The big idea: The Earth stays at a roughly steady temperature because energy in balances energy out.
- Energy in = short-wave solar radiation from the Sun. - Energy out = long-wave terrestrial radiation (heat) re-emitted by the warmed Earth.
When the two are equal, the planet's temperature is stable. This balance is called the global energy budget.
Key terms
- Insolation (solar radiation) — short-wave energy arriving from the Sun.
- Terrestrial radiation — long-wave heat re-emitted by the warmed Earth back towards space.
- Albedo — the share of incoming radiation that a surface reflects straight back, with no warming.
- Terrestrial albedo — the reflectivity of the Earth's surface itself (ice and desert are high; ocean and forest are low).
- Energy budget / balance — the overall accounting of energy in versus energy out for the planet.
Short-wave in, long-wave out: The Sun's energy arrives as short-wave radiation. The Earth is much cooler than the Sun, so it re-emits energy as long-wave radiation.
This difference is the whole reason the greenhouse effect works.
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Not all incoming radiation warms the Earth. Some is reflected straight back to space (the albedo), some is absorbed by the atmosphere, and the rest is absorbed by the surface. The energy budget tracks each part.
| Part of the budget | Roughly how much | What happens |
|---|---|---|
| Incoming solar radiation | 100% | Short-wave energy arriving from the Sun |
| Reflected by clouds, air and the surface | about 30% | Bounced straight back to space (the albedo) |
| Absorbed by the atmosphere | about 20% | Warms the air directly |
| Absorbed by the surface | about 50% | Warms the land and oceans |
| Outgoing long-wave radiation | about 70% | Re-emitted by the warmed Earth, balancing the input |
Albedo: bright reflects, dark absorbs
- High albedo (reflective) surfaces — fresh snow and ice, light desert sand and cloud tops — bounce most radiation back, so they warm little.
- Low albedo (dark) surfaces — ocean, forest and dark soil — absorb most radiation, so they warm a lot.
- Terrestrial albedo is the average reflectivity of the land and sea surface; changing it (e.g. melting ice) changes how much energy the Earth absorbs.
Outline two parts of the planet's energy budget.
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How the natural greenhouse effect works: Some long-wave radiation leaving the Earth is absorbed by greenhouse gases in the atmosphere instead of escaping to space.
Those gases then re-radiate some of that heat back down to the surface, keeping the lower atmosphere warm. Without this natural greenhouse effect the planet would be about 33C colder and largely frozen.
The greenhouse gases
- Water vapour (H2O) — the most abundant natural greenhouse gas.
- Carbon dioxide (CO2) — released naturally by respiration, volcanoes and oceans.
- Methane (CH4) — from wetlands, animal digestion and decay.
- Nitrous oxide (N2O) — from soils and natural processes.
- Ozone (O3) — present naturally in the lower and upper atmosphere.
Why it is called a 'greenhouse': Like the glass of a greenhouse, the gases let the short-wave sunlight in but trap some of the long-wave heat trying to get out.
The glass and the gases both warm the inside by slowing the escape of heat.
Name the gases, not just 'CO2': When a question says two naturally present greenhouse gases that are not carbon dioxide, the safe answers are water vapour and methane (also nitrous oxide or ozone). Don't write CO2 if it is excluded.
How this is tested: An energy-budget diagram — showing what happens to each share of incoming solar radiation — is the usual stimulus here.
The command terms tested are Outline (e.g. the greenhouse mechanism, two parts of the energy budget, terrestrial albedo, the enhanced greenhouse effect) and State (name two natural greenhouse gases), each worth [1–2 marks]. Read the percentages straight off the figure before you write.
Read the key and axis first. Which bar shows the albedo, and how big is it?
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Using the chart, estimate the percentage of incoming solar radiation that is reflected straight back to space.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
| Part of the budget | Roughly how much | What happens |
|---|---|---|
| Incoming solar radiation | 100% | Short-wave energy arriving from the Sun |
| Reflected by clouds, air and the surface | about 30% | Bounced straight back to space (the albedo) |
| Absorbed by the atmosphere | about 20% | Warms the air directly |
| Absorbed by the surface | about 50% | Warms the land and oceans |
| Outgoing long-wave radiation | about 70% | Re-emitted by the warmed Earth, balancing the input |
Outline the mechanism by which the natural greenhouse effect keeps the Earth warm.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Easy marks: (1) Say long-wave radiation is absorbed. (2) Say the gases re-radiate heat back down. Two clear steps = two marks.