Hi Tara. Topic 4 is four separate problems that share one sky, and the marks are lost almost entirely in the joins between them. A student who has revised properly can still throw away half a question by writing that the ozone hole causes global warming, or that acid rain comes from carbon dioxide, or that the greenhouse effect is a bad thing. None of those is true, and each one is a very common answer.
So hold these four apart from the start. 4.1 is the atmosphere itself — what it is made of, how it is layered, and the natural greenhouse effect, which keeps the Earth warm enough to live on and has been running for billions of years. 4.2 is climate change: extra greenhouse gases, the enhanced greenhouse effect, twelve listed impacts, twelve listed strategies, and adaptation. 4.3 is acid rain, caused by sulfur dioxide and oxides of nitrogen. 4.4 is ozone depletion, caused by CFCs. Different gases, different mechanisms, different fixes.
One more thing before you start, because it is worth marks in every single “discuss” question in this topic. Cambridge is politically neutral, and the mark scheme for an evaluation question has points on both sides. If you write four sentences about why a carbon tax is a good idea and nothing about who pays it or why a country might refuse, you have written half an answer and you will be capped. Every strategy in this topic has a benefit, a limitation, someone who bears the cost, and a timescale over which it works. Learn all four for each one and the six-markers become easy.
The Four Layers, In Order
The atmosphere is not one thing. It is layered, and the layers are defined by what happens to temperature as you climb. You are not asked about the temperature profile. You are asked for the names and the relative order, so learn them as a list that goes upwards from the ground:
Troposphere → Stratosphere → Mesosphere → Thermosphere. The initials spell T–S–M–T, and the one sentence that fixes the order for me is “The Sky Muddles Together”. Whatever hook you use, rehearse it in the direction the question asks. A question that says “name the layer immediately above the troposphere” is testing order, not names.
| Layer | Where it sits | What you should be able to say about it |
|---|---|---|
| Troposphere | lowest layer, from the ground upwards | the layer we live in and breathe; weather, clouds and nearly all the water vapour are here |
| Stratosphere | directly above the troposphere | contains the ozone layer, in the LOWER stratosphere, which absorbs ultraviolet radiation |
| Mesosphere | third layer up | above the stratosphere and below the thermosphere — the one people forget when they list the four |
| Thermosphere | the outermost of the four | very thin air, the top of the four layers named in the syllabus |
The syllabus is specific: the ozone layer is in the lower stratosphere. Two other answers appear constantly and both score nothing. The first is “in the troposphere” — that is where the weather is, and it is the layer below. The second is “above the atmosphere” or “in space” — ozone is a gas in the atmosphere, not a shell around the outside of it. If the question gives you a diagram with unlabelled bands, count up from the ground: the ozone band sits in the bottom part of the second layer.
What Clean Air Is Made Of
“Clean air” means air without pollutants, and its composition is a straight recall mark. Two numbers do almost all the work:
| Gas | Proportion of clean air | Note |
|---|---|---|
| Nitrogen | 78% | much the largest share — and unreactive, until it meets the temperature inside a vehicle engine (see 4.3) |
| Oxygen | 21% | the gas used in respiration and in combustion |
| Argon | a small share of the remaining 1% | a noble gas; named in the syllabus, so it belongs in your list |
| Carbon dioxide | a small share of the remaining 1% | small in quantity, large in effect — this is a greenhouse gas |
| Water vapour | variable | also a greenhouse gas; the amount changes with place and weather, which is why no fixed percentage is given |
Notice what the numbers do not say. Nitrogen and oxygen together are 99% of clean air, and neither of them is a greenhouse gas. Every gas that matters for 4.2 is inside the last 1%. That is the single most useful thing to understand about this table: a gas can be a tiny fraction of the air and still change the temperature of the planet, because what counts is not how much of it there is but what it does to radiation.
Climate Is Not Weather
The syllabus gives you a definition to learn, and it gives it in a particular form. Learn it in that form:
Climate is the weather conditions in a location based on the weather over many years.
Two ideas are doing the work: a location, and many years. Weather is what the sky is doing now, or this week. Climate is the long-run pattern for a place. “It rained yesterday” is weather. “This region has a dry season from November to March” is climate.
