Environmental science can feel like a list of problems that never get solved. This page is the exception. A global threat was identified, the world agreed to act, the chemicals were phased out, and the layer began to recover. It is the strongest evidence available that international cooperation can work — which is exactly why examiners like it so much.
📚 What you need to know
UNEP (the United Nations Environment Programme) raised awareness of the depletion, its causes and its impacts, and drove the creation of international agreements.
The Montreal Protocol on Substances that Deplete the Ozone Layer was initiated in 1987 under UNEP’s guidance.
It regulates the production, trade and use of CFCs and other ODSs. 24 countries signed initially; the total now stands at 197.
It has been strengthened over time — the Copenhagen amendment (1992) tightened restrictions further.
ODS emissions fell from around 1.5 million tonnes in 1987 to about 400,000 tonnes in 2010. UNEP aims to end all HCFC production by 2040.
Phased reductions gave industry time to adapt and to develop alternatives.
Stratospheric ozone depletion is one of the nine planetary boundaries, and action under the protocol prevented that boundary being crossed.
What UNEP did
UNEP’s contribution came in two parts. First, information: through research and by sharing what was found, it made governments, industries and the public aware that the ozone layer was being rapidly depleted, what was causing it, and what the environmental and health consequences would be. Second, agreement: it was actively involved in creating the international agreements that reduced ODS use. Awareness alone would have achieved nothing without a treaty; a treaty would have been unsignable without the awareness.
What happened to emissions
Drawn from the published pattern rather than exact annual figures. The shape is the point: a long rise, a sharp peak within a couple of years of the treaty, then a sustained decline that has continued for three decades.
The protocol itself
Initiated in 1987 under the guidance of UNEP, as the Montreal Protocol on Substances that Deplete the Ozone Layer.
It is a landmark agreement regulating the production, trade and use of CFCs and other ODSs.
24 countries signed initially; the total now stands at 197, which makes it one of the most widely adopted treaties in existence.
It has been updated and strengthened, notably at a summit in Copenhagen in 1992, which tightened restrictions further.
Phased reductions were deliberate: gradual schedules allowed industries to adapt and gave time for alternatives to ODSs to be developed and adopted.
National governments then enacted laws and regulations to cut consumption and production of halogenated organic gases, enforcing the targets and pushing the transition to ozone-friendly alternatives.
Phased reductions are the design detail worth understanding. An immediate ban would have been fought by every industry that depended on these chemicals. A phased schedule turned the question from “will you accept a ban” into “how quickly can you switch”, which is a far easier negotiation and gave manufacturers a commercial reason to invent replacements.
What it bought us
Schematic, showing the shape of published projections rather than specific case numbers. The lesson is that the amendments were not a formality — a treaty that had been signed once and left alone would have delivered far less.
Why this one succeeded
It is worth asking directly why the ozone treaty worked when climate agreements have struggled, because the comparison is a strong evaluation point in almost any question about international cooperation.
Feature
Ozone: the Montreal Protocol
Climate: the Paris Agreement
Number of substances
A small group of specific chemicals, made by a limited number of companies
Emissions from energy, transport, farming and industry — effectively the whole economy
Alternatives available
Substitutes existed or could be developed relatively quickly
Replacing fossil fuels means rebuilding entire energy systems
Cost of compliance
Moderate, and industry could profit from selling the replacements
Very large, spread unevenly across countries and sectors
Nature of the harm
Direct, visible and personal — skin cancer and cataracts
Diffuse, delayed and shared, which weakens the sense of urgency
Legal force
Binding phase-out schedules with monitoring and enforcement
Self-set national targets with review but no enforcement
Use this table carefully rather than as a criticism of climate policy. The honest reading is that Montreal was an easier problem: fewer chemicals, cheaper substitutes, clearer personal risk. That does not make the achievement small — it took real diplomacy — but it does mean you should not write “the world just needs to copy Montreal for climate change”. The most rewarded answers say what is transferable: binding targets, phased schedules, regular strengthening, and support for developing countries to comply.
