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Mark ở vũ trụ khác · Another Universe Big Events Revisited #7 · Environment · Decision-making

The ozone hole

From a warning in 1974 to the Montreal Protocol in 1987, and the recovery since

What happenedCFC gases from spray cans and fridges thinned the ozone layer that blocks ultraviolet light
The numberOctober ozone at Halley: 300 DU (1957–72) down to 150 DU (1987)
What the world didsigned the Montreal Protocol on 16 Sep 1987; every country had ratified by 2009
Main sourcesNature 1974 and 1985, BAS Halley measurements, UNEP/WMO, NASA, EPA

Source: Molina and Rowland, Nature 249 (1974); British Antarctic Survey October ozone at Halley (Shanklin 2001); Montreal Protocol 1987; Decision XXI/1 of the Parties (UNEP, 2009).

One-minute recap

Essence

Warned in 1974Two chemists worked out that chlorine from CFCs could destroy ozone in the stratosphere.
Measured in 1985The instrument at Halley station, Antarctica: October ozone down from about 300 to 194 DU (1984).
Signed before 100% certainty16 Sep 1987: the Montreal Protocol cut CFCs while the cause of the hole was still being proven.
The whole world signedIn 2009 it became the first UN treaty ratified by every country.
Recovering, not healedProjected back to 1980 levels around 2066 over Antarctica, if commitments hold.

Source: Molina and Rowland, Nature 249, 810–812 (1974); BAS, October ozone at Halley (Shanklin 2001) and BAS bulletin 95/96 (1957–72 mean 300 DU); Montreal Protocol (16 Sep 1987); UNEP Decision XXI/1 (2009); WMO/UNEP Scientific Assessment of Ozone Depletion 2022 (2066 projection).

Part 1

The question of the event

In 1987 nobody had fully proven that CFCs were punching the hole in the ozone layer. Why did almost the whole world agree to cut them anyway? Because waiting cost more than being wrong: an ozone layer, once lost, cannot be bought back.

The answer is Mark's view, based on the sourced milestones in Parts 2 and 3.

1974–1985WarningA theory on paper, then real measurements in Antarctica.
→
1985–1990DecisionA framework convention, a protocol, then step-by-step tightening.
→
1990–nowSlow recoveryCFCs last for decades, so the hole shrinks only very slowly.
Part 2

Timeline

The first picture is real data: mean October ozone (Antarctic spring) at the British Antarctic Survey's Halley station.

