MadSci Network: Environment/Ecology

Re: enviroment

Area: Environment/Ecology
Posted By: John Christie, Faculty, School of Physical Chemistry, La Trobe University, Bundoora, Victoria, Australia
Date: Thu May 2 19:41:03 1996

A good way to think of the ozone supply in the stratosphere is that it is like an old bathtub, with the tap running full in, and no plug in it. After a while, the water finds a steady level - perhaps half full. Now if that bathtub springs a new leak, it will not empty completely, but find a new steady level a bit lower down.

There are natural processes that continuously supply and remove ozone from the stratosphere (like the tap and the plug-hole). The way that people used chlorine containing compounds between about 1950 and 1987 made an important extra removal process (like a large new leak in the tub), and so it lowered the level of ozone by a few percent. And there is a special process that started up near the South pole that removes nearly all of the ozone from that region for a month or two each year -- recently a similar process has started to occur near the North pole, but it is not nearly so severe.

There is good news and bad news. The good news you will understand from my bathtub model -- if we plug the leak, the water will return to the level it was at before. And people decided in 1987 to stop using the substances that were causing the problem in that way, so the ozone level should eventually return to what it was before. The bad news is that those chlorine compounds have spread throughout the atmosphere, and they have a very long lifetime there. So it might take about 5 to 10 years before the ozone hole stops getting worse, and about 50 to 100 years before the ozone levels return to what they were in the 1960s before the problem started.

The ozone levels have not changed in the equatorial zone. They are down by 2 to 4% in the North temperate zone, and 5 to 8 % in the South temperate zone. In the polar regions the change is very seasonal -- 60 to 80% in the Antarctic spring, but 5% or less in the other seasons; perhaps 20% in the Arctic spring.

There is a lot of good information about stratospheric ozone on the Web. You might enjoy some cartoons that my wife and I made on:

But the real information can be found from links at

Regards, John.

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