Key Idea: Topic D4.3 is one cause-and-effect chain: humans add greenhouse gases → those gases trap more heat → the planet warms → ecosystems and oceans change. It starts with how greenhouse gases warm the planet — they absorb the longwave infrared the Earth radiates and send some back down (the greenhouse effect). Then the human causes that raise those gases — burning fossil fuels, deforestation, agriculture and cattle — which tip the carbon cycle out of balance so CO₂ rises. From there the warming feeds on itself through positive-feedback loops (melting ice, thawing permafrost) that can push the system past tipping points. The consequences follow: species shift their ranges, communities change, phenology (the timing of events) shifts, and dissolved CO₂ causes ocean acidification that harms coral and other shell-builders. It ends with mitigation — how we limit the warming. This is a regular on Paper 1 (graph/data MCQs) and Paper 2 (describe/explain questions).
☀️ Greenhouse gases & the greenhouse effect (4.12.1)
The main greenhouse gases are carbon dioxide (CO₂), methane (CH₄) and water vapour — they share one key property: they absorb longwave infrared (heat) radiation. Sunlight (short-wave radiation) passes through the atmosphere and warms the Earth's surface; the surface then re-emits longer-wave infrared. The greenhouse gases absorb that infrared and re-radiate some back down, so heat is trapped instead of escaping to space. Adding more of these gases traps more heat — that is the enhanced greenhouse effect, i.e. global warming.
How the greenhouse effect warms the planet (cause → effect)
- Short-wave radiation (mostly visible light) arrives from the Sun and passes through the atmosphere to warm the Earth's surface.
- The warmed surface re-emits energy as longer-wave infrared (heat) radiation.
- Greenhouse gases absorb this longwave infrared and re-radiate some of it back towards the surface, instead of letting it escape to space.
- This traps heat and keeps the lower atmosphere warm — the natural greenhouse effect that makes Earth habitable.
- Adding more greenhouse gases absorbs more outgoing infrared, so the planet warms further — the enhanced greenhouse effect we call global warming.
| Greenhouse gas | Main human source | Note for the exam |
|---|---|---|
| Carbon dioxide (CO₂) | Burning fossil fuels; deforestation | The largest overall contributor because there is so much of it |
| Methane (CH₄) | Cattle (and other livestock), rice paddies, landfill, leaking gas | Far more warming per molecule than CO₂, but present in smaller amounts |
| Water vapour (H₂O) | Mostly natural; rises as the air warms | A powerful greenhouse gas and part of a positive-feedback loop |
| Nitrous oxide (N₂O) | Nitrogen fertilisers and agriculture | A minor but potent contributor |
When a graph shows rising atmospheric CO₂ alongside rising global temperature, the marked points are: (1) there is a positive correlation — as CO₂ rises, temperature rises; (2) the link is causal because CO₂ is a greenhouse gas that absorbs the longwave infrared radiated by the Earth and re-radiates it back, trapping heat. State both — a bare 'they go up together' is only half the answer.
Greenhouse gases absorb longwave infrared (heat) radiation and re-radiate it back to the surface. Sunlight in is short-wave; heat trying to leave is longwave infrared — that is what the gases catch.
🏭 Anthropogenic causes & the carbon cycle (4.12.2)
Normally photosynthesis removes CO₂ from the air and respiration, decomposition and combustion return it — and these roughly balance. Human activity tips that balance: we add CO₂ faster than it can be removed, so atmospheric CO₂ rises. The main causes are burning fossil fuels (releasing carbon locked away for millions of years), deforestation (removing the trees that absorb CO₂ — this lowers carbon sequestration), and agriculture and cattle (which also release methane).
Visual recap of 4.12.1–4.12.2: photosynthesis (down arrow) removes CO₂ from the air, while respiration, decomposition and combustion (up arrow) add it. Normally these balance. Burning extra fossil fuels — plus deforestation, which removes the trees that absorb CO₂ — adds CO₂ faster than photosynthesis can remove it, so atmospheric CO₂ rises and traps more heat.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Human activity | How it raises atmospheric CO₂ (or other gases) |
|---|---|
| Burning fossil fuels | Releases carbon that was locked away for millions of years back into the air as CO₂ |
| Deforestation | Removes trees that absorbed CO₂; burning or rotting the wood also releases it — so it both adds CO₂ and removes a sink |
| Agriculture & rice paddies | Release methane and nitrous oxide; clearing land for crops reduces carbon sequestration |
| Raising cattle (livestock) | Cattle release methane from digestion; more cattle means more methane and often more deforestation for pasture |
Why CO₂ wobbles each year on a Mauna Loa graph
- Spring/summer (Northern Hemisphere): lots of leaves photosynthesise, removing CO₂ — the curve dips.
