How did the world manage to start healing the ozone layer?
Topic 9.2 Reducing Ozone Depletion: describe the strategies and international agreements used to reduce ozone depletion and how the ozone layer is recovering.
A focused answer to APES Topic 9.2, covering the Montreal Protocol, the phase-out of CFCs, substitutes (HCFCs and HFCs) and their trade-offs, why ozone recovery is slow, the success of international cooperation, and the lesson for other global problems, with a worked recovery-timescale reasoning example.
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What this topic is asking
The College Board (Topic 9.2) wants you to describe the strategies and international agreements used to reduce ozone depletion and how the ozone layer is recovering.
The Montreal Protocol and the phase-out
Substitutes and their trade-offs
Why recovery is slow
Why this matters
Topic 9.2 is the success story that completes Topic 9.1, and the AP exam contrasts it with the harder problem of climate change: ozone depletion had a clear cause, available substitutes and global agreement, so action worked. It links to greenhouse gases (HFC trade-off) and is the model for the international cooperation needed for global change.
Try this
Q1. Identify the international agreement that phased out ozone-depleting substances. [1 point]
- Cue. The Montreal Protocol.
Q2. Explain why the ozone layer recovers slowly even after CFCs are banned. [2 points]
- Cue. The CFCs already released are very persistent and remain in the atmosphere for decades, so the chlorine they release keeps destroying ozone long after production and emissions have stopped.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). (a) Identify the international agreement that phased out ozone-depleting substances. (b) Explain how it reduces ozone depletion. (c) Explain why the ozone layer recovers slowly even after CFCs are banned. (d) Identify one trade-off of a CFC substitute.Show worked answer →
A 4-point FRQ on reducing ozone depletion.
(a) Identify (1 point): the Montreal Protocol.
(b) Explain (1 point): it phases out the production and use of CFCs and other ozone-depleting substances, so less chlorine reaches the stratosphere over time.
(c) Explain (1 point): CFCs already released are very persistent and remain in the atmosphere for decades, so chlorine keeps destroying ozone long after emissions stop.
(d) Identify (1 point): HCFCs still deplete some ozone (a transitional substitute), and HFCs do not deplete ozone but are potent greenhouse gases.
Markers reward the Montreal Protocol, the phase-out mechanism, the persistence of existing CFCs for slow recovery, and a valid substitute trade-off.
AP 2018 (style)1 marksSection I (multiple choice). The Montreal Protocol is widely regarded as a success because it: (A) reduced carbon dioxide emissions worldwide (B) phased out ozone-depleting substances through international cooperation (C) banned all fossil fuels (D) eliminated acid rain. Justify your choice.Show worked answer →
A 1-point MCQ on the Montreal Protocol. The answer is (B).
The Montreal Protocol is celebrated for phasing out CFCs and other ozone-depleting substances through near-universal international cooperation, putting the ozone layer on a path to recovery. It did not target carbon dioxide (A), ban all fossil fuels (C), or address acid rain (D). The trap is confusing the Montreal Protocol (ozone) with climate agreements (carbon dioxide); they tackle different problems.
Related dot points
- Topic 9.1 Stratospheric Ozone Depletion: explain how CFCs deplete stratospheric ozone and describe the consequences of a thinner ozone layer.
A focused answer to APES Topic 9.1, covering the protective role of stratospheric ozone, how CFCs release chlorine that catalytically destroys ozone, the Antarctic ozone hole, the consequences of increased UV (skin cancer, cataracts, harm to ecosystems), and the difference from ground-level ozone, with a worked catalytic-destruction reasoning example.
- Topic 9.5 Global Climate Change: describe the evidence and effects of global climate change and explain the role of positive feedback loops.
A focused answer to APES Topic 9.5, covering the evidence for global climate change, its effects (rising temperatures, melting ice, sea-level rise, extreme weather, shifting species), positive feedback loops (ice-albedo, permafrost methane, water vapor), the difference between weather and climate, and mitigation and adaptation, with a worked sea-level reasoning example.
- Topic 9.4 Increases in the Greenhouse Gases: identify the human activities that increase greenhouse gases and explain why their concentrations are rising.
A focused answer to APES Topic 9.4, covering the human activities that raise greenhouse gases (fossil fuel combustion, deforestation, agriculture, landfills, industry), the specific sources of carbon dioxide, methane and nitrous oxide, the role of the carbon cycle, and the Keeling Curve evidence, with a worked emissions calculation.
- Topic 9.10 Human Impacts on Biodiversity: identify the major human causes of biodiversity loss (HIPPCO) and explain why declining biodiversity matters.
A focused answer to APES Topic 9.10, covering the HIPPCO causes of biodiversity loss, why habitat loss is the largest, the importance of biodiversity for ecosystem services and resilience, the sixth mass extinction, and conservation responses, with a worked species-loss reasoning example.
- Topic 7.6 Reduction of Air Pollutants: describe methods used to reduce air pollution, including regulation, scrubbers, catalytic converters and cleaner fuels.
A focused answer to APES Topic 7.6, covering methods to reduce air pollution including the Clean Air Act and regulation, scrubbers and electrostatic precipitators, catalytic converters, vapor recovery, cleaner fuels and renewable energy, with a worked scrubber efficiency calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)