The distinction matters because of a trap you will meet in 4.2. A single cold winter is weather, and it is not evidence against a long-term warming trend, in the same way that one tall student is not evidence about the average height of a class. When a question hands you a graph of temperature against year and asks you to describe the trend, describe the trend — the overall direction over the whole period — and mention the short-term ups and downs separately as fluctuations around it. Students who fasten on to one dip and say “the temperature is falling” lose the mark.
The Natural Greenhouse Effect — Five Steps, In Order
This is the most important thing in 4.1, and it is examined as an ordered chain, so learn it as one. The word natural is not decoration. This process has always happened, it is the reason the Earth is warm enough to live on, and it is not the thing that causes climate change. The thing that causes climate change is the same process running harder because there is more greenhouse gas in the way, and that is 4.2.4. Keep them apart.
| Step | What happens |
|---|---|
| (a) | Solar radiation passes through the Earth’s atmosphere. |
| (b) | Some solar radiation is absorbed by the land and oceans, heating the Earth. |
| (c) | Some solar radiation is reflected back into space. |
| (d) | Some radiation is absorbed and re-emitted back to the Earth’s surface by greenhouse gases and clouds. |
| (e) | This natural process warms the Earth enough to support life. |
Describe the natural greenhouse effect. [5] Five marks, five steps — that is not a coincidence, and it tells you exactly how much to write. One sentence per step, in order, and stop. Do not start explaining fossil fuels; that is a different objective and it earns nothing here. And do not finish with “and this is why the planet is warming”, because the question said natural. The last mark is “warms the Earth enough to support life”, and it is the mark most often missed, because it feels like a nice ending rather than a marking point. It is a marking point.
Two details inside step (d) are worth pausing on. First, the greenhouse gases do not trap radiation the way a bag traps a ball, however often you see that word used. They absorb it and then re-emit it, in all directions — and the share that goes downwards is what warms the surface. Second, the syllabus says “greenhouse gases and clouds”. Clouds are in the list. It costs you nothing to include them and it is a mark on some schemes.
This is the largest sub-topic in the whole of 0680 — nine objectives, one of them a list of twelve impacts and another a list of twelve strategies. It is far too much to hold as one blur, so work through it as five separate questions: what is a greenhouse gas, why is there more of it, what does that do, what could reduce it, and how do people live with what has already changed. That last one is adaptation, and it has its own definition.
What a Greenhouse Gas Actually Is
A greenhouse gas is a gas that absorbs radiation and emits the energy as thermal (heat) energy.
Two verbs, in this order: absorbs, then emits. A definition that says a greenhouse gas “traps heat” or “stops heat escaping” is describing a blanket, and it does not contain either verb the mark scheme wants.
The three named in the syllabus are carbon dioxide (CO₂), water vapour (H₂O) and methane (CH₄). Water vapour surprises people, because it is not a pollutant and nobody emits it deliberately — but it absorbs and re-emits radiation, so by the definition it qualifies, and the syllabus lists it. Do not leave it out because it feels like the odd one.
Why the Concentrations Have Risen — the Six Causes
Learn these as six separate causes with six separate mechanisms, because a question asking for three will not accept “burning things” three times in different words. Notice that two of them release methane rather than carbon dioxide, and that one of them — deforestation — works by removing a process rather than by releasing a gas.
| Cause | Gas | The mechanism — the “because” that earns the mark |
|---|---|---|
| (a) Combustion of fossil fuels | carbon dioxide | coal, oil and gas contain carbon; burning them combines that carbon with oxygen and releases carbon dioxide that had been stored underground for millions of years |
| (b) Agriculture | methane | three named sources: digestion by cattle, decomposition of manure, and rice fields. Say which one you mean — “farming” on its own is too vague to mark |
| (c) Deforestation | carbon dioxide | fewer trees means less removal of carbon dioxide by photosynthesis, so more of it stays in the atmosphere. The mechanism is a loss of uptake, not an emission |
| (d) Changes in land use | methane | drainage of wetlands releases methane trapped in the waterlogged ground (the syllabus answer); the drained peat then decays in air and also releases carbon dioxide |
| (e) Cement manufacture | carbon dioxide | calcium carbonate (limestone) is heated to produce calcium oxide (lime), and that reaction releases carbon dioxide — on top of any released by the fuel used to heat it |
| (f) Rising population and energy use | carbon dioxide | more people, each using more energy, means more combustion and so more carbon dioxide |
The commonest wrong answer is “cutting down trees releases carbon dioxide”. Burning the felled timber does release it, but that is not what the syllabus lists. The listed mechanism is that deforestation reduces the removal of carbon dioxide by photosynthesis. Trees are a sink; removing them removes the sink. Write the word photosynthesis and the mark is safe.
