Greenhouse gases (AQA GCSE Chemistry Combined Science): Revision Notes
Greenhouse gases
What are greenhouse gases?
Greenhouse gases are special gases found in our atmosphere that help keep Earth warm enough for life to exist. These essential atmospheric components trap heat and maintain Earth's temperature at levels suitable for life.
The main greenhouse gases you need to know are:
- Carbon dioxide (CO₂) - the most common greenhouse gas
- Methane (CH₄) - very powerful at trapping heat
- Water vapour - water in gas form
Without these gases, Earth would be far too cold for plants, animals and humans to survive. The average temperature would be about -18°C instead of the current +15°C!
The greenhouse effect
The greenhouse effect is the natural process by which greenhouse gases trap heat in Earth's atmosphere. This mechanism is essential for life as we know it.
How the Greenhouse Effect Works:
Step 1: The Sun sends out short-wavelength radiation (light energy) to Earth.
Step 2: This short-wavelength radiation passes easily through the atmosphere and warms up Earth's surface.
Step 3: Earth's surface then gives off long-wavelength radiation (heat energy) back towards space.
Step 4: Greenhouse gases in the atmosphere absorb this long-wavelength radiation and trap it. This keeps the heat close to Earth instead of letting it escape to space.
This process is actually essential for life. If there were no greenhouse gases, all the heat would escape and Earth would be too cold to support life.
How humans increase greenhouse gases
Human activities are adding extra greenhouse gases to the atmosphere. This is causing more heat to be trapped, which leads to global warming.
Activities that increase carbon dioxide (CO₂):
- Burning fossil fuels - coal, oil and gas release CO₂ when burned for energy
- Deforestation - cutting down trees means less CO₂ is absorbed from the atmosphere
Activities that increase methane (CH₄):
- Animal farming - cows and other farm animals release methane when they digest grass
- Rubbish in landfill sites - bacteria break down food waste and release methane gas
The enhanced greenhouse effect caused by human activities is the main driver of climate change. While the natural greenhouse effect keeps Earth habitable, the extra greenhouse gases we're adding are causing dangerous warming.
For example, when bacteria in landfill sites decompose food waste, they produce methane gas. Farm animals like cows also produce methane in their digestive systems when they break down grass.
Key Points to Remember:
- Greenhouse gases keep Earth warm - without them, our planet would be too cold for life
- The three main greenhouse gases are carbon dioxide, methane and water vapour
- Short-wavelength radiation comes in from the Sun, long-wavelength radiation tries to go out but gets trapped
- Human activities are increasing greenhouse gas levels through burning fossil fuels, farming and waste disposal
- This extra trapping of heat is causing global warming and climate change