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While the previous lesson showed that climate has always changed naturally, the scientific consensus is overwhelming: the rapid warming observed since the mid-20th century is primarily caused by human activities. This lesson examines the enhanced greenhouse effect, the evidence for human-caused climate change, its impacts on people and environments, and the strategies being used to address it. This is one of the most important topics in GCSE Geography because it connects physical processes with human decision-making and global consequences.
The natural greenhouse effect is essential for life on Earth. Without it, the average surface temperature would be approximately -18°C instead of the current +15°C.
The enhanced greenhouse effect occurs when human activities increase the concentration of greenhouse gases in the atmosphere beyond their natural levels. More greenhouse gases trap more heat, causing the planet to warm beyond its natural temperature.
graph TD
A["Sun emits short-wave radiation"] --> B["Passes through atmosphere<br/>Heats Earth's surface"]
B --> C["Earth emits long-wave<br/>(infrared) radiation"]
C --> D["Greenhouse gases absorb<br/>and re-radiate heat"]
D --> E["Some heat escapes to space"]
D --> F["More heat trapped by<br/>INCREASED greenhouse gases<br/>→ Enhanced warming"]
F --> G["Global temperatures rise"]
| Gas | Chemical Formula | Main Human Sources | Contribution to Enhanced Greenhouse Effect | Atmospheric Lifetime |
|---|---|---|---|---|
| Carbon dioxide | CO₂ | Burning fossil fuels (coal, oil, gas), deforestation, cement production | ~65% of total warming | 300–1,000 years |
| Methane | CH₄ | Agriculture (rice paddies, livestock — especially cattle), landfill sites, fossil fuel extraction | ~16% of total warming | ~12 years |
| Nitrous oxide | N₂O | Agricultural fertilisers, burning fossil fuels, industrial processes | ~6% of total warming | ~114 years |
| Fluorinated gases | HFCs, PFCs, SF₆ | Industrial processes, refrigeration, air conditioning | ~2% of total warming | Up to 23,000 years |
| Water vapour | H₂O | Not directly produced by humans, but warming increases evaporation, amplifying the greenhouse effect | Acts as a feedback mechanism | Days to weeks |
Exam Tip: CO₂ gets the most attention because it is the most abundant human-produced greenhouse gas and persists in the atmosphere for centuries. But methane is approximately 80 times more potent as a greenhouse gas than CO₂ over a 20-year period, even though it has a much shorter atmospheric lifetime.
The evidence that current warming is primarily human-caused is extensive and comes from multiple independent sources:
Exam Tip: When presenting evidence for climate change in an exam, organise your answer around different types of evidence (temperature, ice, sea level, CO₂). For each one, give specific data — examiners reward precise figures.
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