The Enhanced Greenhouse Effect
Here is the join between 4.1 and 4.2, and it is only two sentences long in the syllabus. Greenhouse gases absorb solar radiation causing global warming, and global warming can result in climate change. That is the whole of 4.2.4. What makes it worth marks is the word enhanced: the mechanism has not changed at all since 4.1. There is simply more greenhouse gas in the atmosphere, so more radiation is absorbed and re-emitted downwards, so the surface warms further.
Cambridge separates them deliberately, and so should you. Global warming is the rise in average temperature. Climate change is what that rise then does to the long-run weather patterns of places — shifting rainfall, changing seasons, altering the frequency of extreme events. The syllabus phrasing is that global warming can result in climate change. Warming is the cause; the change in climate is the consequence.
The Twelve Impacts
Twelve is a lot to memorise as a flat list, so group them. Physical changes come first, they drive ecological changes, and the ecological changes plus the physical ones drive the human consequences. If you can rebuild the chain you do not have to remember the list.
| Group | Impact | How it works |
|---|---|---|
| Physical | (a) increase in temperature of the Earth’s surface | the direct result of the enhanced greenhouse effect |
| (b) increase in temperature of the ocean surface | the oceans absorb a large share of the extra energy, so surface waters warm too | |
| (c) melting of ice sheets, glaciers and permafrost, leading to a rise in sea level | water held on land as ice returns to the sea; warmer sea water also expands. Name all three — ice sheets, glaciers and permafrost | |
| (d) ocean acidification | more carbon dioxide in the atmosphere means more of it dissolves in sea water, making the water more acidic | |
| (i) more frequent and more severe extreme weather events | leading to flooding and loss of land, and to drought and wildfires. The syllabus names both directions — too much water in some places, too little in others | |
| Ecological | (e) disruption to food chains | if a species at one trophic level declines or shifts its range, everything that feeds on it is affected |
| (f) change in biodiversity | species that cannot tolerate the new conditions or move fast enough decline; others spread. “Change” is the syllabus word, and it is more accurate than “loss” | |
| (g) loss of habitat | habitats are lost outright as ice, coastline, wetland and coral are altered | |
| (k) increased pest outbreaks and invasive species | warmer conditions let pests survive winters and extend their range into areas that were previously too cold for them | |
| Human | (h) forced migration of humans and other animals | people move away from flooded, drought-affected or unproductive land; animals shift their ranges. The objective covers both |
| (j) change in crop yields | note change, not “fall”. Yields drop where it becomes too hot or too dry, and can rise where a growing season lengthens | |
| (l) food shortages | the end of the chain: lower or less reliable yields, plus disrupted food chains and fisheries, reduce the food available |
Ocean acidification is not caused by acid rain. It is caused by carbon dioxide from the atmosphere dissolving in sea water. Acid rain (4.3) comes from sulfur dioxide and oxides of nitrogen and acidifies lakes, rivers and soils. Different gases, different water bodies, different sub-topic.
And melting sea ice does not raise sea level the way melting land ice does. The syllabus lists ice sheets, glaciers and permafrost — ice sitting on land. If you want the safe answer, quote those three.
