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.
Where 100 units of incoming solar radiation go
| Part of the budget | Roughly how much | What happens |
|---|---|---|
| Reflected (the albedo) | about 30% | Bounced straight back to space by clouds, air and bright surfaces |
| Absorbed by the atmosphere | about 20% | Warms the air directly |
| Absorbed by the surface | about 50% | Warms the land and oceans |
| Outgoing long-wave radiation | about 70% | Re-emitted by the warmed Earth, balancing the input |
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.
Natural drivers of the global energy balance
| Natural driver | What it does | Effect on temperature |
|---|---|---|
| Solar output (sunspot cycle) | Energy from the Sun rises and falls slightly | Warmer when output is high |
| Earth's orbit (Milankovitch) | Slow changes in orbit, tilt and wobble over 1000s of years | Drives ice ages and warm periods |
| Large volcanic eruption | Throws ash and sulfate aerosols high into the air | Cooling for 1-3 years (global dimming) |
| Feedback loops | A change triggers more of the same change | Amplifies warming OR cooling already underway |
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.
Human causes — two routes to a warmer planet
| Route | How humans do it | Effect |
|---|---|---|
| More greenhouse gases | Burning fossil fuels (CO2); livestock & rice (methane); industry, deforestation | Traps more outgoing heat — the enhanced greenhouse effect |
| A changed land surface | Dark cities and tarmac, clearing forest, lost ice | Lowers albedo, so more sunlight is absorbed |
| Development & trade | Industry, transport and export manufacturing grow | Can raise emissions, or fall in wealthy decarbonising economies |
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.
Read the axis units first, then describe the trend: flat to about 1960, then a steep rise to 2020.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Key terms to use precisely
- Global energy balance — incoming solar (short-wave) radiation balancing outgoing terrestrial (long-wave) radiation; it sets Earth's temperature.
- Greenhouse effect — gases absorbing outgoing long-wave heat and re-radiating it down, warming the lower atmosphere.
- Enhanced greenhouse effect — the extra warming when human activity raises greenhouse-gas levels.
- Albedo — the share of incoming radiation a surface reflects (high for ice/desert, low for ocean/forest/tarmac).
- Milankovitch cycles — slow changes in Earth's orbit, tilt and wobble that alter incoming sunlight and drive ice ages.
- Global dimming — a fall in sunlight reaching the ground because particles (e.g. volcanic aerosols) reflect or block it.
- Feedback loop — a change that amplifies itself (positive, e.g. ice-albedo) or damps itself (negative).
✍️ IB-style questions
Explain how the natural greenhouse effect keeps the Earth warm, and how human activity has enhanced it.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Examine the view that today's changes in the global energy balance are caused mainly by human activity rather than natural processes.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick self-check
Tap each card to reveal the answer.
What is the global energy balance? Incoming short-wave solar radiation balancing outgoing long-wave terrestrial radiation — keeping the planet's temperature steady.
How does the natural greenhouse effect work? Greenhouse gases absorb outgoing long-wave heat and re-radiate some back down, warming the lower atmosphere. Without it Earth would be about 33 °C colder.
Name one warming and one cooling natural cause. Warming: higher solar output or Milankovitch orbital cycles. Cooling: a large volcanic eruption → sulfate aerosols → global dimming.
What is a positive feedback loop? Give one. A change that amplifies itself. Ice-albedo: warming melts bright ice, the dark surface absorbs more heat, so it warms further and melts more ice.
What are the two routes by which humans warm the climate? (1) Emitting greenhouse gases (CO2 from fuels, methane from livestock/rice); (2) changing the land surface (dark cities, lost ice) to lower albedo.
🎯 Highest-yield exam reminders
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.