I've been seeing small news items over the past week or two saying that this year's Antarctic ozone hole has matched the previous record, and that the amount of ozone is the lowest ever [e.g., LINK]. While this is interesting and important news, I'm wondering if it might confuse people. Afterall, a lot, way more than you think, of people think there is a direct link between anthropogenic global warming and the ozone hole. Not just laypeople on the street either, smart and usually-informed people think this. Climate scientists everywhere are constantly being forced to correct people at cocktail parties and other social events. "No, the ozone hole is due to chemicals called CFCs in the upper atmosphere; global warming has to do with burning fossil fuels."
After the gigantic ozone hole of 2000, the size has actually decreased, leading most to believe that the Montreal Protocol of 1987 was a smashing success, and that the hole would disappear in 50 years or so. Actually, that is still what most people in the know are thinking.
So what's with the new big ozone hole? Well, it may have something to do with global warming. Sigh.
Basically, every winter (in Antarctica the winter is during June-July-August) it gets really, really, really cold around and over Antarctica. Because of the geography of the southern hemisphere, there are incredibly strong winds that essentially circle around the continent of Antarctica. Cold air basically gets trapped inside this huge votex, and has nothing better to do than get even colder, all winter. During this deep freeze, polar stratospheric clouds (PSCs) can form, in which molecular chlorine can form (Cl2), and the stronger the vortex, the larger the PSCs and the more Cl2 can form.
When the spring comes, sunlight is added to the equation. Sunlight easily breaks the molecular chlorine into atmoic chlorine. The atomic chlorine (Cl) quickly reacts in a chain of events that destroys ozone; it's a catalytic reaction; a single chlorine atom can tear apart many ozone molecules. This is why the ozone hole appears suddenly in September, when the sun finally shines on the pole.
How is this related to global warming? Well, the same course of events happens year in and year out, but there is variability, of course. Because of the international agreement to eliminate the use of CFCs, every year the amount of CFCs decreases. As a side note, CFCs get absorbed by the upper ocean, and are used as a great passive tracer to study ocean motion. Even with the decrease, the coldness of the winter is still quite variable. The colder the winter, the stronger the polar vortex, the more PSCs can form and condition the stratosphere for ozone depletion. It is possible that the large-scale circulation pattern of the southern hemisphere could adjust to make the polar night colder even as the global surface temperature rises. A paper from 2000 explores some of these issues of synergy between stratospheric ozone depletion and greenhouse gas warming (Hartmann et al, 2000, PNAS). This is ongoing research, as the question of how the circulation will adjust to a warmer world is hard to answer, but my feeling is that more and more people seem to think that the change might favor these extremely cold winters with a strong polar vortex and favorable conditions for ozone depletion.
By the way, Cambridge has a nice ozone hole web site: LINK