Reducing Carbon Footprints — the Twelve Strategies
A carbon footprint is the amount of greenhouse gas released by a person, an organisation or an activity. The twelve strategies below are the ones the syllabus lists, and they fall into four families: use less fossil fuel, change what is farmed and eaten, take carbon back out of the air, and change the rules people operate under. Sorting them that way also tells you who has to act, which is exactly what an evaluation question wants.
| Family | Strategy | How it reduces greenhouse gas |
|---|---|---|
| Use less fossil fuel | (a) reduced use of fossil fuels | less combustion, so less carbon dioxide released |
| (e) increase in energy efficiency | the same task done with less energy — insulation, efficient appliances, better industrial processes — so less fuel burnt for the same result | |
| Food and farming | (b) more sustainable food choices | three named: a plant-based diet, seasonal food and locally produced food. Less livestock methane, less energy for heated greenhouses, less fuel for transport |
| (c) reduction in livestock farming | fewer cattle means less methane from digestion and from manure decomposition | |
| (d) reduction in the number of children per woman | a smaller future population means fewer people using energy, so lower total emissions — but over decades rather than years. See 7.3 for how this is actually approached | |
| Take carbon back out | (f) reforestation and afforestation | replanting cleared land (reforestation) and planting where there was no forest (afforestation) increases removal of carbon dioxide by photosynthesis |
| (g) carbon sequestration | capture, remove and store carbon dioxide — for example capturing it from power station flue gases and storing it underground | |
| Change the rules and the knowledge | (h) transport policies | public transport, cycle routes, low-emission zones, electric vehicles — anything that cuts the fuel burnt per journey |
| (i) international agreements and policies | emissions are shared between countries, so targets agreed between governments address what no single country can fix alone | |
| (j) taxation | taxing fuel or emissions makes the high-carbon option the expensive one, so people and firms choose the lower-carbon alternative | |
| (k) research and climate models | models project what different emission paths would lead to, so policies can be chosen on evidence rather than guesswork | |
| (l) use of artificial intelligence (AI) | used to predict the impacts of climate change and to develop strategies for reducing them — for example finding the most efficient way to run an electricity grid |
Adaptation — Living With What Has Already Changed
Climate change adaptation is altering behaviour, practices and infrastructure to adapt to the impacts of climate change.
The distinction to hold on to: the twelve strategies above try to reduce the cause. Adaptation accepts that some change is already happening and reduces the harm. A sea wall does not lower anyone’s carbon footprint by a gram, and it is still a correct answer — to an adaptation question.
| Adaptation strategy | What it involves |
|---|---|
| (a) crops that resist warmer, drier or wetter conditions | growing varieties suited to the conditions that are now arriving, so yields hold up |
| (b) improved flood defences | sea walls, embankments, barriers and drainage that keep water off land and property |
| (c) legislation about building near coasts and on floodplains | a law preventing new building in places likely to flood — cheaper than defending them later |
| (d) building design and materials | houses on stilts, reflective roofs, shading and ventilation, materials that cope with heat or flooding |
4.2.9 — Discussing Benefits and Limitations
This objective is worth learning as a table rather than as prose, because that is the shape the marks come in. In an evaluation question the examiner is looking for four things about each strategy: the benefit, the limitation, who bears the cost, and over what timescale it works. Very few strategies fail on all four, and none of them succeeds on all four — which is precisely why the question is worth six marks and not two.
