AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/9/9-2)
Unit 9 · Topic 9.2
9.2 Reducing Ozone Depletion
The 1987 Montreal Protocol phased out CFCs and other ozone-destroying chemicals worldwide, and the ozone layer is now slowly recovering. The replacements came in stages: HCFCs, which harm ozone less, then HFCs, which don't harm ozone but are strong greenhouse gases.
Key terms
- Montreal Protocol
- phase-out
- HCFCs
- HFCs
The Montreal Protocol
After scientists showed in the 1970s that CFCs could destroy ozone, and the Antarctic ozone hole was discovered in 1985, countries negotiated the Montreal Protocol, signed in 1987. It set deadlines to phase out the production of CFCs, halons, carbon tetrachloride, methyl bromide and other ozone-depleting substances. Developed countries ended CFC production by 1996, and developing countries, given more time and financial help, by 2010.
Every country in the world has ratified the Montreal Protocol, and it is often called the most successful international environmental agreement. Its success came from strong scientific agreement, available substitutes and a fund to help developing countries switch.
The replacements: HCFCs, then HFCs
| Chemical group | Contains chlorine? | Ozone damage | Greenhouse effect | Status |
|---|---|---|---|---|
| CFCs | Yes | High | Very strong | Phased out |
| HCFCs (hydrochlorofluorocarbons) | Yes | Much lower, because they mostly break down before reaching the stratosphere | Strong | Being phased out (developed countries by 2030, developing by 2040) |
| HFCs (hydrofluorocarbons) | No | None | Many are very strong | Being phased down under the 2016 Kigali Amendment |
Why recovery is slow
CFCs already in the atmosphere last a long time; some take 50 to more than 100 years to be removed. Old refrigerators, air conditioners and foam insulation still release them as they are thrown away. So even though production stopped decades ago, chlorine levels in the stratosphere are falling only gradually.
Scientific assessments expect the ozone layer to return to its 1980 condition around 2040 over most of the world, around 2045 over the Arctic, and around 2066 over Antarctica, as long as current rules are followed. The Antarctic ozone hole still forms each spring, but measurements show signs of healing.
A bonus for the climate
CFCs are not only ozone destroyers; they are also extremely powerful greenhouse gases (topic 9.3). By phasing them out, the Montreal Protocol also prevented a large amount of global warming that would otherwise have happened. Researchers estimate it has done more to slow climate change so far than many agreements designed specifically for climate.
The Protocol is often held up as a model for solving global environmental problems: scientists identified the cause, governments agreed on firm deadlines, industry developed substitutes, and richer countries funded poorer countries' transitions. Climate change is harder to solve in the same way, because CO₂ comes from fossil fuels that power nearly every part of the economy, while CFCs came from a small set of products.
Other ways to reduce ozone depletion
- Recover and recycle refrigerants instead of venting them when appliances are repaired or scrapped.
- Use refrigerants with low ozone and climate impact, such as some natural refrigerants like ammonia, CO₂ and hydrocarbons, where they are safe to use.
- Enforce the bans, since illegal production of CFCs has been detected and stopped in the past.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Evaluating a replacement chemical
A company switches its air conditioners from an HCFC refrigerant to an HFC. Evaluate this change for (a) the ozone layer and (b) climate change.
Show the solutionHide the solution
- Step 1: (a) HFCs contain no chlorine, so they don't destroy stratospheric ozone. Switching from an HCFC removes the remaining ozone harm, which is good for the ozone layer.
- Step 2: (b) Many HFCs absorb infrared radiation very strongly, with global warming potentials in the hundreds to thousands. If the HFC leaks, it adds to the greenhouse effect.
- Step 3: So the switch solves one problem while potentially making another worse, which is why the Kigali Amendment now phases down HFCs too.
Answer: (a) Good for the ozone layer: HFCs contain no chlorine. (b) Possibly bad for climate: many HFCs are potent greenhouse gases.
Common mistakes
- Calling HFCs harmless. They don't hurt ozone, but many are powerful greenhouse gases.
- Saying the ozone layer has already fully recovered. It is recovering slowly and isn't expected to fully recover until the middle of this century.
- Mixing up the Montreal Protocol (ozone-depleting chemicals) with climate agreements like the Kyoto Protocol or Paris Agreement (greenhouse gases).
On the exam
- Know the name and year: the Montreal Protocol, 1987, phased out CFCs.
- Expect questions about the trade-off between ozone protection and climate when CFCs were replaced by HCFCs and HFCs.
Connected topics
Videos
Check yourself
4 questions on 9.2 Reducing Ozone Depletion. Pick an answer to see if you got it, and why.
| Year | Effective chlorine in the stratosphere (relative to 1980 = 100) |
|---|---|
| 1980 | 100 |
| 1990 | 130 |
| 2000 | 140 |
| 2010 | 133 |
| 2020 | 125 |
Hypothetical index based on observed trends
The Montreal Protocol was signed in 1987, but chlorine levels kept rising into the late 1990s. Which statement best explains this?
CFCs were largely replaced by HFCs. Which statement best describes the trade-off of this switch?
The Montreal Protocol is often called the most successful international environmental agreement. Which factor most helped it succeed?
HCFCs replaced many CFCs in the 1990s, but they are now being phased out too. Which statement best explains why?
0 of 4 answered