What are the natural and human causes of climate change, and how does the enhanced greenhouse effect work?
Explain the natural and human causes of climate change, including the enhanced greenhouse effect
A focused answer to the O-Level Geography outcome on the causes of climate change. The natural greenhouse effect, natural causes (solar and volcanic), and human causes (burning fossil fuels, deforestation) that drive the enhanced greenhouse effect, with a worked walkthrough.
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What this dot point is asking
SEAB wants you to explain the natural and human causes of climate change, with the enhanced greenhouse effect at the centre. The central insight is that the greenhouse effect is a natural and necessary process that keeps the Earth warm; the problem is that human activities are strengthening it by adding greenhouse gases, tipping a life-supporting balance into harmful warming.
The answer
The natural greenhouse effect
The greenhouse effect is natural and essential:
- The sun's energy (shortwave radiation) passes through the atmosphere and warms the Earth's surface.
- The warm surface gives off heat as longwave radiation.
- Greenhouse gases in the atmosphere, such as carbon dioxide, water vapour and methane, absorb some of this outgoing heat and re-radiate it back toward the surface.
- This traps warmth, keeping the planet at an average of about rather than a frozen .
Without the natural greenhouse effect, the Earth would be too cold for life.
Natural causes of climate change
The climate has always varied naturally, through causes such as:
- Changes in the sun's output, which alter how much energy reaches the Earth.
- Volcanic eruptions, which can throw ash and gases high into the atmosphere, sometimes cooling the Earth for a year or two by blocking sunlight.
- Long-term changes in the Earth's orbit that shift the seasons over tens of thousands of years.
These explain past natural swings, but they are too slow or small to account for the rapid recent warming.
Human causes and the enhanced greenhouse effect
Human activities are adding greenhouse gases to the atmosphere, strengthening the natural effect, the enhanced greenhouse effect:
- Burning fossil fuels (coal, oil and gas) for electricity, industry and transport releases large amounts of carbon dioxide, the main human-caused greenhouse gas.
- Deforestation removes trees that absorb carbon dioxide (a carbon sink) and releases stored carbon when trees are burned or rot (a carbon source), doubly raising carbon dioxide.
- Agriculture and waste release methane (from livestock, rice paddies and landfills), a powerful greenhouse gas.
More greenhouse gases trap more outgoing heat, so the surface warms further. This human strengthening of the natural effect is the main cause of present-day global warming.
Examples in context
Example 1. Singapore's emissions and the switch to natural gas. As a small but industrialised and densely populated state, Singapore's greenhouse gas emissions come mainly from burning fossil fuels for electricity and industry. To curb its contribution, Singapore generates most of its power from natural gas, the cleanest fossil fuel, and is investing in solar and imported low-carbon electricity. It illustrates how a country's emissions stem from fossil-fuel use and how the response targets that source.
Example 2. Deforestation and burning in Southeast Asia. The clearing and burning of forests and peatlands in parts of Indonesia, often for plantations, releases huge amounts of carbon dioxide and causes the regional haze. It is a clear regional example of deforestation acting as both a lost carbon sink and a major carbon source, strengthening the greenhouse effect while also harming air quality and health.
Try this
Q1. Name the main greenhouse gas released by burning fossil fuels. [1 mark]
- Cue. Carbon dioxide, the principal human-caused greenhouse gas, released when coal, oil and gas are burned for energy and transport.
Q2. Explain why the natural greenhouse effect is important for life on Earth. [2 marks]
- Cue. It traps some of the heat radiated by the Earth's surface and keeps the average temperature around ; without it the planet would be far too cold (around ) to support life as we know it.
Q3. Explain how deforestation strengthens the greenhouse effect. [3 marks]
- Cue. Trees absorb carbon dioxide as they grow, so clearing them removes this carbon sink and leaves more carbon dioxide in the air; burning or rotting the cleared trees also releases their stored carbon as carbon dioxide, so deforestation both reduces absorption and adds emissions, raising atmospheric carbon dioxide.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marks(a) Explain how the natural greenhouse effect keeps the Earth warm. (b) Explain how human activities are strengthening this effect to cause global warming.Show worked answer →
(a) The natural greenhouse effect: the sun's energy passes through the atmosphere and warms the Earth's surface. The warm surface gives off heat (longwave radiation). Greenhouse gases in the atmosphere, such as carbon dioxide and water vapour, absorb some of this outgoing heat and re-radiate it back toward the surface, trapping warmth and keeping the planet warm enough to support life. Without it, the Earth would be far too cold.
(b) Human activities are increasing the amount of greenhouse gases in the atmosphere, especially carbon dioxide from burning fossil fuels (coal, oil and gas) for energy and transport, and methane from agriculture and waste. Deforestation adds to the problem by removing trees that absorb carbon dioxide. More greenhouse gases trap more heat, so the surface warms further. This is the enhanced greenhouse effect.
Markers reward the natural mechanism (sunlight warms surface, surface emits heat, greenhouse gases absorb and re-radiate it back), and the human additions (fossil fuels, deforestation) that trap extra heat as the enhanced greenhouse effect.
Original5 marksExplain why deforestation contributes to climate change in two different ways.Show worked answer →
First, trees absorb carbon dioxide from the atmosphere as they grow, storing carbon in their wood (they act as a carbon sink). When forests are cleared, this removal of carbon dioxide stops, so more of the gas stays in the atmosphere where it traps heat.
Second, when trees are cut and especially when they are burned or left to rot, the carbon stored in them is released back into the atmosphere as carbon dioxide. Burning to clear land (as in slash-and-burn agriculture) releases large amounts at once.
So deforestation both removes a carbon sink (less carbon dioxide is taken up) and acts as a carbon source (stored carbon is released), increasing atmospheric carbon dioxide and strengthening the greenhouse effect.
Markers reward the two distinct mechanisms: losing the carbon sink (less absorption) and releasing stored carbon (a source), both raising atmospheric carbon dioxide.
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