| Strategy | Benefits | Limitations |
|---|---|---|
| Reduced use of fossil fuels | addresses the largest single source of carbon dioxide; also reduces sulfur dioxide and oxides of nitrogen, so it helps with acid rain at the same time | fossil fuels currently supply most of the world’s energy; replacing them needs large investment in generation and grids, and countries whose economies depend on producing or using them face higher costs |
| Energy efficiency | cuts emissions and fuel bills together, so it can pay for itself; no change of lifestyle is required | the initial cost of insulation or new equipment falls on the household or firm; savings arrive over years; and cheaper running costs can lead people to use more, offsetting part of the saving |
| Sustainable food choices and reduced livestock farming | methane has a strong warming effect per unit released, so reducing livestock numbers acts relatively quickly; also frees land and water | diet is cultural and personal, and change is slow and uneven; livestock is the livelihood of many farming families and in some regions grazing land cannot grow crops; “buy local” is not always lower-carbon than importing from a place that grows the crop more efficiently |
| Reduction in the number of children per woman | a smaller future population means lower total energy demand and emissions; the measures usually associated with it — education and access to healthcare — bring benefits of their own | works over decades, so it does little about emissions this decade; population growth is highest in countries whose emissions per person are lowest, so it targets the smaller part of the problem; and family size is a personal matter, which makes anything beyond providing education and services contested (see 7.3) |
| Reforestation and afforestation | removes carbon dioxide from the air, restores habitat, reduces soil erosion and can provide timber and income | trees take decades to absorb much carbon; land planted with trees cannot be farmed; a plantation of one species stores carbon but supports far less biodiversity than the forest it replaced; and the carbon is released again if the trees burn or are felled |
| Carbon sequestration | allows emissions from industries that are hard to run without fossil fuel, such as cement, to be captured at the source | expensive; uses energy itself, so a plant must burn more fuel to run the capture; and the store has to remain sealed indefinitely, which has to be monitored |
| Transport policies | transport is a large and growing share of emissions; better public transport also reduces congestion and local air pollution | infrastructure is costly and slow to build; it works best in dense cities and poorly in rural areas; and restrictions such as low-emission zones can fall hardest on people who cannot afford a newer vehicle |
| International agreements and policies | the atmosphere is shared, so an agreement is the only way to act on emissions that cross borders; agreements also create the monitoring and reporting that make progress checkable | agreements are voluntary in practice and hard to enforce; countries disagree about who should reduce most, since the countries emitting most today are not always those that emitted most historically; and a government can withdraw |
| Taxation | makes the polluter pay, changes behaviour without banning anything, and raises revenue that can fund alternatives | a flat tax on fuel takes a larger share of a low income than a high one, so it can be regressive unless the revenue is returned; industries may relocate to countries without the tax, moving the emissions rather than removing them; and taxes are politically difficult to sustain |
| Research, climate models and AI | improves the projections that policy depends on, identifies which measures give the largest reduction per unit spent, and can optimise systems such as electricity grids in real time | reduces no emissions by itself — it informs decisions rather than making them; models carry uncertainty, particularly for small regions; and large AI systems consume significant electricity themselves |
| Adaptation (flood defences, resistant crops, building rules, design) | protects people from impacts that are already arriving, and does so on a timescale of years rather than decades; a country can act alone without waiting for an agreement | treats the effects, not the cause, so emissions continue; defences are expensive and can fail if conditions exceed what they were designed for; and the countries facing the largest impacts often have the least money to spend on defences |
Describe — say what happens. No reasons needed. Explain — give the mechanism; every sentence needs a “because” or a “so”. Discuss — both sides, properly weighted, then a judgement. Evaluate — weigh the evidence and come to a supported conclusion; a conclusion with no reasons attached scores nothing.
For a six-mark “discuss the benefits and limitations” question, a reliable structure is: two benefits with reasons, two limitations with reasons, then one sentence of judgement that says which and under what conditions — for example “taxation acts faster than reforestation, but only where an affordable alternative already exists, otherwise it raises costs without changing behaviour.” That last clause is what separates a top answer from a list.
Acid rain is the tidiest sub-topic in Topic 4: a four-step chain of formation, six impacts, three strategies. It is also the one where a small number of specific details separate a full answer from a half one, and all of them are in the formation chain. Start there.
Formation — Two Gases, Three Origins
| Step | What happens | The detail that earns the mark |
|---|---|---|
| (a) | Sulfur dioxide is released by the combustion of fossil fuels | because those fuels contain sulfur compounds. Say that, or the answer is incomplete |
| (b) | Sulfur dioxide is released by volcanic gases | a natural source. Acid rain existed before people did, and this is the item that is nearly always forgotten |
| (c) | Oxides of nitrogen form in vehicle engines | nitrogen from the atmosphere reacts with oxygen in the high temperature of the engine. The nitrogen is not in the fuel — it is in the air drawn into the engine |
| (d) | The two gases react with oxygen and water vapour in the atmosphere to form acid rain | the reaction happens in the atmosphere, which is why the rain often falls a long way downwind of the source |
Spelling. Cambridge uses sulfur and sulfur dioxide. Write it that way.
