B.2 Greenhouse effect

UNIT SYLLABUS

SL/HL 6 hours

This unit applies radiation physics to the most consequential system there is: Earth's climate. The Sun delivers a measurable intensity (the solar constant); geometry spreads it over the globe to an average of $S/4$; and the planet's albedo reflects a fraction straight back to space. What remains warms the surface, which re-radiates in the infrared — and there the greenhouse gases (water vapour, CO₂, methane and nitrous oxide) intercept the outgoing energy because their molecular energy levels resonate at infrared frequencies, re-emitting it in all directions including back down. The physics is careful and quantitative, and it explains precisely why enhancing this natural effect shifts the planet's energy balance.

Guiding Questions

? How does the greenhouse effect arise from the interaction between radiation and molecules?
? What physical factors determine the equilibrium temperature of a planet?

✓ What the IB expects you to master

Apply conservation of energy to a planet's radiation balance.
Define emissivity as radiated power per unit area compared with a black body at the same temperature.
Define albedo as scattered power over incident power, and explain why Earth's albedo varies with clouds and latitude.
Use the solar constant $S$ , and explain why the mean incoming intensity over the whole Earth is $S/4$ (projected disc versus full sphere).
Name the main greenhouse gases — CH $_4$ , H $_2$ O, CO $_2$ , N $_2$ O — and recognise both natural and human origins.
Explain infrared absorption by greenhouse gases through molecular energy levels (and the resonance model), with re-emission in all directions.
Distinguish the natural greenhouse effect from the enhanced greenhouse effect caused by human activity.

1 Key Formulas

Intensity

I = \frac{P}{A}

Emissivity

e = \frac{\text{power radiated per unit area}}{\sigma T^{4}}

Albedo

\alpha = \frac{\text{total scattered power}}{\text{total incident power}}

Mean incoming intensity

I_{\text{mean}} = \frac{S}{4}

2 Exam Preparation & Topic Explanations

Energy-balance calculations

Exam questions build a chain: solar constant → divide by 4 → multiply by $(1-\alpha)$ → equate to $e\sigma T^4$ for equilibrium temperature. Practise the full chain until each link is automatic; questions stop at different links.

Be precise with language: "absorb and re-emit in all directions" earns the mark; "trap" or "reflect" heat does not.

Pro Exam Strategy

Know the four named greenhouse gases and one natural + one human source for each.
The factor 4 is geometry (disc vs sphere), not absorption — a favourite MCQ distractor.
Equilibrium means absorbed power = emitted power; any imbalance changes the temperature until balance is restored.
Both the resonance model and the energy-level model are named in the guide — be ready to describe either.

3 MCQ Practice

Q1. The mean intensity of solar radiation arriving at Earth is one quarter of the solar constant because:

The atmosphere absorbs three quarters of the radiation
Earth presents a disc of area $\pi R^2$ to the Sun but has surface area $4\pi R^2$
Earth's albedo reflects three quarters of the radiation
The Sun only illuminates a quarter of the Earth at any time

Q2. Greenhouse gases absorb infrared radiation because:

They are denser than other atmospheric gases
Their molecular energy levels match infrared photon energies
They reflect infrared radiation back to Earth
They have higher specific heat capacities

Q3. Fresh snow has an albedo of about 0.85. This means that fresh snow:

Absorbs 85% of incident radiation
Scatters 85% of incident radiation
Emits 85% as much as a black body
Transmits 85% of incident radiation

4 Short Answer Questions

PDF

Download the practice worksheet

All questions from this unit + answer key — free, printable.

↓

Formula Booklet →

Every data-booklet equation, searchable.

Simulations →

See this physics in motion.

Daily Question →

Practise one concept a day.

← Previous unit

B.1 Thermal energy transfers

Next unit →

B.3 Gas laws

Stuck on Greenhouse effect?

Get it explained properly — 1-on-1, by a specialist who has taught this unit for 13+ years. Newtonine students average 6.8 in IB Physics.

Book a Free Consultation