Section 1 of 5Radiation Balance
The Earth's temperature stays steady when energy in equals energy out.
1
The Sun emits radiation, mostly visible light, ultraviolet and some infrared, and some of it reaches the Earth.
2
Part of the incoming radiation is reflected by clouds, ice and the surface, and the rest is absorbed.
3
The absorbed energy warms the surface and the atmosphere.
4
Because the Earth is warm, it emits radiation of its own, mainly infrared, which travels out to space.
5
When the radiation absorbed matches the radiation emitted, the average temperature stays constant.
Section 2 of 5The Greenhouse Effect
Greenhouse gases
- Carbon dioxide, methane and water vapour absorb infrared radiation leaving the Earth.
Re-emission
- These gases radiate the energy in all directions, and some of it returns to the surface.
Natural effect
- This natural process keeps the Earth about 33 degrees Celsius warmer than it would be without an atmosphere.
Enhanced effect
- Burning fossil fuels and deforestation add more carbon dioxide, which traps more radiation and raises average temperature.
Section 3 of 5Factors That Change Temperature
Factors That Change Temperature
Reflection
- Clouds, snow and ice reflect sunlight, so more reflection lowers the amount absorbed.
Greenhouse gases
- More of these gases in the atmosphere reduces the infrared lost to space, so the Earth warms.
Sun's output
- Changes in the energy the Sun emits can alter the radiation received by the Earth.
Volcanic dust
- Large eruptions put particles into the atmosphere, which reflect some sunlight and cool the Earth for a time.
| Change | Effect on absorbed or emitted radiation | Effect on temperature |
|---|---|---|
| More carbon dioxide | Less infrared escapes to space | Rises |
| More cloud or ice cover | Less sunlight absorbed | Falls |
| Less greenhouse gas | More infrared escapes | Falls |
Section 4 of 5Evidence and Responses
Measurements
- Records of temperature, carbon dioxide levels in air and ice cores show that both have risen in recent centuries.
Computer models
- Scientists use models of the climate to predict future temperatures, but results depend on many assumptions.
Consequences
- Warming can cause sea level rise from melting ice and more extreme weather in some regions.
Reducing the effect
- Using renewable energy, saving energy and planting trees can lower the carbon dioxide added to the atmosphere.
Section 5 of 5Exam Checklist & Tips
8.3.6 – Key Points for the Exam
- Greenhouse gas A gas in the atmosphere that absorbs infrared radiation and re-emits some of it back towards the surface.
- Equilibrium temperature The steady temperature reached when the energy absorbed equals the energy emitted.
- Infrared radiation Electromagnetic radiation emitted by warm objects, including the Earth's surface.
- Reflection The bouncing of radiation off a surface, so that it is not absorbed.
- Global warming The rise in average temperature of the Earth's surface and atmosphere.
Common Mistakes
- Saying the greenhouse effect is entirely harmful; without it the Earth would be too cold for life.
- Claiming that greenhouse gases trap visible light from the Sun;
- they mostly absorb the infrared the Earth emits.
- Confusing the greenhouse effect with the hole in the ozone layer.
- Thinking temperature keeps rising only when energy is absorbed; it is steady when absorbed equals emitted.