We banned CFCs to save the ozone layer and replaced them with HFCs. HFCs do not harm ozone at all. They are also powerful greenhouse gases. This page is about that pattern — how a genuine environmental success created a second problem that took another treaty to fix — and it is one of the most useful case studies in the whole course.
📚 What you need to know
CFCs contain chlorine, fluorine and carbon. Used through the 20th century in aerosols, refrigerators and air conditioning.
CFCs release chlorine radicals in the stratosphere, depleting ozone and increasing UV at the surface. The Montreal Protocol (1987) banned them.
HFCs contain hydrogen, fluorine and carbon — no chlorine, so they do not significantly deplete ozone.
But HFCs are potent greenhouse gases with a high global warming potential (GWP), so they contribute to climate change.
The Kigali Amendment (2016) set targets to reduce HFC production and consumption globally, with a gradual phase-down.
HFOs (hydrofluoroolefins) have much lower GWP and are being adopted; natural refrigerants such as ammonia and carbon dioxide are also being explored.
Alternatives to air conditioning: improved building design, cool roofs, urban greening and passive cooling.
Solving one problem, creating another
GWP compares a gas to carbon dioxide over a fixed period, so a GWP of 1,430 means one tonne of HFC-134a traps roughly 1,430 times as much heat as a tonne of carbon dioxide. That is why a chemical used in small quantities still matters for the climate.
The three generations
CFCs
Chlorofluorocarbons contain chlorine, fluorine and carbon, and were commonly used through the 20th century in aerosols, refrigerators and air conditioning systems. Their impact is the chlorine: released as radicals in the stratosphere, it depletes ozone and increases the UV radiation reaching the surface. The Montreal Protocol of 1987 banned them globally.
HCFCs
Hydrochlorofluorocarbons were the first substitutes. They still contain chlorine, so they still deplete ozone — but far less, because the added hydrogen makes them break down more readily in the troposphere, so most never reach the stratosphere. They were always intended as a transitional chemical, buying time while something better was developed, and UNEP aims to end all HCFC production by 2040.
HFCs
Hydrofluorocarbons contain hydrogen, fluorine and carbon — and crucially no chlorine at all, which is why they do not significantly deplete ozone. That made them look like the perfect answer. The problem emerged later: HFCs are potent greenhouse gases with a high global warming potential, and their emissions contribute to climate change by trapping heat in the atmosphere. The Kigali Amendment of 2016 added them to the Montreal Protocol, setting targets to cut production and consumption, with developed and developing countries phasing them down gradually.
This sequence is the reason environmental scientists insist on systems thinking. Each replacement was a genuine improvement judged against the problem it was designed for, and each created a cost that only became visible once it was deployed at scale.
This is the single best example in the course of an unintended consequence, and you can use it far beyond this sub-topic. If a question asks you to evaluate any technological fix — biofuels, carbon capture, geoengineering — the CFC to HFC story is the evidence for why a solution should be assessed against the whole system rather than one target. It also has a genuinely encouraging ending: the problem was spotted and the treaty was amended, which shows the framework worked as designed.
Air conditioning and where this all matters
Air conditioning regulates indoor temperature and humidity, and is widely used in homes, workplaces, vehicles and public buildings. It carries two separate environmental costs. It is energy-intensive, so it raises electricity consumption and the emissions from generating that electricity. And refrigerants leak, releasing gases that were historically ozone-depleting and are now potent greenhouse gases instead.
There is an uncomfortable feedback here worth stating in an answer: a warming climate increases demand for cooling, which increases both the electricity used and the quantity of refrigerant in circulation, which contributes to further warming.
Approach
How it works
Benefit
Improved building design
Natural ventilation, insulation and reflective materials stop buildings overheating in the first place
Less cooling needed, so lower energy use and emissions
Cool roofs
Reflective paint, tiles or shingles, usually lighter in colour, reflect sunlight and reduce indoor heat absorption
Reduces the urban heat island effect and improves comfort in warm climates
Window shading and glazing
Overhangs, awnings and double glazing minimise solar heat gain through windows
Cuts the largest single source of unwanted heat in many buildings
Urban greening
Trees, parks and green roofs cool the air through shade and evapotranspiration, and absorb carbon dioxide
Lowers city temperatures, improves air quality, reduces demand for cooling
Thermal mass
Concrete and stone store and release heat slowly, moderating temperature swings across the day
Keeps interiors cooler through the hottest hours without any energy input
Passive cooling generally
Design that combines shading, ventilation and insulation to minimise heat gain and maximise heat loss
Comfortable interiors with no reliance on artificial cooling systems
Note which category these fall into. Cool roofs, urban greening and passive cooling reduce the need for energy, so they count as mitigation. They also make buildings liveable in hotter conditions, which makes them adaptation as well. Strategies that do both jobs are unusual and worth pointing out.
