Solar radiation & Earth's energy budget
Big idea: Earth receives energy from the Sun as short-wave radiation and releases it back to space as long-wave radiation. The balance between these determines our climate.
What happens to incoming solar radiation?
When sunlight reaches Earth, it doesn't all reach the surface. The energy is distributed in several ways:
- ~30% is reflected back to space (by clouds, ice, and light surfaces) — this is called albedo
- ~20% is absorbed by the atmosphere (by ozone, clouds, water vapour)
- ~50% is absorbed by Earth's surface (land and oceans)
Energy in vs energy out
For Earth's temperature to stay stable, energy in must equal energy out. The surface absorbs short-wave radiation and re-emits it as long-wave (infrared) radiation.
Short-wave = from Sun (visible light, UV) → passes through atmosphere easily. Long-wave = from Earth (infrared/heat) → absorbed by greenhouse gases.
Exam tip: Questions often ask you to explain the difference between short-wave and long-wave radiation. Remember: Sun = short, Earth = long.
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The natural greenhouse effect
Big idea: The greenhouse effect is a natural process that makes Earth habitable. Without it, Earth would be about 33°C colder!
How does it work?
- Step 1: Short-wave solar radiation passes through the atmosphere and warms Earth's surface
- Step 2: Earth's surface emits long-wave (infrared) radiation back toward space
- Step 3: Greenhouse gases absorb some of this long-wave radiation
- Step 4: Greenhouse gases re-emit the radiation in all directions — including back toward Earth
- Step 5: This 'traps' heat in the lower atmosphere, warming the planet
The main greenhouse gases
Natural greenhouse gases
- Water vapour (H₂O) — most abundant
- Carbon dioxide (CO₂)
- Methane (CH₄)
- Nitrous oxide (N₂O)
- Ozone (O₃)
Human-made additions
- CFCs and HFCs (refrigerants)
- Extra CO₂ from fossil fuels
- Extra CH₄ from agriculture
- Extra N₂O from fertilisers
The natural greenhouse effect is essential for life. The problem is the enhanced greenhouse effect caused by human emissions increasing greenhouse gas concentrations.
Exam tip: In 7-mark questions, make sure you describe the complete mechanism — incoming short-wave, surface absorption, long-wave emission, greenhouse gas absorption, and re-emission.
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IB-style question — the greenhouse effect [4]
Explain how the natural greenhouse effect keeps the Earth's surface warm enough to support life. [4]
How to answer it, step by step
- Sunlight comes in and warms the surface
• Short-wave solar radiation passes through the atmosphere and heats the ground.
• The warmed surface re-emits energy as long-wave (infrared) radiation. - Greenhouse gases trap the outgoing heat
• Gases like CO₂ and water vapour absorb the outgoing long-wave radiation.
• They re-emit it in all directions, sending some back down and keeping the surface warm (about +33 °C warmer than without it).
Final answer
Mark the difference between short-wave in and long-wave out — the trick is that greenhouse gases trap the long-wave radiation, not the incoming sunlight.
IB-style question — energy balance change [2]
A planet currently receives 340 W m⁻² of incoming solar energy and radiates 340 W m⁻² back to space. Rising greenhouse gases cut the energy radiated to space to 338 W m⁻². Calculate the resulting energy imbalance and state what happens to the planet's temperature. [2]
How to answer it, step by step
- Subtract output from input
• Imbalance = energy in − energy out.
• = 340 − 338 = 2 W m⁻². - Say what the imbalance does
• More energy comes in than leaves, so energy builds up.
• The planet warms (surface temperature rises).
Final answer
Show the subtraction with the unit (W m⁻²) and link a positive imbalance (in > out) to warming — a bare number misses a mark.