- Autumn/winter: plants drop leaves and photosynthesis slows, while respiration and decomposition continue to release CO₂ — the curve rises.
- These seasonal dips and rises sit on top of a steady long-term upward trend caused by burning fossil fuels.
Key Idea: Cutting down forests harms the carbon balance twice: It removes a sink — fewer trees means less CO₂ absorbed by photosynthesis (reduced carbon sequestration). It adds a source — burning or rotting the felled wood releases its stored carbon as CO₂. So deforestation both stops CO₂ being taken in and puts more in.
🔁 Positive feedback & tipping points (4.12.3)
A positive-feedback loop is a self-amplifying cycle: warming triggers a change that causes even more warming. These loops can push the climate past a tipping point — a threshold beyond which the change becomes self-sustaining and very hard to reverse. The two examiners ask about most are ice–albedo (melting ice exposes dark surfaces that absorb more heat) and permafrost methane (thawing ground releases trapped methane, a greenhouse gas).
| Positive-feedback loop | Why warming feeds on itself |
|---|---|
| Ice–albedo (melting ice) | Bright ice reflects sunlight; as it melts, dark ocean/land is exposed, which absorbs more heat → more warming → more melting |
| Permafrost methane | Warming thaws frozen ground, releasing trapped methane (and CO₂) → more greenhouse gas → more warming → more thawing |
| Water vapour | Warmer air holds more water vapour, itself a greenhouse gas → more warming → still more water vapour |
A loop is positive if the effect feeds back to increase the original cause. Trace it as a circle: warming → ice melts → darker surface absorbs more heat → more warming. Because the arrow comes back round to more warming, it is positive feedback — the opposite of the stabilising negative feedback you met in homeostasis.
🌍 Consequences: distribution & phenology (4.12.4)
As the climate warms, organisms must track the conditions they can tolerate. This shows up in three ways examiners test: shifting distribution (range), changed community structure, and altered phenology (the timing of life-cycle events). Species move towards the poles and up mountains, and hardiness zones for trees shift northwards. Communities change as some species arrive and others are lost. And events like flowering, breeding and migration happen earlier — and can fall out of sync with the food they depend on.
Visual recap of 4.12.4: each biome sits at a particular mean temperature and rainfall. As warming shifts the climate, the band a region falls into moves — so species ranges and hardiness zones shift (towards the poles and up mountains) to track the conditions they can tolerate.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Consequence | What it means |
|---|---|
| Shifting distribution (range) | Species move towards the poles and up mountains to track the temperatures they can tolerate; hardiness zones for trees shift northwards |
| Changed community structure | Some species arrive, others are lost or out-competed, so the mix of species in a community changes (e.g. cold-water fish replaced by warm-water ones) |
| Altered phenology (timing) | The timing of life-cycle events shifts earlier — earlier flowering, breeding or migration |
| Loss of habitat | Melting sea ice and rising seas remove habitat (e.g. ice-dependent species lose hunting/breeding ground) |
| Mismatch | If a predator and its food shift their timing by different amounts, they fall out of sync — food is unavailable when offspring need it |
Distribution = where — species shift in space (poleward, uphill) to find tolerable temperatures. Phenology = when — the timing of events (flowering, breeding, migration) shifts, usually earlier. A 'predict a consequence' answer can use either: trees spread northwards (distribution), or flowers open earlier in spring (phenology).
🐚 Ocean acidification & marine ecosystems (4.12.5)
The ocean absorbs a large share of the extra CO₂ — and this changes its chemistry. Dissolved CO₂ forms carbonic acid, which lowers the ocean's pH (makes it more acidic) and reduces the carbonate ions that organisms need to build calcium-carbonate shells and skeletons. This hits calcifying organisms hardest — corals, molluscs and some plankton — whose shells and skeletons form more slowly or even dissolve. Because coral reefs are habitats for vast numbers of species, weakening them threatens entire reef communities.
Ocean acidification (cause → effect)
- Some of the extra atmospheric CO₂ dissolves into the ocean.
