By the end of this lesson
- Describe human activities that increase each greenhouse gas.
- Explain the biological basis of methane and nitrous oxide release.
- Suggest ways to reduce emissions.
BIG QUESTION
How does a hamburger contribute to greenhouse gas emissions?
Cattle release methane, land may be cleared for grazing, and fuel is burned in farming and transport.
Carbon dioxide
combustionBurning a fuel in oxygen, releasing energy and carbon dioxide. for electricity, transport, heating and industry releases carbon that had been stored for millions of years.
deforestationCutting down large areas of forest, which reduces carbon dioxide uptake by photosynthesis and can cause soil erosion and loss of biodiversity. reduces the number of trees removing carbon dioxide by photosynthesis, and burning or decomposing the cleared wood releases more.
Methane and nitrous oxide
methaneA greenhouse gas released by cattle, rice fields and decomposing waste in landfill. is released by bacteria in the guts of cattle and other ruminants, by anaerobic decomposition in flooded rice paddies and landfill sites, and from leaks during fossil-fuel extraction.
nitrous oxideA greenhouse gas released when soil bacteria act on nitrogen fertilisers, especially in waterlogged soils, and from some industrial processes. is released when soil bacteria act on nitrogen fertilisers, especially when soils are waterlogged, and from some industrial processes.
CFCs and water vapour
CFCsChlorofluorocarbons: man-made greenhouse gases, once used in fridges and aerosols, that also damage the ozone layer. were used in refrigerators, air-conditioning and aerosols. They are now banned in most countries because they also damage the ozone layer, but long-lived molecules remain in the atmosphere.
Humans do not directly control water vapour much, but warming increases evaporation.
Reducing emissions
Options include renewable energy, energy efficiency, reforestation, reducing food waste, improved livestock feeds, careful fertiliser timing and capturing methane from landfill.
The effectiveness of each depends on scale, cost and local conditions.
Human sources of greenhouse gases
Link each activity to the gas and the process.
| Activity | Gas released | Process |
|---|---|---|
| Burning fossil fuels | Carbon dioxide. | Combustion of stored carbon. |
| Deforestation | Carbon dioxide. | Less photosynthesis; burning and decay of wood. |
| Cattle farming | Methane. | Gut bacteria in ruminants. |
| Rice paddies and landfill | Methane. | Anaerobic decomposition. |
| Nitrogen fertilisers | Nitrous oxide. | Soil bacteria act on excess nitrate. |
| Old refrigerants | CFCs. | Leakage of synthetic gases. |
Language that earns precision
- deforestation
- Removal of large areas of forest.
- ruminant
- A grazing mammal, such as a cow, whose gut bacteria digest cellulose and release methane.
- anaerobic decomposition
- Breakdown of organic matter without oxygen, which can produce methane.
Common misconceptions
Claim: Deforestation only matters because it releases CO₂ when trees are burned.
Correction: It also reduces future CO₂ removal by photosynthesis.
Claim: Methane comes mainly from cows’ breathing.
Correction: It is produced by gut bacteria and released mainly by belching.
Claim: Fertilisers only cause water pollution.
Correction: Bacterial action on fertilisers also releases nitrous oxide, a greenhouse gas.
Match the answer to the command word
Name an activity, the gas and the process; one-word answers like “cars” score less.
Give both less photosynthesis and release of CO₂ by burning or decomposition.
Recall, apply, then justify
Explain how deforestation increases the concentration of carbon dioxide in the atmosphere.
Hint
Two effects, then the overall change.
Suggested marking points
- Fewer trees means less photosynthesis.
- So less carbon dioxide is removed from the air.
- Burning or decomposition of the wood releases carbon dioxide.
Give two human activities that release methane.
Hint
Farming and waste.
Suggested marking points
- Cattle (ruminant) farming.
- Rice paddies / landfill sites / fossil-fuel extraction.
Evaluate the proposal that replacing beef with rice in diets would reduce greenhouse gas emissions.
Planning hint
Both foods release methane; consider trophic levels and land.
What a strong design includes
- Beef cattle release large amounts of methane.
- Producing beef requires more land and feed per kg of food because of energy losses between trophic levels.
- So switching could reduce emissions per unit of food.
- Flooded rice paddies also release methane.
- Rice farming changes such as intermittent drainage can reduce this.
- The overall effect depends on farming methods, transport and diet quantities.