Where refrigerants are heading
HFOs (hydrofluoroolefins) have much lower global warming potential than HFCs and are being increasingly adopted in air conditioning systems. Natural refrigerants such as ammonia and carbon dioxide are also being explored, and have the advantage of being substances the atmosphere already handles rather than novel industrial chemicals whose long-term behaviour has to be discovered the hard way.
WORKED EXAMPLE
Explain why HFCs were introduced and why they are now being phased down. [4]
Point 1 — why introducedHFCs contain hydrogen, fluorine and carbon but no chlorine, so they do not release chlorine radicals and do not significantly deplete stratospheric ozone.Point 2 — the role they filledThey replaced CFCs in refrigeration and air conditioning after the Montreal Protocol banned CFCs in 1987.Point 3 — the problemHFCs are potent greenhouse gases with a high global warming potential, so their emissions trap heat and contribute to climate change.Point 4 — the responseThe Kigali Amendment of 2016 added HFCs to the Montreal Protocol, setting targets for a gradual global phase-down.4 / 4Notice the structure mirrors the story: good for ozone, bad for climate, so the treaty was amended.
WORKED EXAMPLE
Evaluate alternatives to air conditioning as a way of keeping buildings cool. [6]
Point 1 — the problem being solvedAir conditioning is energy-intensive and leaks refrigerants that are potent greenhouse gases, so reducing reliance on it addresses two impacts at once.Point 2 — building designInsulation, natural ventilation, shading and thermal mass reduce heat gain, cutting cooling demand with no ongoing energy use.Point 3 — cool roofsReflective roofing lowers indoor heat absorption and reduces the urban heat island effect across a whole neighbourhood.Point 4 — urban greeningTrees and green roofs cool through shade and evapotranspiration while also improving air quality and absorbing carbon dioxide.Point 5 — limitationsThese measures are far easier to build into new construction than to retrofit, require upfront investment, and may not be sufficient during extreme heatwaves where cooling becomes a health necessity.Point 6 — judgementThey should be treated as reducing rather than eliminating the need for cooling; combined with low-GWP refrigerants they substantially cut the environmental cost of keeping buildings habitable.6 / 6Point 5 is essential. Passive cooling alone does not keep vulnerable people safe in a severe heatwave, and saying so is honest rather than negative.
💡 Exam tip
Learn the composition of each: CFC has chlorine, HFC has hydrogen instead. The chlorine is the ozone problem.
Pair the treaties with the chemicals: Montreal 1987 for CFCs, Kigali 2016 for HFCs.
Define global warming potential as a comparison with carbon dioxide over a set period.
Use CFC to HFC as your go-to example of an unintended consequence in any evaluation question.
For cooling alternatives, name cool roofs, urban greening and passive cooling and give the mechanism for each.
Point out that these alternatives count as both mitigation and adaptation. Examiners notice.
⚠️ Common mix-up
Thinking HFCs damage the ozone layer. They contain no chlorine, so they do not significantly deplete ozone.
Thinking CFCs were harmless to climate. They damaged ozone and were powerful greenhouse gases.
Confusing ODP with GWP. One measures ozone damage, the other heat trapping relative to carbon dioxide.
Mixing up HCFCs and HFCs. HCFCs still contain chlorine and still deplete ozone slightly; HFCs do not.
Assuming Kigali replaced Montreal. It is an amendment to it, not a separate treaty.
Treating passive cooling as a complete replacement for air conditioning. It reduces demand; in extreme heat, active cooling can be a health necessity.
That completes Topic 6. Before you move on, try connecting its two halves: 6.2 and 6.3 are climate, 6.4 is ozone, and the most rewarding exam answers move between them. The Montreal Protocol shows what a well-designed treaty achieves; the climate agreements show why the same design is harder to apply; and the Kigali Amendment is the point where the two stories become one.
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