Greenhouse effect and greenhouse gases
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Question
What is the greenhouse effect?
Answer
Greenhouse gases let **sunlight in** but absorb the **infrared** the warm surface radiates out, sending some **back down** — so the surface stays **warmer**.
Question
Name the four main greenhouse gases.
Answer
**Carbon dioxide (CO₂)**, **methane (CH₄)**, **water vapour (H₂O)** and **nitrous oxide (N₂O)**.
Question
Which radiation do greenhouse gases trap — incoming or outgoing?
Answer
**Outgoing infrared** from the warm surface. Incoming sunlight (mostly visible) passes straight through.
Question
Outline the mechanism (2-mark answer).
Answer
Greenhouse gases **absorb** the **infrared** the surface emits, then **re-emit** it in all directions, so some returns **back down** to the surface, keeping it warmer.
Question
Why do CO₂ and CH₄ absorb infrared but N₂ and O₂ don't?
Answer
CO₂/CH₄ bonds **resonate** (vibrate) at infrared frequencies, so they absorb infrared; the simple N₂/O₂ bonds do not.
Question
What does 'resonate' mean here?
Answer
The infrared radiation's frequency **matches** the natural vibration frequency of the gas molecule's bonds, so the bond absorbs the energy.
Question
Natural vs enhanced greenhouse effect?
Answer
**Natural** = warming from gases always present (Earth ~33 °C warmer, needed for life). **Enhanced** = **extra** warming from human-added gases.
Question
Main human cause of the enhanced greenhouse effect?
Answer
**Burning fossil fuels** (coal, oil, gas), which releases extra **CO₂**.
Question
Roughly how much warmer is Earth because of the greenhouse effect?
Answer
About **33 °C** warmer than it would be with no atmosphere — without it, Earth would be far too cold for life.
Question
Common greenhouse-effect mistake to avoid?
Answer
Saying the gases block **incoming sunlight**. They don't — sunlight passes in; the gases trap the **outgoing infrared**.
Question
Where does the extra methane (CH₄) mostly come from?
Answer
**Farming** (cattle), **rice fields**, **landfill** and **gas leaks** — a strong infrared absorber per molecule.
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