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Question: "Describe how human activities are causing the enhanced greenhouse effect." (4 marks)
Burning fossil fuels releases large amounts of carbon dioxide into the atmosphere, because the carbon stored in coal, oil, and natural gas is oxidised to CO₂. [1] Deforestation also increases CO₂ levels, because fewer trees are available to absorb carbon dioxide through photosynthesis, and burning the trees releases the carbon they stored. [1] Agriculture — particularly cattle farming and flooded rice paddies — produces methane (CH₄) through anaerobic decomposition, adding another greenhouse gas to the atmosphere. [1] As the concentrations of these greenhouse gases increase, more infrared radiation emitted by Earth's surface is absorbed and re-emitted back towards Earth, trapping more heat and causing global temperatures to rise above the natural level. [1]
Examiner note: For a "describe" question, you do not need to explain the molecular mechanism of IR absorption — that is for an "explain" question. The four mark points here are: CO₂ from fossil fuels, CO₂ from deforestation, methane from agriculture, and the consequence (more IR trapped, temperatures rise). Students who only list gases without linking them to human activities typically score 2 marks. Use the word "infrared" rather than just "heat" for precision.
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