The Mechanism
Halley Bay is a British Antarctic Survey base on the edge of the Brunt Ice Shelf. Since 1957 the base had operated a Dobson ozone spectrophotometer — a wooden, prism-based instrument that measures the total ozone column above the observer by comparing two ultraviolet wavelengths of sunlight. From the late 1970s the October readings — early antarctic spring — began drifting downward. By 1984 the October column was around 200 Dobson units, down from a 1960s baseline of roughly 320: a near-halving. Joseph Farman, the head of the BAS atmospheric programme, his deputy Brian Gardiner, and the young meteorologist Jonathan Shanklin spent over a year cross-checking the instrument, the calibration plates, and the data before they would publish. NASA's NIMBUS-7 satellite, carrying the Total Ozone Mapping Spectrometer, had been measuring antarctic ozone from space since 1978 and had recorded the same drop — but the satellite's data-processing pipeline had flagged any column below 180 Dobson units as instrument error and discarded those readings. The British team's paper appeared in *Nature* on 16 May 1985. Within weeks NASA had retrieved the satellite data from tape archives and reprocessed it without the cutoff; the entire history of the hole appeared on the maps, starting around 1979. Susan Solomon at NOAA led an expedition to McMurdo in 1986 that proved the mechanism: chlorine released from chlorofluorocarbon refrigerants was catalysing the destruction of ozone on the surfaces of polar stratospheric clouds. The Montreal Protocol was signed in September 1987. It is the only environmental treaty ever ratified by every country on Earth, and the ozone hole is now slowly closing.
Why It Matters
This discovery is remarkable because the ozone hole was not found by a brand-new machine or a dramatic expedition. It was uncovered by careful routine measurements at one place, then confirmed by a satellite dataset that had partly hidden the signal. The surprise was not only that the ozone was disappearing so fast, but that the evidence had been sitting in the data for years. The story also shows how science can depend on patient checking, not just one flashy result.
Wait — That's Not Quite Right
A common mistake is to think NASA discovered the ozone hole first and British scientists merely repeated it. In fact, the Halley Bay team identified the pattern from ground measurements and published it first, while NASA later recovered and corrected satellite data that had been filtered too aggressively. Another wrong idea is that the ozone hole is a literal gap in the sky; it is really a strong thinning of ozone high in the stratosphere.
Vocabulary
- ozone
- ozone hole
- dobson spectrophotometer
- dobson units
- halley bay
- antarctica
- nimbus-7
- total ozone mapping spectrometer
- chlorofluorocarbons
- polar stratospheric clouds
- stratosphere
- montreal protocol
Quick Quiz
5 questions · For classroom or kitchen table
The Experiment
Measure Sunlight Like an Ozone Scientist
Go outside on a sunny day with a white sheet of paper and a pencil. Look at how bright the sunlight is in different places - full sun, shade, near a wall, and under a tree. Write down where the light feels strongest and where it feels dimmest.
Then compare this to the ozone story. The Halley Bay scientists were not measuring brightness with their eyes, but they were comparing ultraviolet light at two wavelengths to figure out how much ozone was above them. You cannot measure ozone this way at home, but you can notice how scientists turn changes in light into information about the air.
If you want, make a simple table with three columns: place, brightness, and reason you think the light changed. This is a safe way to practice the same habit the Antarctic team used - careful observation before drawing a conclusion.
paper, pencil, sunny outdoor space, adult supervision recommended
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