- Dissolved CO₂ reacts with water to form carbonic acid, which lowers the ocean's pH — it becomes more acidic.
- The more acidic water reduces the carbonate ions that organisms need to build shells and skeletons.
- Calcifying organisms (corals, molluscs, some plankton) struggle to make or maintain their calcium-carbonate shells/skeletons, which can even dissolve.
- Weaker corals → coral reefs decline, and because reefs are habitats for huge numbers of species, whole reef communities are threatened.
Key Idea: Corals build a hard skeleton from calcium carbonate. Acidification makes the carbonate harder to obtain, so reefs grow more slowly and weaken. (Warming also causes coral bleaching — corals expel their symbiotic algae and lose their food and colour.) Losing reefs removes habitat and food for the huge community that depends on them, so reef biodiversity falls.
🌱 Mitigation of climate change (4.12.6)
Mitigation means reducing the cause of warming — lowering greenhouse-gas emissions and protecting the sinks that absorb CO₂. (That is different from adaptation, which means coping with the warming that is already happening.) The main routes are to cut fossil-fuel use (switch to renewables, improve efficiency), protect and restore carbon sinks (stop deforestation, plant trees, protect peatland), reduce other gases (methane, nitrous oxide), and capture and store carbon.
| Mitigation strategy | How it limits warming |
|---|---|
| Cut fossil-fuel use | Switch to renewable energy (solar, wind, hydro) and improve efficiency so less CO₂ is released |
| Protect & restore carbon sinks | Stop deforestation, plant trees (reforestation) and protect peatlands — more CO₂ is absorbed and stored |
| Reduce other gases | Cut methane (e.g. from livestock and landfill) and nitrous oxide (from fertilisers) |
| Carbon capture & storage | Capture CO₂ from power stations and store it underground instead of releasing it |
Every mitigation idea is one of two moves: add less CO₂ (burn less fossil fuel) or take more CO₂ out (protect forests, plant trees, capture carbon). If you can sort a strategy into source or sink, you can explain why it works.
✍️ Worked examples
IB-style question — explain a CO₂–temperature correlation
A graph shows that, between 1960 and 2020, both atmospheric carbon dioxide and mean global temperature rose steadily together. Explain the relationship shown. [3]
Model answer:
Describe the correlation. There is a positive correlation — as atmospheric CO₂ rises, mean global temperature also rises.
Give the mechanism. CO₂ is a greenhouse gas: it absorbs the longwave infrared (heat) radiation emitted by the Earth's surface and re-radiates some of it back, trapping heat in the lower atmosphere.
Make it causal. So more CO₂ traps more heat, which raises the temperature — the rise in CO₂ is a cause of the rise in temperature, not just a coincidence. (Mark 1: positive correlation. Mark 2: CO₂ absorbs longwave infrared / greenhouse gas. Mark 3: traps heat → warming, link is causal.)
There is a positive correlation: as CO₂ rises, temperature rises. CO₂ is a greenhouse gas that absorbs the longwave infrared radiated by the Earth and re-radiates it back, trapping heat — so more CO₂ traps more heat and raises the temperature, making the link causal.
IB-style question — predict consequences of warming for a lake community
A temperate lake currently holds mainly cold-water fish. Predict, with reasons, two ways the fish community might change as the climate warms. [4]
Model answer:
Distribution / community change. As the water warms, cold-water species decline or move away (poleward or to cooler, deeper water), while warm-water species spread in — so the community structure changes.
Why. Each species can only tolerate a certain temperature range; warming pushes the cold-water fish outside their tolerance, while it brings conditions into range for warm-water fish.
Phenology change. The timing of life-cycle events may shift — e.g. fish may breed earlier in the year as the water warms sooner.
A knock-on (mismatch). If breeding shifts but the food supply does not shift by the same amount, young fish may hatch when there is little food, reducing survival. (1 mark per distinct, reasoned point, up to 4 — at least one distribution/community point and one phenology point.)
Cold-water fish decline or move away while warm-water species spread in, changing the community structure, because warming pushes the cold-water fish outside their temperature tolerance. Timing also shifts — fish may breed earlier — and if breeding no longer matches the food supply (a mismatch), young-fish survival falls.
