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.
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.