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 are:
- 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.
A strong answer names a human activity, then explains the mechanism — exactly which gas it releases, or how it changes the land surface.
| 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.
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Reflect or absorb: Albedo is how much sunlight a surface reflects.
- 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.
When humans change the land surface, they change its albedo — and so how much heat it absorbs.
| 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 |
How this is tested: On Paper 2 the climate question (Q2) often opens with a map or graph of emissions — you Describe a pattern or State a value off the axes — and then asks you to Explain or Suggest human causes (trade, development, globalization, methane, albedo) for [4 marks] (2 per developed point).
| 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).
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).