3.4.2Ecology and the Environment
Greenhouse Gases — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 12
Section 1 of 6What is a greenhouse gas?
Definition
- Definition A greenhouse gas is a gas in the atmosphere that absorbs infrared (heat) radiation and so helps to keep energy in the Earth's atmosphere.
- Short-wave radiation from the Sun passes through the atmosphere and warms the ground; the ground then gives out longer-wavelength infrared radiation,
- which greenhouse gases absorb and re-emit in all directions.
A natural and necessary effect
- A natural and necessary effect Without any greenhouse effect the Earth would be far too cold for most living things,
- so the effect itself is beneficial.
- The problem is the enhanced greenhouse effect, where extra gas in the atmosphere traps more heat.
Consequence of higher levels
- Consequence of higher levels If the concentration of greenhouse gases rises, more infrared radiation is trapped and the average temperature of the Earth increases.
- This is called global warming.
Section 2 of 6The main greenhouse gases
Greenhouse gases
Water vapour
- Water vapour Present naturally in large amounts; it absorbs infrared radiation strongly.
Carbon dioxide
- Carbon dioxide Released when fossil fuels and wood are burned and by respiration;
- it is the gas whose rise is most closely linked to recent warming.
Methane
- Methane Released by microorganisms in the guts of cattle and in waterlogged rice paddy fields, and from rubbish tips.
Nitrogen oxides
- Nitrogen oxides Formed in vehicle engines and by some fertilised soils.
CFCs
- CFCs Man-made compounds once used in fridges and aerosols; they are also greenhouse gases.
Section 3 of 6How human activity raises the levels
| Gas | Human source | Why levels rise |
|---|---|---|
| Carbon dioxide | Combustion of fossil fuels in power stations, vehicles and homes | Carbon locked away for millions of years is released quickly as CO2 |
| Carbon dioxide | Deforestation | Fewer trees remain to remove CO2 by photosynthesis, and burning cleared wood releases more |
| Methane | Cattle farming | Cattle produce methane as microorganisms break down the grass they eat |
| Methane | Rice paddy fields | Waterlogged soil has little oxygen, so microorganisms release methane |
Section 4 of 6From extra gas to a warmer planet
1More emissions
- More emissions Burning fuels, farming more animals and growing more rice add greenhouse gases faster than they are removed.
2More heat trapped
- More heat trapped A higher concentration absorbs more of the infrared radiation leaving the surface.
3Temperature rises
- Temperature rises The average global temperature increases over time.
4Wider effects
- Wider effects Possible results include melting ice caps, rising sea levels, changing rainfall and shifts in where species can live.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Greenhouse gas | A gas that absorbs infrared radiation and so reduces heat loss from the Earth to space. |
| Global warming | The gradual increase in the average temperature of the Earth's surface and atmosphere. |
| Fossil fuel | A fuel such as coal, oil or natural gas formed from the remains of ancient living things. |
| Deforestation | The large-scale removal of trees from an area. |
| Infrared radiation | Heat radiation given out by warm objects such as the ground. |
Section 6 of 6Exam technique
1
Saying greenhouse gases 'trap the Sun's light' — it is infrared radiation from the warmed Earth that is absorbed.
2
Thinking the greenhouse effect is always harmful — it is natural and essential; the harm comes from the increase caused by humans.
3
Confusing the ozone layer with the greenhouse effect — they are separate issues, although CFCs affect both.
4
Forgetting methane when listing sources — cattle and rice paddy fields are both specification examples.
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