The volcano. If a question asks for two sources of sulfur dioxide and you give “coal-fired power stations” and “factories”, you have given one source twice. The second listed source is volcanic gases.
Where the nitrogen comes from. “The fuel contains nitrogen” is wrong and it is very common. Nitrogen is 78% of the air; an engine draws air in; at high temperature that nitrogen reacts with oxygen. That is why oxides of nitrogen come from vehicles and not from a low-temperature fire.
The Six Impacts
The impacts follow the water. Acid falls on lakes and rivers, and on soil; from the soil it reaches plant roots; and it attacks stone directly.
| Impact | What happens, and why |
|---|---|
| (a) acidification of bodies of water | lakes, rivers and ponds become more acidic as acid rain falls on them and drains into them |
| (b) reduced fish populations | many fish cannot tolerate the lower pH; eggs and young fish are affected first, so numbers fall over time even where adults survive |
| (c) effect on aquatic food webs | fewer fish, and fewer of the invertebrates they eat, so the species that depend on them are affected too — the whole web shifts, not just one species |
| (d) acidification of soils | acid rain lowers soil pH and washes nutrients out of reach of roots |
| (e) damage to crops and vegetation | the syllabus names two effects: plant defoliation (leaves are damaged and drop) and reduced crop yields |
| (f) damage to buildings | acid reacts with the calcium carbonate in limestone and marble, so stonework, statues and carvings are eroded |
One consequence of step (d) is worth remembering because it makes acid rain a political problem as well as a scientific one. The reaction happens up in the atmosphere while the gases are being carried by the wind, so the rain can fall hundreds of kilometres from the chimney that released the gas — often in a different country from the one that burnt the fuel. The country bearing the damage is frequently not the country that caused it, which is why 4.3.3 includes measures agreed between governments and why enforcement is difficult.
The Three Strategies
| Strategy | How it works |
|---|---|
| (a) flue-gas desulfurisation | removes sulfur dioxide from the waste gases of a power station by reacting it with calcium carbonate or calcium oxide, before the gas leaves the chimney |
| (b) catalytic converters | fitted to vehicle exhausts; they remove oxides of nitrogen, converting them to less harmful gases before they reach the air |
| (c) transport policies | fewer vehicle journeys, or cleaner vehicles, mean fewer oxides of nitrogen produced in the first place |
Flue-gas desulfurisation deals with sulfur dioxide. Catalytic converters deal with oxides of nitrogen. The name gives it away in both cases — “de-sulfur-isation”, and a converter sits on a vehicle, which is where the oxides of nitrogen are made. Swapping them round is a wasted mark on a question you knew.
4.3.4 — Discussing Benefits and Limitations
| Strategy | Benefits | Limitations |
|---|---|---|
| Flue-gas desulfurisation | removes most of the sulfur dioxide at the point of release, so it works on the largest single source without asking anyone to change behaviour; the calcium sulfate produced can be sold for use in plasterboard | expensive to install and to run, and it uses energy, so the station burns more fuel; it does nothing about oxides of nitrogen; it produces a solid waste that has to go somewhere; and it does not reduce carbon dioxide, so it does not help with 4.2 |
| Catalytic converters | address the source that flue-gas desulfurisation cannot reach, and are cheap enough per vehicle to be required by law in many countries | only work once warmed up, so short journeys benefit least; they add to the price of a vehicle; they contain scarce metals that must be mined; and they do not reduce the number of vehicles, so a growing fleet can offset the gain |
| Transport policies | reduce oxides of nitrogen, carbon dioxide, congestion and local air pollution together, so one measure addresses several problems | public transport is costly to build and takes years; the measures work far better in dense cities than in rural areas; and charges or restrictions can fall hardest on people who cannot afford a newer vehicle |
| Any of them, considered together | the technologies exist, are proven, and countries that adopted them have recorded large falls in sulfur dioxide emissions | none of them removes acid already in lakes and soils, which can take decades to recover; and because the gases cross borders, a country can reduce its own emissions and still receive acid rain from its neighbours |
Ozone depletion is not global warming, and it is not a cause of global warming. They are separate problems with separate causes, separate mechanisms, separate impacts and separate solutions. Ozone depletion is caused by CFCs and its consequence is more ultraviolet radiation reaching the surface. Climate change is caused by greenhouse gases and its consequence is warming. A hole in the ozone layer does not let in extra heat, and carbon dioxide does not destroy ozone.