Mean October ozone at Halley, Antarctica, 1956 to 2000: around 300 DU until the mid-1970s, down to 194 DU in 1984, 150 DU in 1987, lowest 122 DU in 1993. The Montreal Protocol was signed in September 1987. 100 200 300 0 1957–72 mean: 300 DU Sep 1987 · Montreal 1984: 194 1987: 150 1993: 122 1960 1970 1980 1990 2000 October ozone at Halley, Dobson units (DU) BAS provisional values; the table ends in 2000; years without data are skipped
Falling from the mid-1970s, bottoming out after the treaty was signed. October 1984: 194 DU. October 1987: 150 DU. After the Protocol was signed the numbers kept falling, to a low of 122 DU in 1993, because CFCs already released stay in the air for decades. DU (Dobson units) measure the total ozone in a column of air.Source: British Antarctic Survey, monthly mean ozone at Halley 1956–2000 (Shanklin 2001, provisional values), via the "ozone" dataset of the DAAG statistics package; 1957–72 mean = 300 DU from BAS ozone bulletin 95/96.
Timeline from 1974 to around 2066: warning 1974, US bans CFCs in spray cans 1978, Vienna Convention and discovery of the hole 1985, Montreal Protocol 1987, London Amendment 1990, Nobel 1995, largest hole 2006, universal ratification 2009, Kigali 2016, 2025 fifth smallest hole since 1992, projected recovery around 2066. 1974warningNature 1978US bansspray cans 1985Vienna,the hole 1987Montrealcut 50% 1990Londonphase-out 1995NobelChemistry 2006largesthole 2009all nationsratify 2016KigaliHFCs 20255th smallestsince 1992 ~2066recovery(projected) spacing between milestones is not to time scale
13 years from warning to treaty, more than 90 years from warning to healing. Green marks recovery; 2066 is a projection, not a measurement.Source: Nature 1974 and 1985; FDA and EPA rules (Federal Register, 17 Mar 1978); UNEP Ozone Secretariat; Nobel Foundation; NASA/NOAA (largest hole 2006, 2025 fifth smallest since 1992); UNEP Decision XXI/1; WMO/UNEP Scientific Assessment 2022 (2066).
1974 · A warning on paperMario Molina and Sherwood Rowland publish in Nature: CFCs drift up to the stratosphere, ultraviolet light frees chlorine atoms, and each chlorine atom destroys ozone in a chain reaction.
1978 · Spray cans bannedThe US bans CFCs as propellants in "non-essential" aerosols (rule of 17 Mar 1978, manufacturing ban from 15 Oct 1978). Fridges, air conditioners and insulating foam still use CFCs.
1985 · Vienna ConventionCountries sign a framework convention: commitments to research and cooperate, no cuts yet.
May 1985 · The hole appearsFarman, Gardiner and Shanklin (British Antarctic Survey) publish in Nature: spring ozone over Halley down by about a third. In 1986 Nimbus-7 satellite data confirm it.
16 Sep 1987 · Montreal ProtocolFreeze CFCs and halons at 1986 levels, then cut by 50% within about 10 years. A clause to revise the cuts based on regular scientific assessments.
1 Jan 1989 · In forceCondition met: at least 11 parties representing two-thirds of 1986 consumption.
1990 · TightenedLondon Amendment: developed countries phase out CFCs by 2000; a Multilateral Fund is set up to help developing countries switch.
1995 · Nobel Prize in ChemistryAwarded to Paul Crutzen, Mario Molina and Sherwood Rowland for their work on the formation and decomposition of ozone.
2006 · Largest holeThe largest seasonal average area ever measured.
16 Sep 2009 · Every country ratifies196 parties. The Vienna Convention and the Montreal Protocol become the first treaties in UN history to achieve universal ratification.
2016 · Kigali AmendmentThe Montreal machinery is used to cut HFCs, CFC substitutes that spare ozone but warm the planet.
2025 · Fifth smallest since 1992NASA/NOAA: average 18.71 million km² (7 Sep to 13 Oct), about 30% smaller than in 2006. WMO ranks it fourth to fifth smallest.
Around 2066 · Projected recoveryBack to 1980 levels: around 2066 over Antarctica, 2045 over the Arctic, 2040 for the rest of the world.

Source: Molina and Rowland, Nature 249 (1974); FDA and EPA, 43 FR 11301 (17 Mar 1978) and EPA release on the ban from 15 Oct 1978; Vienna Convention 1985 (UNEP Ozone Secretariat); Farman, Gardiner and Shanklin, Nature 315, 207–210 (1985), "a third" per UKRI (secondary source); Stolarski et al. (1986), Nimbus-7 satellite; Montreal Protocol 1987, cuts per Britannica (secondary source); entry-into-force condition from the Protocol text (Article 16), via Wikipedia (secondary source); London Amendment 1990; Nobel Foundation (1995); NASA/NOAA 2025; UNEP Decision XXI/1 (2009); Kigali Amendment 2016; WMO Ozone and UV Bulletin No. 4 (16 Sep 2026); WMO/UNEP Scientific Assessment 2022, years per AP 9 Jan 2023 (secondary source).

Part 1

Timeline

1974300 → 150 DU16 Sep 19872066
Part 2

Key decisions

spray cans 1978framework firstrole-play 1987myths
Part 3

What the world learned

regular reviewsbring the slow alongMark's score
Part 3

Key decisions

Five decisions, seen through the eyes of the time: who decided, what they knew, what they chose, what followed.

1 · Ban CFCs in spray cans US, 1978

Who decided
The US FDA and EPA (with the Consumer Product Safety Commission).
What they knew
The Molina–Rowland theory and studies by the US National Academy of Sciences. Nobody had measured an ozone decline yet.
What they chose
Ban the easiest part to replace: CFC propellants in non-essential aerosols. Keep essential uses such as inhalers.
What followed
It showed CFCs could be replaced without economic collapse. But fridges, air conditioning and foam still used CFCs, and this was one country.

