower down in the atmosphere also results in cooling higher up. Large ozone losses themselves also lead to cooling there because ozone heats the local atmosphere by absorbing both incoming solar ultraviolet radiation and outgoing infrared radiation. So lower temperatures cause still lower temperatures. That’s an example of a dreaded positive feedback.

At the altitudes of the ozone layer, the decrease in temperature in recent decades has been twice as large as the increase at the surface. Those temperature decreases increase the likelihood of ice crystal formation, so in the next decades - while the concentrations of atmospheric chlorine remain high - relatively severe ozone holes should occasionally be expected (as for this year).

Hopefully, because chlorine is on the decline, future ozone holes probably won’t ever again be as severe as they were at their peak. But it came close this year, and it might be worse in the future if there are:

1. injections of aerosols into the stratosphere (e.g., from volcanic eruptions, or some suggested climate intervention strategies), or

2. recurrences of the illegal fugitive emissions of CFCs (e.g., from China, as reported in 2019).

The good news is that, because of our successful actions to protect the ozone layer, large reductions in ozone have been confined to polar regions (mainly Antarctica). Consequently, any long term increases in UV caused by ozone depletion have been small over populated lower latitudes. The observed changes in UV at the southermost tip of New Zealand, Invercargill (latitude 46 S), are dominated by seasonal effects, with scarcely detectable long term increases due to ozone depletion in the 1980s and 1990s. Invercargill is the closest New Zealand site to Antarctica, and the only UV measurement site in the country that was operational over the growth period of the Antarctic ozone hole (see above plot). Measurements from the other 4 greyed-out sites in the legend of the time series plot below started only in the mid-1990s.

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