Examiners see “the ozone hole lets in more heat and warms the Earth” every year and it scores nothing. If you can keep these two apart you are already ahead of most candidates in this sub-topic.
The Cause
The syllabus is short here, and precise. Chlorofluorocarbons (CFCs), used in aerosols and refrigerants, are the cause of ozone depletion in the ozone layer. That is the whole of 4.4.1 — a statement, not a mechanism. You are not asked for the chemistry of how a chlorine atom breaks ozone apart, so do not spend exam time on it. You are expected to name the two uses, because “CFCs come from chemicals” is not an answer and “CFCs from aerosols and refrigerants” is.
Recall from 4.1 where the ozone layer sits: the lower stratosphere, the second layer up. Its job is to absorb ultraviolet radiation from the Sun before that radiation reaches the ground. Thin the layer and less ultraviolet is absorbed, so more of it arrives at the surface. Every impact in 4.4.2 follows from that one sentence.
The Three Impacts
| Impact | The link back to ultraviolet radiation |
|---|---|
| (a) higher levels of solar radiation reaching the Earth’s surface | this is the first impact and the cause of the other two — less ultraviolet is absorbed in the stratosphere, so more of it arrives at ground level |
| (b) increased rates of cancer and cataracts | ultraviolet radiation damages skin cells, so rates of skin cancer rise; it also damages the lens of the eye, so rates of cataracts rise. Name both — they are two separate items in one line |
| (c) damage to vegetation, resulting in reduced crop yields | ultraviolet radiation damages plant tissue and reduces growth, so harvests are smaller |
More ultraviolet gets through → skin, eyes, plants. If you can only remember one thing, remember that all three impacts are consequences of the same single change. In an exam you can then rebuild the list from the mechanism instead of recalling three unrelated facts.
The Three Strategies
| Strategy | What it involves |
|---|---|
| (a) international agreements and policies | governments agreeing together to phase out the production and use of CFCs, with timetables and reporting |
| (b) a CFC ban, and alternatives to CFCs | prohibiting CFCs in new products, and using replacement chemicals in aerosols and refrigeration instead |
| (c) safe disposal of CFCs | old fridges and air-conditioning units still contain CFCs. If they are scrapped carelessly the gas escapes, so the gas is recovered and destroyed instead |
It is easy to assume that banning a chemical solves the problem, and this is the objective that shows why not. CFCs sealed inside equipment made decades ago are still in use and still in scrapyards. A ban stops new CFCs being made; only careful disposal stops the existing ones being released. If a question asks why a ban alone was not enough, that is the answer.
4.4.4 — Discussing Benefits and Limitations
| Strategy | Benefits | Limitations |
|---|---|---|
| International agreements | the stratosphere is shared, so no country can protect its own ozone alone; agreement on CFCs has been unusually widely adopted, and measurements since show the depletion has stopped worsening and is slowly reversing | agreements need every significant producer to join and to keep to the timetable; monitoring and enforcement cost money; and a country can leave. Success also depended on there being an affordable substitute, which is not true of every environmental problem |
| Banning CFCs and using alternatives | removes the cause at source, and substitutes for aerosols and refrigerants were developed relatively quickly and cheaply | some replacements turned out to be strong greenhouse gases, so one problem was partly traded for another; equipment has to be redesigned; and enforcement is difficult where CFCs are still traded illegally because they are cheap |
| Safe disposal | deals with the large stock of CFCs already sealed inside older fridges, freezers and air-conditioning units, which a ban cannot reach | requires collection points, trained handlers and specialist equipment, all of which cost money; and it depends on people not simply dumping old appliances, which is hard to police |
| All three together | ozone depletion is the clearest example in this syllabus of an international response producing a measurable improvement, which is why it is often used as a model | ozone recovery takes decades because CFCs already released remain in the atmosphere for a long time, so the impacts continue for years after emissions stop |