Source: FDA and EPA, 43 FR 11301 (17 Mar 1978); EPA release "Government Ban on Fluorocarbon Gases in Aerosol Products Begins October 15" (1978); CPSC 1977 (via search excerpts, full text not opened).

2 · Frame first, cut later Vienna, 1985

Who decided
Countries meeting under the UN Environment Programme (UNEP).
What they knew
Models predicted a slow ozone decline. The Halley paper was not yet published.
What they chose
A framework convention: commitments to research, share data and cooperate. No cuts.
What followed
Nothing was cut yet. But when news of the hole arrived a few months later, the negotiating table already existed.

Source: Vienna Convention 1985 (UNEP Ozone Secretariat); Farman et al., Nature 315 (May 1985). The "table already existed" point is Mark's view.

3 · Sign cuts before 100% certainty Montreal, 16 Sep 1987

Who decided
The negotiators: 24 countries and the European Economic Community signed on the first day, including European countries, Japan, the Soviet Union and the US.
What they knew
The hole was real (Halley, satellites). But why it appeared was still debated: chlorine, or the winds and temperatures of the polar region.
What they chose
Freeze at 1986 levels, cut 50% within about 10 years, and revise the cuts based on regular scientific assessments.
What followed
The 50% cut later proved far too small. But thanks to the review clause, the parties could tighten as soon as the evidence grew stronger.

Source: Montreal Protocol 1987; number of signatories per Nature and UNEP (some sources such as Britannica say 46, counting countries that signed later); memorandum to President Reagan, FRUS 1981–88 vol. XLI, doc. 375 (Europe, Japan, the Soviet Union signed); four-yearly assessments per UNEP/WMO 2022.

4 · Tighten to a phase-out, and pay the slow London, 1990

Who decided
The parties to the Protocol, at their second meeting.
What they knew
In 1988 a NASA assessment confirmed ozone loss beyond Antarctica and tied it to CFCs. At Halley, October 1987 and October 1989 were both down to 150 DU.
What they chose
Developed countries phase out CFCs entirely by 2000. A Multilateral Fund to help developing countries, which had already been given a 10-year delay in 1987.
What followed
Large developing countries joined. This laid the ground for universal ratification in 2009.

Source: London Amendment 1990 and Article 5 of the Protocol (UNEP Ozone Secretariat); BAS, October ozone at Halley (Shanklin 2001); UNEP Decision XXI/1 (2009).

5 · Reuse the machinery for climate Kigali, 2016

Who decided
The parties to the Montreal Protocol.
What they knew
HFCs, the CFC substitutes, do not destroy ozone but are very strong greenhouse gases.
What they chose
Add HFCs to the list of substances to be cut on a schedule.
What followed
The 2022 scientific assessment estimates 0.3–0.5 °C of warming avoided by 2100.

Source: Kigali Amendment 2016 (UNEP); WMO/UNEP Scientific Assessment 2022, the 0.3–0.5 °C figure per Al Jazeera/AP 9 Jan 2023 and IIFIIR (secondary sources).

Part 4

What the world learned

What changed after the event: laws, institutions, habits.

The Montreal Protocol loop: scientific panels assess every four years, the parties adjust the cuts, countries implement, the atmosphere is measured again, then the next assessment. Alongside is the Multilateral Fund helping developing countries. scientific review tighten the cuts countries act measure the air →↓←↑ every 4 years Multilateral Fund helps developing countries illustrative diagram, no figures
Not signed once and done. The Protocol is a loop: science measures, the parties adjust, then measure again. That is how the 50% cut of 1987 became a full phase-out.Source: assessments prepared every four years under the Protocol (UNEP/WMO 2022); London Amendment 1990 and the Multilateral Fund (UNEP Ozone Secretariat).

Law: cuts on a schedule

About 99% of controlled ozone-depleting substances have been phased out (WMO/UNEP Scientific Assessment 2022, per UNEP).

Institutions: science has a seat

The four-yearly assessment is a required input for every adjustment. The 2026 edition is in preparation, with the Executive Summary expected late 2026 (NOAA CSL).