Enforcement and the illegal market
An illegal market in ozone-depleting substances remains a real challenge to the effectiveness of ozone protection. Where a chemical is banned but still cheap and useful, smuggling follows. UNEP recognises the need for consistent monitoring and enforcement, and works with national authorities, customs agencies and other stakeholders to stop illegal trade and ensure compliance with international regulations.
The planetary boundary
Stratospheric ozone depletion is one of the nine planetary boundaries — thresholds which, if crossed, lead to significant environmental change. Action taken in response to the Montreal Protocol prevented this boundary being crossed, and the protocol is generally regarded as the most successful example yet of international cooperation resolving a major environmental problem.
The evidence comes from data. Measurements show a decrease in the size of ozone holes over time, and continuous monitoring indicates that recovery is underway. Recovery is slow, because CFC molecules already released persist in the atmosphere for decades, but the direction of travel is clear.
WORKED EXAMPLE
Explain the role of UNEP in protecting the stratospheric ozone layer. [3]
Point 1 — raising awarenessUNEP researched and shared information, educating governments, industries and the public that the layer was being rapidly depleted and why it mattered.Point 2 — creating agreementIt was directly involved in developing international agreements to reduce ODS use, and the Montreal Protocol of 1987 was initiated under its guidance.Point 3 — monitoring and enforcementIt continues to work with national authorities and customs agencies to tackle the illegal trade in ODSs and ensure compliance.3 / 3Awareness, agreement, enforcement. Three verbs, three marks.
WORKED EXAMPLE
Evaluate the success of the Montreal Protocol. [6]
Point 1 — participationIt grew from 24 signatories to 197 countries, making it one of the most widely adopted environmental treaties ever agreed.Point 2 — measurable outcomeODS emissions fell from around 1.5 million tonnes in 1987 to roughly 400,000 tonnes by 2010.Point 3 — environmental resultMonitoring shows ozone holes shrinking and recovery underway, and the planetary boundary for ozone depletion was not crossed.Point 4 — why the design workedBinding but phased reduction schedules let industry adapt and develop alternatives, and the treaty was strengthened over time, as at Copenhagen in 1992.Point 5 — limitationAn illegal market in ODSs persists, recovery is slow because CFCs remain in the atmosphere for decades, and the Antarctic hole still forms each spring.Point 6 — judgementOn balance it is a clear success, though partly because the problem was more tractable than climate change: fewer substances, cheaper substitutes and a directly visible risk to human health.6 / 6Point 5 stops this being an uncritical answer and point 6 gives the judgement an evaluate question demands.
💡 Exam tip
Memorise four numbers: 1987, 24 to 197 countries, 1.5 million to 400,000 tonnes, and HCFCs gone by 2040.
Name UNEP and the Copenhagen amendment. Specific names beat “an international agreement”.
Explain phased reductions as a reason for success, not just a feature of the treaty.
Mention the planetary boundary framing if the question is about management and intervention.
Always include a limitation: the illegal market, slow recovery, or the hole that still forms each spring.
The Montreal comparison is your best evidence in any “can international cooperation work?” question.
⚠️ Common mix-up
Saying the ozone layer is fixed. Recovery is underway, not complete, and the Antarctic hole still appears annually.
Confusing Montreal with Kyoto or Paris. Montreal is ozone; the others are greenhouse gases.
Forgetting Kigali. The 2016 amendment made Montreal a climate treaty too by targeting HFCs.
Thinking the ban was immediate. Reductions were phased over years, which is precisely why they worked.
Crediting the treaty alone. National governments had to pass their own laws to enforce it.
Assuming success means no problems remain. The illegal trade in ODSs is an active enforcement issue.
Up next: How Ozone Is Destroyed — the higher level material starts here, with the catalytic chemistry that lets one chlorine atom destroy thousands of ozone molecules.
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