The big idea: Earth's climate is warming mainly because human activity adds extra greenhouse gases to the atmosphere. These gases trap heat. The more we add, the more heat is trapped — so global temperatures rise.
The two human changes you must be able to explain
- More greenhouse gases — from burning fuels, farming and industry.
- A changed land surface — clearing forests and building cities alters how much sunlight Earth reflects.
Key terms
- Greenhouse gas (GHG) — a gas that traps heat in the atmosphere; the main ones are carbon dioxide (CO2) and methane (CH4).
- The enhanced greenhouse effect — extra warming caused by the human-added greenhouse gases on top of the natural effect.
- Albedo — how reflective a surface is; bright surfaces (ice, sand) reflect sunlight, dark surfaces (forest, tarmac, ocean) absorb it.
- Carbon footprint — the total greenhouse gases caused by a person, product or activity.
Two routes, one result: Humans warm the planet in two ways:
(1) by emitting greenhouse gases, and
(2) by changing the land surface (and so its albedo). Exam questions test both routes.
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A strong answer names a human activity, then explains the mechanism — exactly which gas it releases, or how it changes the land surface.
World greenhouse-gas emissions by sector (approximate shares)
| Sector | Share of emissions | Main human cause |
|---|---|---|
| Energy (power + heat) | about 35% | Burning coal, oil and gas for electricity |
| Transport | about 15% | Petrol and diesel vehicles, shipping, flights |
| Industry | about 20% | Factories, cement and steel making |
| Agriculture & land use | about 20% | Livestock, rice paddies, deforestation |
| Buildings | about 10% | Heating and cooling homes and offices |
Where the emissions come from
- Burning fossil fuels — coal, oil and gas for electricity, transport and industry release CO2.
- Agriculture — cattle and rice paddies release methane; fertilisers release nitrous oxide.
- Deforestation — clearing forests removes trees that absorbed CO2, and burning them releases more.
- Industry — cement and steel making release CO2 from both fuel and chemical reactions.
Why methane (CH4) is rising
- More livestock — growing demand for meat and dairy means more cattle, which release methane as they digest food.
- More rice farming — flooded paddy fields let bacteria produce methane.
- Fossil-fuel extraction — methane leaks from gas pipelines, oil wells and coal mines.
- Landfill waste — rotting rubbish in landfill sites gives off methane.
Explain two different reasons why methane emissions are rising.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
CO2 from fossil fuels and industry, 2022 (approximate)
| Country / bloc | Share of world total | Per person (tonnes a year) | Development |
|---|---|---|---|
| China | about 31% | about 8 | upper-middle income, the world's factory |
| USA | about 14% | about 15 | high income, car- and energy-heavy |
| India | about 8% | about 2 | lower-middle income, rising fast |
| European Union | about 7% | about 6 | high income, emissions falling |
| World average | 100% | about 4.7 |
What the table shows
- Totals and per person tell different stories — China emits most in total, but an American emits about twice as much as a Chinese person and over seven times as much as an Indian.
- Development raises emissions — industrialising MICs (China, India) are where emissions grow; many HICs have peaked and are falling as they switch from coal and move into services.
- History matters — since 1850 the USA and Europe have emitted the most in total, which is why LICs argue that rich countries should cut first and pay for adaptation.
Emissions embedded in trade: Territorial emissions are counted where goods are made; consumption-based emissions are counted where they are bought. Roughly a fifth to a quarter of world CO2 is released making goods that are then exported. So the UK and EU look cleaner by the territorial count than by the consumption count, and China looks dirtier: part of its emissions are made for shoppers in Europe and North America. Globalization has moved emissions, not removed them.
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Reflect or absorb: Albedo is how much sunlight a surface reflects. When humans change the land surface, they change its albedo — and so how much heat it absorbs.
Bright vs dark surfaces
- Bright surfaces (ice, snow, light sand) have high albedo — they reflect sunlight and stay cool.
- Dark surfaces (forest, dark soil, tarmac, ocean) have low albedo — they absorb sunlight and warm up.
Human changes to the land surface and their effect on albedo
| Human activity | Surface change | Effect on albedo and warming |
|---|---|---|
| Deforestation | Bright cleared/farmed ground replaces dark forest | Albedo can rise locally, but lost trees stop absorbing CO2 |
| Building cities | Dark tarmac and roofs replace fields | Lower albedo — more heat absorbed (urban heat island) |
| Melting ice (then human warming) | Dark ocean/land replaces bright ice | Lower albedo — more heat absorbed, speeding warming |
| Irrigating deserts | Dark crops replace bright sand | Lower albedo over the irrigated area |
Suggest two ways human activity has altered how much sunlight Earth's land surface reflects.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
How this is tested: A line graph of CO2 emitted per person against age, or a shaded map of farming emissions, is the usual way in here.
The question sequence
- First State or Estimate a value straight off the axis (or Describe the map pattern).
- Then the question turns to Explain or Suggest the human causes (trade, development, globalization, methane, albedo).
- These are worth [4 marks] — 2 per developed point.
Read the axes first. Find the age band at the highest point, and where the line climbs most steeply.
Interactive diagram
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Estimate the CO2 emitted per person at the age band where the line peaks.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Average CO2 emitted per person in a high-income country, by age
| Age (years) | CO2 per person (tonnes/year) |
|---|---|
| 0-9 | 8 |
| 10-19 | 12 |
| 20-29 | 18 |
| 30-39 | 22 |
| 40-49 | 24 |
| 50-59 | 21 |
| 60-69 | 16 |
IB-style question — read the graph
Using the data above: (a) state the age band at which the average person emits the most CO2 [1]; (b) state the age band over which emissions rise most steeply [1].
How to answer each part
- (a) Find the maximum. Scan the CO2 column for the largest value — it is 24 tonnes, in the 40-49 age band.
- (b) Find the steepest rise. Compare the jump between bands: 0-9 to 10-19 rises 4; 10-19 to 20-29 rises 6 (the biggest jump); 20-29 to 30-39 rises 4. So emissions rise most steeply over the 10-19 to 20-29 bands.
Final answer
(a) 40-49 years (24 tonnes); (b) the 10-19 to 20-29 bands (the steepest increase, +6 tonnes).
Explain two reasons why expanding trade in emerging economies has raised greenhouse-gas emissions.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Easy marks: (1) Name the gas (CO2 or methane). (2) Give the mechanism, not just the activity. (3) State the direction — emissions rise (or, for development, can rise or fall).