Habit: monitor to catch cheating

Atmospheric networks detected an unexpected rise in CFC-11 emissions after 2012; once traced, emissions fell again from 2019 (Montzka et al., Nature 2018 and 2021).
Two benefits estimated with models. Health: EPA estimates that for people born in the US between 1890 and 2100 the Protocol avoids about 443 million skin cancer cases, 2.3 million deaths and 63 million cataract cases. Climate: Velders et al. 2007 conclude that Montreal's climate benefit is far larger than the Kyoto first-period target. health (people born in the US only, 1890–2100, EPA model) 443 million skin cancer cases avoided 63 million cataract cases 2.3 million skin cancer deaths climate: Montreal's benefit is far larger than Kyoto's first-period target (Velders et al., 2007)
These are estimates, not counts. The EPA figures cover only Americans and compare against a no-controls scenario.Source: EPA (May 2020), AHEF model, and peer-reviewed paper in ACS Earth and Space Chemistry (2021); Velders et al., PNAS 104(12), 4814–4819 (2007) (via search excerpts, full text not opened).
Part 5

Fact check: truth and myths

14 things often said about the ozone layer, checked against measurements and scientific reports. Tap each line for the reasoning.

October ozone at Halley three years running: 1987 was 150 DU, 1988 was 216 DU, 1989 was 150 DU again. The hole does not shrink steadily every year. Oct 1987150 DU Oct 1988216 DU Oct 1989150 DU a "good" year can sit right next to a "bad" one: weather makes the numbers swing
One small year does not prove healing, one large year does not prove failure. Look at the trend over many years.Source: BAS, October ozone at Halley (Shanklin 2001); WMO, Ozone and UV Bulletin No. 4 (16 Sep 2026): year-to-year swings are mainly meteorological.
SOLIDScientists warned in 1974 that CFCs destroy ozone

Molina and Rowland, Nature 249, 810–812 (1974): CFCs are long-lived, rise to the stratosphere, ultraviolet light frees chlorine, and chlorine destroys ozone in a chain. In 1995 both, with Paul Crutzen, won the Nobel Prize in Chemistry.

SOLIDThe hole was found by a ground instrument, and satellites confirmed it later

Farman, Gardiner and Shanklin used a Dobson spectrophotometer at Halley (Nature, May 1985). Stolarski et al. (1986) confirmed it with TOMS and SBUV data from the Nimbus-7 satellite.

SOLIDMontreal was signed before the cause of the hole was fully proven

By September 1987 the hole had been measured, but the chlorine explanation still competed with hypotheses about polar meteorology. A NASA assessment in 1988, after the signing, clearly tied the loss to CFCs.

SOLIDIt was the first treaty ratified by every country

On 16 Sep 2009, with 196 parties, the Vienna Convention and the Montreal Protocol became the first treaties in UN history to achieve universal ratification (UNEP, Decision XXI/1).

SOLIDAlmost all controlled ozone-depleting substances have been phased out

About 99%, according to the WMO/UNEP Scientific Assessment 2022 (as summarised by UNEP).

SOLIDMontreal did more for the climate than Kyoto's first-period target

CFCs are also strong greenhouse gases. Velders et al. (PNAS, 2007): Montreal's climate benefit is "far larger" than Kyoto's first commitment target; the authors presented it as about 5–6 times in 2010.

CAUTION"Montreal saved hundreds of millions from skin cancer"

The EPA's 443 million cases is a model estimate, only for people born in the US between 1890 and 2100, against a no-controls scenario. The large benefit is real, but it is not a count and not global.

CAUTION"Without Montreal, two-thirds of the ozone layer would be gone"

This is the "world avoided" model scenario of Newman et al. (ACP, 2009): if CFCs had grown 3% a year, 17% of global ozone would be lost by 2020 and 67% by 2065. Useful to see the scale, not a certainty.

CAUTION"The ozone hole causes global warming"

Two different problems: the ozone hole is about ultraviolet light, warming is about greenhouse gases. But they are linked: CFCs and HFCs are greenhouse gases, so cutting them also helps the climate (Velders 2007; Kigali Amendment).