IB-style question — explain ocean acidification and its effect on coral
Explain how rising atmospheric carbon dioxide leads to ocean acidification, and describe one effect on coral reefs. [4]
Model answer:
CO₂ dissolves. Some of the extra atmospheric CO₂ dissolves into the ocean.
Acid forms. The dissolved CO₂ reacts with water to form carbonic acid, which lowers the pH of the water — it becomes more acidic.
Carbonate falls. The more acidic water reduces the carbonate ions available to build calcium-carbonate structures.
Effect on coral. Corals struggle to build (and may even lose) their calcium-carbonate skeletons, so reefs grow more slowly and weaken — which threatens the many species that depend on the reef habitat. (1 mark each: CO₂ dissolves; forms carbonic acid / lowers pH; reduces carbonate; corals can't build skeletons / reefs weaken.)
Extra atmospheric CO₂ dissolves in the ocean and forms carbonic acid, lowering the water's pH (acidification). This reduces the carbonate ions corals need, so they struggle to build their calcium-carbonate skeletons; reefs grow more slowly and weaken, threatening the species that depend on them.
✅ Quick self-check
Tap each card to check yourself.
What type of radiation do greenhouse gases absorb? Longwave infrared (heat) radiation — the energy the warmed Earth's surface re-emits. The gases re-radiate some back down, trapping heat and warming the lower atmosphere.
Name three human activities that raise atmospheric CO₂ (or greenhouse gases). Burning fossil fuels (releases stored carbon as CO₂), deforestation (removes a CO₂ sink and releases stored carbon), and agriculture/cattle (release methane and nitrous oxide).
Why does atmospheric CO₂ fall each summer on a Mauna Loa graph? In summer abundant leaves photosynthesise and remove CO₂ (the curve dips); in winter leaves are shed and respiration/decomposition release CO₂ (the curve rises) — on top of a steady upward trend from fossil fuels.
Give one positive-feedback loop in global warming and explain it. Ice-albedo: warming melts bright ice, exposing dark ocean/land that absorbs more heat, causing more warming and more melting. (Or permafrost thaw releasing methane.)
What is the difference between a shift in distribution and a shift in phenology? Distribution is a change in WHERE species live (poleward, uphill) to track tolerable temperatures; phenology is a change in WHEN events happen (earlier flowering, breeding, migration).
How does rising CO₂ harm coral reefs? CO₂ dissolves in seawater to form carbonic acid, lowering pH and reducing carbonate ions; corals can't build their calcium-carbonate skeletons, so reefs weaken — threatening reef communities. (Warming also causes bleaching.)
Give two ways to mitigate climate change. Cut fossil-fuel use (switch to renewables) and protect/restore carbon sinks (stop deforestation, plant trees). Every strategy either adds less CO₂ (reduce the source) or removes more CO₂ (protect the sink).
Exam Tips
- The whole topic is one chain: humans add greenhouse gases → gases trap more heat → planet warms → ecosystems and oceans change → mitigation reduces it.
- Memorise the key phrase: greenhouse gases ABSORB LONGWAVE INFRARED (heat) radiation and re-radiate it back. Sunlight in is short-wave; the trapped heat is longwave infrared.
- For a CO₂-temperature graph, give BOTH the positive correlation AND the causal mechanism (CO₂ is a greenhouse gas that absorbs longwave infrared) — a bare 'they rise together' is half the answer.
- Human causes to list: burning fossil fuels, deforestation, agriculture/fertiliser, and cattle (methane). Deforestation is a 'double hit' — removes a sink AND releases stored carbon.
- Seasonal CO₂ wobble on a Mauna Loa graph: summer photosynthesis removes CO₂ (dip), winter respiration/decomposition releases it (rise), on top of the fossil-fuel upward trend.
- A positive-feedback loop feeds back to MORE warming (ice-albedo melting, permafrost methane). Trace the circle back to the start to confirm it's positive, not negative feedback.
- Consequences split into distribution (WHERE — poleward, uphill, shifting hardiness zones) and phenology (WHEN — earlier flowering/breeding/migration). 'Predict a consequence' can use either.
- Ocean acidification chain: CO₂ dissolves → carbonic acid → lower pH → less carbonate → calcifiers (coral, molluscs, plankton) can't build shells/skeletons → reefs weaken. Don't confuse it with bleaching (caused by warming).
- Mitigation = reduce the source (burn less fossil fuel) or protect the sink (forests, peatland, carbon capture). Sort any strategy into source or sink to justify it.