The big idea: Atmospheric carbon dioxide (CO₂) is normally kept roughly steady by the carbon cycle — a balance between CO₂ being removed (mostly by photosynthesis) and CO₂ being added (respiration, decomposition and combustion).
Human (anthropogenic) activity tips this balance, adding CO₂ faster than it is removed. The result is rising atmospheric CO₂, which strengthens the greenhouse effect and warms the planet.
Four human activities do most of the damage: burning fossil fuels, deforestation, agriculture, and cattle farming.
The carbon cycle as a balance: photosynthesis REMOVES CO₂ from the air, while respiration, decomposition and combustion ADD it back. When the two flows are equal, atmospheric CO₂ is steady.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
- Anthropogenic
- Caused by human activity (rather than by natural processes).
- Carbon cycle
- The movement of carbon between the atmosphere (as CO₂) and living things, soil and fossil fuels, driven by photosynthesis, respiration, decomposition and combustion.
- Carbon sink
- A store or process that REMOVES CO₂ from the air, e.g. a forest photosynthesising.
- Carbon source
- A store or process that ADDS CO₂ to the air, e.g. burning fossil fuels.
- Carbon sequestration
- The long-term locking-away of carbon out of the atmosphere — for example, carbon stored in forest wood or in fossil fuels.
- Fossil fuels
- Coal, oil and natural gas — carbon stored from ancient organisms; burning them releases that carbon as CO₂.
Sink vs source — the words examiners want: A sink takes CO₂ out of the air (photosynthesis).
A source puts CO₂ into the air (respiration, decomposition, combustion).
Human activity increases sources (burning fuel, cattle) and removes sinks (deforestation) — both push CO₂ up.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it free for 7 days:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Read each human activity as a cause → effect chain on the carbon cycle. Some activities add a source of CO₂, some remove a sink, and the worst — deforestation — does both.
Then there is a separate, smaller pattern on top: a seasonal up-and-down in CO₂ that graph questions love to test.
The four anthropogenic causes, step by step
- Burning fossil fuels releases carbon locked in coal, oil and gas for millions of years → this is the largest source of the extra CO₂.
- Deforestation cuts down trees: the lost forest can no longer photosynthesise (a sink is removed), and burning or rotting the wood releases its stored carbon → a double effect.
- Agriculture clears and ploughs land (releasing soil carbon) and burns fuel in machinery → more CO₂.
- Cattle (livestock) farming releases methane from the animals' digestion (a potent greenhouse gas), and pasture often replaces cleared forest → more CO₂ and methane.
| Human activity | What it does to the carbon cycle | Effect on atmospheric CO₂ |
|---|---|---|
| Burning fossil fuels (combustion) | Releases carbon that was locked away in coal, oil and gas for millions of years | Adds CO₂ — the single biggest cause of the rise |
| Deforestation | Removes trees that act as a carbon SINK (they photosynthesise); burning/rotting the wood also releases its stored carbon | Adds CO₂ AND removes the sink that took CO₂ out → a double hit |
| Agriculture (crops & soil) | Ploughing and clearing land releases stored soil carbon; machinery burns fuel | Adds CO₂ (and other greenhouse gases) |
| Cattle (livestock) farming | Cattle release methane (CH₄) from digestion; pasture often replaces cleared forest | Adds methane (a potent greenhouse gas) and CO₂ from the lost forest |
Humans tip the balance: burning fossil fuels (combustion) adds CO₂ faster than photosynthesis can remove it, while deforestation cuts the down-arrow — so atmospheric CO₂ climbs.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Deforestation reduces sequestration: A favourite 1-mark exam point: name an action that reduces carbon sequestration.
The answer is deforestation / clearing forest. A growing forest is a carbon sink — photosynthesis locks carbon away in wood.
Cut it down and you stop that sequestration AND release the stored carbon. So deforestation both removes a sink and adds a source.
The seasonal wobble (a graph favourite): On a long-term CO₂ graph (like the famous Mauna Loa record) the line rises overall but also has a yearly zig-zag. Two processes cause the zig-zag:
Spring/summer (growing season): plants photosynthesise hard, so the sink wins and CO₂ dips.
Autumn/winter: many plants stop photosynthesising, but respiration and decomposition carry on, so the source wins and CO₂ rises each winter.
The long-term upward trend underneath is from human activity (mainly burning fossil fuels).
The seasonal wobble lives here too: in the growing season the down-arrow (photosynthesis) is strong and CO₂ dips; in winter plants stop, so the up-arrow (respiration/decomposition) wins and CO₂ rises.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Increases a SOURCE
- Burning fossil fuels → releases stored carbon
- Cattle → release methane + CO₂
- Burning/clearing forest → releases stored carbon
- Winter respiration & decomposition > photosynthesis
Removes a SINK
- Deforestation → fewer trees photosynthesising
- Less carbon sequestration (carbon not locked away)
- Agriculture clearing land → less plant cover
- Result: less CO₂ pulled out of the air
A memory hook: FDAC — Fossil fuels, Deforestation, Agriculture, Cattle.
Three add a source; deforestation does both (adds a source AND removes a sink).
Stop wasting time on topics you know
Our AI identifies your weak areas and focuses your study time where it matters. No more overstudying easy topics.
How this is tested: The big one is a 7-mark Explain on Paper 2: explain how human activity drives climate change. Score separate points — fossil-fuel combustion releases CO₂; deforestation removes a sink and releases carbon; agriculture and cattle add CO₂ and methane; rising greenhouse gases → enhanced greenhouse effect → warming.
On Paper 1A you may get a 1-mark graph item: why does CO₂ rise each winter (photosynthesis stops, respiration/decomposition continue), or which processes drive the monthly up-and-down (photosynthesis down, respiration/decomposition up).
Also common: identify an action that reduces sequestration (deforestation), or suggest how cattle farming warms the climate (methane + lost forest).
IB-style question — explain how human activity raises atmospheric CO₂
Explain how human activities have increased the concentration of carbon dioxide in the atmosphere. [4]
How to score all four marks
- Fossil fuels. Burning fossil fuels (combustion) releases carbon that was locked away in coal, oil and gas, adding CO₂ to the atmosphere — the largest cause.
- Deforestation removes a sink. Cutting down forests removes trees that photosynthesise, so less CO₂ is removed from the air (reduced sequestration).
- Deforestation also adds a source. Burning or rotting the cleared wood releases its stored carbon as CO₂.
- Agriculture and cattle. Clearing/ploughing land and cattle farming add CO₂ (and methane) — so the total CO₂ added now outpaces the CO₂ removed by photosynthesis. (Award 1 mark per distinct point, up to 4.)
Final answer
Burning fossil fuels releases stored carbon as CO₂; deforestation removes a photosynthesising carbon sink AND releases stored carbon; and agriculture/cattle add further CO₂ — so CO₂ is added faster than photosynthesis can remove it, and atmospheric CO₂ rises.
✓ Why this scores full marks: Each sentence is a separate, distinct point — combustion, the sink removed by deforestation, the carbon released by deforestation, and the extra CO₂ from agriculture/cattle.
A high-mark 'Explain' needs the mechanism (sink removed / source added / balance tipped), not just a list of activities.
| Carbon SINK (takes CO₂ out) | Carbon SOURCE (puts CO₂ in) | |
|---|---|---|
| Main process | Photosynthesis | Respiration, decomposition, combustion |
| Examples | Forests, phytoplankton, growing crops | Burning fossil fuels, burning/clearing forest, cattle digestion |
| Human action that hurts it | Deforestation removes a sink → reduces sequestration | Burning fossil fuels and clearing land increase the source |