FALSE"The ozone hole has healed itself"

Not healed, and not "by itself": the ozone layer is recovering because people cut CFCs. In 2025 the hole still averaged 18.71 million km² (NASA/NOAA). Return to 1980 levels is projected around 2066 over Antarctica (2022 Assessment).

FALSE"The ozone hole is made up"

Continuous measurements at Halley since 1956: October mean 300 DU (1957–72), down to 150 DU in 1987 and 122 DU in 1993. Satellites confirmed it independently. The chemistry earned a Nobel Prize in 1995.

FALSE"The hole shrinks steadily every year"

At Halley: 150 DU (1987), 216 DU (1988), 150 DU (1989). WMO says year-to-year swings are mainly meteorological; the long-term trend is what shows recovery.

FALSE"CFCs are heavier than air, so they cannot reach the stratosphere"

Winds and convection mix the atmosphere, so heavy and light gases alike are carried up. CFCs have been measured directly in the stratosphere (WMO/UNEP, "Twenty Questions and Answers About the Ozone Layer").

FALSE"Volcanoes put more chlorine into the stratosphere than people do"

Volcanic chlorine mostly dissolves in water and is rained out in the lower atmosphere. Stratospheric chlorine comes mainly from human-made substances (WMO/UNEP, "Twenty Questions and Answers About the Ozone Layer").

Source: as noted in each line; full list in Part 9.

Part 6

How well did the world respond?

Scores are Mark's assessment based on the sources in Part 9, not data from a study.

Early detection
4/5
Acting before certainty
5/5
Global coordination
5/5
Speed
3/5
Learning and adjusting
5/5
Done right
  • Signed cuts in 1987 when evidence was strong enough, not certain
  • Built in a clause to revise based on science
  • Paid to help developing countries instead of only pressuring them
Not done right
  • 13 years from warning (1974) to treaty (1987)
  • The initial 50% cut was too small and had to be tightened
  • Illegal emissions still happened, caught only by monitoring the air

One of the rare times the whole world got it right: it did not wait for 100% certainty, and it did not sign once and stop. The minus is speed, and that slowness is paid for with more than half a century of waiting to heal.

More: why did it work this time, when many other problems did not?

Some conditions helped: few chemicals to cut, few producing companies, substitutes already available, and a danger (skin cancer) that is easy to picture. This is Mark's view, not the finding of a study.

But helpful conditions do not turn into a treaty by themselves. What made the difference was the design: frame first, cut small first, review regularly, and pay the slow. That design works at work too.

Part 7

Work lesson: act when the evidence is strong enough, don't wait for 100% certainty

For any team that has to decide before the data is complete: a spreading bug, a customer about to leave, a risk that keeps growing.

Four habits: compare the cost of waiting with the cost of being wrong; decide small with a review clause; set the tightening threshold in advance; bring the slow along with support. 1 · cost of waitingwait vs. wrongwhich can't be undone 2 · decide small firstwith a review datelike the 50% of 1987 3 · written thresholdevidence X, then Ydecide before you need to 4 · bring the slow alongsupport, not just pressurelike the Multilateral Fund "Not certain yet" does not mean "do nothing yet".
Each habit comes from a real decision. A small cut with a review clause (1987), tightening when evidence grew (1990), the Multilateral Fund (1990).Source: the decisions in Part 3; the habits are Mark's suggestions.
Compare the two costsWrite it down: if we wait 3 more months and the risk is real, what do we lose? If we act now and the risk is not real, what do we lose? Which one can't be undone?
Decide small, set a review dateDon't demand a perfect decision. Take the smallest step that works and write down the date the team will look at the data again.
Write the threshold now"If errors exceed X next week, we stop the release." Write it while calm, not while on fire.
Bring the slow alongWhich group will find the change hardest? What do they need: time, tools, a helper? Give them that instead of just a deadline.
Check your own team: 5 questions

1. The last decision your team postponed "to wait for more data": what data, and when will it arrive?

2. When the team reports a risk, does it state a confidence level (low, medium, high), or just "not sure yet"?

3. Which running decision has no review date? Write one today.

4. At what threshold does the team tighten, and at what threshold does it relax? Is it written down?

5. Who in the team (or which partner team) will be slowest to change, and what have you done to help them?

We simulated a world avoided, and it's a world we should be glad we avoided.Paul Newman (NASA) on the "world avoided" scenario (2009), per NASA (via search excerpts).
Part 8

