Key Idea: Topic 2.1 is about why the climate changes — the natural balance of energy, and what tips it. It pulls together three ideas: 2.1.1 — the greenhouse effect & energy balance: Earth's temperature is steady when incoming solar (short-wave) radiation balances outgoing terrestrial (long-wave) radiation. Some energy is reflected (the albedo); greenhouse gases absorb and re-radiate the rest — the natural greenhouse effect. 2.1.2 — natural causes: solar output, Milankovitch (orbital) cycles and volcanic eruptions (global dimming) nudge that balance, sped up or slowed by feedback loops. 2.1.3 — human causes: burning fossil fuels, agriculture, deforestation and land-use change add greenhouse gases and alter albedo — the enhanced greenhouse effect. This is core content, examined on Paper 2 — a data-response read off a climate graph, a short structured Explain, and an extended-response essay.
☀️ 2.1.1 — The greenhouse effect & the global energy balance
Earth stays at a roughly steady temperature because energy in balances energy out: short-wave solar radiation arrives, the warmed Earth re-emits long-wave heat. Some incoming energy is reflected (the albedo), the rest is absorbed. Greenhouse gases absorb some outgoing long-wave heat and re-radiate it down — the natural greenhouse effect. The skill examiners test is reading an energy-budget figure or bar chart, quoting the shares, then outlining the mechanism.
Tip: The Sun sends short-wave energy in; the cooler Earth re-emits long-wave heat out. Greenhouse gases let the short-wave light through but trap some of the long-wave heat — that is the whole mechanism. The enhanced greenhouse effect is the extra warming when human activity raises greenhouse-gas levels.
🌋 2.1.2 — Natural causes of climate change
Climate has always changed, long before humans. Anything that nudges the energy balance changes it — some natural drivers warm the planet, some cool it, and feedback loops amplify any change.
Example: When Mount Pinatubo (Philippines) erupted in 1991, it pumped sulfate aerosols high into the atmosphere. They reflected sunlight back to space, and global temperatures fell by roughly 0.5 °C for about a year — a clear, measured example of natural cooling and global dimming.
🏭 2.1.3 — Human causes of climate change
Today's rapid warming is mainly human: we add extra greenhouse gases and we change the land surface. Economic development, trade and globalization drive both routes.
Over geological time and short-term blips, natural causes dominate. But the rapid present-day rise — CO2 climbing sharply since ~1850 alongside warming — is mainly human. The key Paper 2 skill is being able to separate the two and weigh them up for the time scale in question.
[Diagram: geo-line-chart]
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Exam Tips
- Energy balance = short-wave in equals long-wave out. The greenhouse effect = gases absorb then re-radiate outgoing long-wave heat down.
- For an energy-budget figure: read the shares straight off (≈30% reflected, ≈70% absorbed) BEFORE you write the mechanism.
- Albedo = reflected radiation. Bright surfaces (ice, desert) reflect; dark surfaces (ocean, forest, tarmac) absorb.
- Explain = give the MECHANISM (aerosols reflect sunlight → cooling), not just name the cause.
- Name the gas: CO2 from fossil fuels, methane from livestock and rice. The enhanced effect is the EXTRA, human-driven warming.
- On the [10] natural-vs-human essay, weigh both sides with named evidence (Pinatubo, Milankovitch, the post-1850 CO2 rise) and end on a judgement that names the TIME SCALE.