Greenhouse Gases and Enhanced Greenhouse effect

Sunlight reaches the Earth and warms its surface. The warm surface then gives off heat energy (infrared radiation). Some of this heat would escape back out into space — but certain gases in the atmosphere absorb (trap) the heat and keep the Earth warm. These gases are called greenhouse gases.

For Double Award Biology you must be able to name the greenhouse gases:

Greenhouse gas	Symbol	A main source
Water vapour	H₂O	Evaporation from oceans, lakes and plants
Carbon dioxide	CO₂	Burning fossil fuels; respiration; deforestation
Nitrous oxide	N₂O	Farming (fertilisers), some industry
Methane	CH₄	Cattle (livestock), rice paddy fields, rotting waste
CFCs	—	Old aerosols, refrigerators and air-conditioning units

A small, natural greenhouse effect is a good thing — without it the Earth would be far too cold (around −18 °C on average) for life. The problem comes when human activities increase the amount of these gases, trapping too much heat.

Exam tip. A common one-mark question is simply "name two greenhouse gases". The safest answers are carbon dioxide and methane — and remember that oxygen and nitrogen gas (N₂) are NOT greenhouse gases.

The greenhouse effect happens in a few simple steps:

1. Energy in. Short-wave radiation (light) from the Sun passes through the atmosphere and warms the Earth's surface.
2. Heat out. The warm surface gives off heat (infrared) radiation.
3. Heat trapped. Greenhouse gases in the atmosphere absorb some of this heat and re-radiate it, so it does not all escape into space.
4. Result. The trapped heat keeps the Earth warm.

The diagram below shows the idea.

Exam tip. Use the keywords absorb / trap heat (infrared radiation) and prevent it escaping into space. Do not say greenhouse gases "make heat" — they trap heat that is already there.

Human activities have increased the amount of greenhouse gases in the atmosphere. The two you must focus on for Double Award are carbon dioxide and methane.

Activities that raise carbon dioxide (CO₂):

• Burning fossil fuels — burning coal, oil and natural gas (in power stations, cars, factories and homes) is combustion, which releases CO₂.
• Deforestation — cutting down forests raises CO₂ in two ways:
  • There are fewer trees left to remove CO₂ from the air by photosynthesis.
  • The cut trees are often burned, which releases CO₂ directly.

Activities that raise methane (CH₄):

• Farming livestock — animals such as cattle produce methane during digestion (it is released from the gut).
• Growing rice — flooded rice paddy fields release methane as plant matter rots in the waterlogged soil.
• Rotting waste — decaying rubbish in landfill sites also produces methane.

(Farming and the use of fertilisers also release some nitrous oxide, another greenhouse gas.)

Exam tip. For burning fossil fuels, name the gas (CO₂). For livestock and rice fields, name the gas (methane). A frequent error is to say deforestation "uses up oxygen" — the correct point is that it reduces CO₂ removal by photosynthesis and burning trees adds CO₂.

When human activities increase the amount of greenhouse gases, more heat is trapped in the atmosphere. This stronger-than-natural trapping of heat is called the enhanced greenhouse effect.

The cause-and-effect chain is:

more greenhouse gases → more heat trapped → enhanced greenhouse effect → global warming

Global warming is the gradual rise in the average temperature of the Earth's surface and atmosphere. Even a small rise in average temperature can have large effects on the planet's climate and living things.

Be careful with the wording:

• The enhanced greenhouse effect is the cause (extra greenhouse gases trapping more heat).
• Global warming is the result (the rise in average global temperature).
• The wider, longer-term result is often called climate change.

Exam tip. Questions love a clear chain of reasoning. Link your points: human activity → named gas released → more heat trapped → enhanced greenhouse effect → global warming → a named consequence. Each link can earn a mark.

Global warming has many serious consequences for the planet and for living things. You should be able to describe several:

• Rising sea levels. Higher temperatures melt ice (glaciers and the polar ice caps) and expand seawater as it warms. This raises sea levels, which can flood low-lying land and coastal habitats.
• Changing weather and climate patterns. Some regions become hotter and drier (more droughts), while others get more storms and flooding. These changes make weather less predictable.
• Loss of habitats. As temperatures and rainfall change, some habitats are destroyed or change (for example, melting polar ice removes the habitat of polar animals; coral reefs are damaged). Species that cannot move or adapt may die out (become extinct), reducing biodiversity.
• Effects on food production. Crops may fail in regions that become too hot or too dry, and the distribution of pests and diseases can change.

All of these effects are linked back to the same cause: too many greenhouse gases trapping too much heat.

Exam tip. When a question asks for consequences, give clear, distinct points such as "sea levels rise so low-lying land floods" and "habitats are lost so some species become extinct". Try to explain the link (e.g. ice melts → sea level rises → flooding), not just list a word.