Further reading

WMO/UNEP, "Scientific Assessment of Ozone Depletion: 2022"The latest full assessment. The 2026 edition is expected at the end of 2026.
WMO/UNEP, "Twenty Questions and Answers About the Ozone Layer"Short Q&A explaining each myth in plain language.
Farman, Gardiner and Shanklin (1985), Nature 315, 207–210The three-page paper that told the world about the hole.
Big Events Revisited #08: The 1973 oil crisisDon't let a single source of supply decide your survival.
Part 9

Primary sources

Peer-reviewed papers, original measurements and documents from UNEP, WMO, NASA and EPA come first. The working environment blocked most of these sites, so apart from the Halley measurement table (obtained in full), sources were checked through excerpts of the original pages in search results, without opening the full text. Press sources are marked as secondary.

  1. Molina, M. J. and Rowland, F. S. (1974), "Stratospheric sink for chlorofluoromethanes: chlorine atom-catalysed destruction of ozone", Nature 249, 810–812. doi.org
  2. Farman, J. C., Gardiner, B. G. and Shanklin, J. D. (1985), "Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction", Nature 315, 207–210. doi.org
  3. British Antarctic Survey, monthly mean ozone at Halley 1956–2000 (Shanklin, 2001, file zoz5699.dat), via the "ozone" dataset of the R package DAAG; BAS ozone bulletin 95/96 (1957–72 mean). antarctica.ac.uk
  4. Stolarski, R. S. et al. (1986), confirmation of the Antarctic springtime ozone decline with TOMS and SBUV (Nimbus-7), Nature 322.
  5. FDA and EPA, rules banning CFCs in non-essential aerosols, 43 FR 11301 (17 Mar 1978); EPA release on the ban from 15 Oct 1978. epa.gov
  6. UNEP Ozone Secretariat, Vienna Convention (1985), Montreal Protocol (1987), London Amendment (1990), Kigali Amendment (2016), Decision XXI/1 (2009). ozone.unep.org
  7. US Department of State, Office of the Historian, FRUS 1981–88 vol. XLI, doc. 375: memorandum to President Reagan on the Montreal Protocol. history.state.gov
  8. WMO/UNEP, Scientific Assessment of Ozone Depletion: 2022, Executive Summary. digitallibrary.un.org. Status of the 2026 edition: csl.noaa.gov
  9. WMO, Ozone and UV Bulletin No. 4 (Sep 2026) and release of 16 Sep 2026. wmo.int
  10. NASA and NOAA, ranking of the 2025 ozone hole (Nov 2025). science.nasa.gov
  11. EPA (2020), health benefits of the Montreal Protocol (AHEF model); peer-reviewed paper in ACS Earth and Space Chemistry (2021). pmc.ncbi.nlm.nih.gov
  12. Newman, P. A. et al. (2009), "What would have happened to the ozone layer if chlorofluorocarbons (CFCs) had not been regulated?", Atmospheric Chemistry and Physics 9, 2113–2128; NASA Earth Observatory. svs.gsfc.nasa.gov
  13. Velders, G. J. M. et al. (2007), "The importance of the Montreal Protocol in protecting climate", PNAS 104(12), 4814–4819. pubmed
  14. Montzka, S. A. et al., Nature 557 (2018) and Nature 590 (2021), unexpected rise and later fall of CFC-11 emissions.
  15. WMO/UNEP, Twenty Questions and Answers About the Ozone Layer (2018 update).
  16. Nobel Foundation, Nobel Prize in Chemistry 1995.
  17. Secondary sources: AP and Al Jazeera 9 Jan 2023 (2066, 2045, 2040 and 0.3–0.5 °C); IIFIIR; UKRI and BBC Mundo (Halley, "a third"); Britannica and EBSCO (initial cuts, the figure of 46 countries); Wikipedia (entry-into-force condition); IISD (2009 ratification); Universe Today (2065 scenario).