How We Know
Every claim on this slate rests on measurement. This page covers who measures the ozone layer, how, and where the data is thin. It also covers the time the satellite saw the hole and threw the reading away.
Measuring the Effect
The core instrument is the Dobson spectrophotometer, which measures how much ultraviolet light the ozone column absorbs overhead. The one that matters most sits at Halley in Antarctica, and its record goes back to 1956. Length is what made it useful. A single low reading proves nothing. Thirty years of readings from the same instrument at the same place is what turned October 1984 into an anomaly somebody could publish. [25] [2]
Halley ozone record begins [25]
1956
Seasons lost when Halley closed, 2017 [26]
2
Years of satellite data discarded before 1985 [28]
2
The Near Miss
A satellite was already watching. The Total Ozone Mapping Spectrometer on Nimbus-7 recorded extremely low values over Antarctica in 1983 and 1984. Nobody saw them. The processing software had been written to discard readings that fell far outside the expected range, on the reasonable assumption that such readings are instrument faults. The real hole was filtered out as bad data. [28] After the ground station paper appeared in 1985, NASA went back to the archive, reprocessed it without that filter, and confirmed the hole from space. [24]
This is the most useful thing on the page. The satellite had better coverage, better funding and more data than the ground station, and it still lost. Quality control that throws away outliers will throw away the discovery. A hut in Antarctica with a 1950s instrument is what rescued the satellite record, not the other way round.
Measuring the Cause
Measuring ozone tells you the damage. It does not tell you who caused it. That needs a second network measuring the gases themselves in the air, run by NOAA's Global Monitoring Laboratory and by AGAGE. [27] This is the network that caught CFC-11. Production had been banned worldwide since 2010 and the official reports said everyone was complying. The air disagreed, and the air won. [10] [11]
Turning Measurement Into Policy
Measurement only matters if it reaches the people writing the rules. The WMO and UNEP publish a full scientific assessment every four years, on a fixed schedule that does not move for political convenience. That is what let the treaty be tightened as evidence arrived rather than when negotiators felt like reopening it. The 2022 report is the current one. The 2026 report is being written. [8] [23]
Where the Data Is Weak
Five gaps. None of them is a task somebody is going to close this year, and two of them are not measurement problems at all but the absence of any obligation to measure.
Monitoring coverage is uneven
CFC-11 was traced to eastern China partly because eastern Asia is densely instrumented. Large parts of the world have far fewer stations, and the same breach in one of them would have been much harder to locate, or to attribute at all. [10] [11]
The longest record has a hole in it
A crack in the ice shelf forced Halley to close in February 2017, costing two ozone hole seasons from the very record that found the problem in the first place. Satellite data has been used to fill the gap, which is a repair, not a replacement. [26]
Feedstock emissions are modelled, not measured
The exemption was granted on a leakage estimate that turned out to be wrong. The figures that replaced it are still modelled, so the seven-year delay attributed to feedstocks rests on inference rather than observation. [17]
Illegal HFC trade has no independent measurement
The only figure available is an industry estimate of up to 30% of the EU market. No enforcement body or independent study has published a competing number, so the scale of the problem is known only from the people it costs money. [18]
Two major gases nobody is required to report
Nitrous oxide and the short-lived chlorine gases are measured by research networks, but neither is controlled by the Montreal Protocol, so no country has any reporting duty for them. The largest ozone-damaging emission we make is tracked as science, not c
If you want to strengthen this slate, this page is the highest-value place to do it. A sourced map of monitoring coverage does not exist here yet, and it would make the first item above concrete instead of general.