In simple terms
A friendly intro before the formal notes — no formulas yet.
Earth's Atmospheric Blanket
The atmosphere acts like a giant, complex blanket for our planet. It lets in the sun's light to warm us up, but traps some of the outgoing heat to keep the temperature just right for life.
Think of a car parked in the sun. The sun's light (short-wave radiation) passes easily through the car's windows, heating the seats and dashboard. These surfaces then radiate heat (long-wave radiation), but this heat can't escape through the windows as easily. The trapped heat makes the inside of the car much warmer than the air outside – this is a simple model of the greenhouse effect.
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The sun emits short-wave radiation, including visible light, which travels towards Earth.
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Some of this incoming solar radiation (insolation) is reflected by clouds and the Earth's surface (albedo), but most is absorbed by the ground.
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The warmed Earth's surface re-radiates energy as long-wave infrared radiation (heat).
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Greenhouse gases in the atmosphere absorb this outgoing long-wave radiation, trapping heat and warming the lower atmosphere.
Explore the concept
Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.
Step 1
The sun emits short-wave radiation, including visible light, which travels towards Earth.
2 more simulations for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
2 simulations
- PhETCoreIB 6.1
Molecules and Light
Choose microwave, infrared, visible or ultraviolet light, choose a gas molecule, and turn the lamp on to see whether the molecule absorbs the photons.
Try this
- Select Infrared and fire it at N₂ and O₂: the photons pass straight through.
- Switch to CO₂, CH₄ and H₂O: each one vibrates, then re-emits the photon in a new direction.
- Select Ultraviolet and O₃ and watch the ozone molecule split apart.
Look for Only some gases absorb infrared, so the 99% of the atmosphere that is N₂ and O₂ plays no part in the greenhouse effect. Ozone does a different job: absorbing ultraviolet in the stratosphere.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- PhETIB 6.1
Blackbody Spectrum
Drag the temperature slider between the Sun and Earth markers and watch the peak of the emission curve move along the wavelength axis.
Try this
- Set the temperature to the Sun marker and note the wavelength at which the curve peaks.
- Drag down to the Earth marker and zoom the axes until the curve reappears.
- Compare the two peak wavelengths.
Look for The hot Sun emits mostly short-wave radiation; the cool Earth emits long-wave infrared, which is the radiation greenhouse gases absorb.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Full topic notes
Formal explanation with the rigour you need for the exam.
The Structure and Composition of the Atmosphere
The atmosphere is a thin envelope of gases surrounding the Earth, divided into several distinct layers based on temperature changes. For ESS, we are primarily concerned with the two lowest layers. The troposphere extends from the surface up to about 12 km. It contains 75% of the atmosphere's mass, is where all weather occurs, and its temperature decreases with altitude. Above this lies the stratosphere (from ~12 to 50 km), where temperature increases with altitude due to the absorption of UV radiation by the ozone layer.
Composition: The atmosphere is approximately 78% nitrogen (N₂), 21% oxygen (O₂), 0.9% argon (Ar), and a small fraction of other gases, including the greenhouse gases.
Troposphere: The lowest layer, containing most of the atmosphere's water vapour, clouds, and pollution. Temperature falls with increasing altitude.
Stratosphere: The second layer, containing the vital ozone layer which protects life from harmful UV-B radiation. Temperature rises with increasing altitude.
The Greenhouse Effect: Earth's Natural Thermostat
The greenhouse effect is a natural and essential process. Without it, Earth would be too cold to support life as we know it. The process involves a delicate energy exchange, governed by the properties of different types of electromagnetic radiation and atmospheric gases.
Step 1: Insolation: Short-wave radiation from the Sun passes through the atmosphere.
Step 2: Absorption: Some radiation is reflected by clouds and the surface, but a significant portion is absorbed by the Earth's surface, warming it.
Step 3: Re-radiation: The warmed Earth's surface radiates energy back out as long-wave infrared radiation (heat).
Step 4: Trapping: Greenhouse gases (like CO₂, CH₄, H₂O) in the troposphere are effective at absorbing this outgoing long-wave radiation. They then re-radiate this energy in all directions, including back towards the Earth's surface, trapping heat in the lower atmosphere.
Be precise in your exam answers. Do not say greenhouse gases 'reflect' heat. They 'absorb and re-radiate' long-wave radiation. Also, always distinguish between incoming 'short-wave' solar radiation and outgoing 'long-wave' terrestrial radiation. These details are crucial for securing top marks.
Earth's Energy Balance and Albedo
For Earth's global temperature to remain stable over long periods, the energy entering the system must equal the energy leaving it. This is known as the Earth's energy balance. A key factor influencing this balance is albedo, which is the proportion of incoming solar radiation that is reflected by a surface. Light-coloured, shiny surfaces like ice and snow have a high albedo, while dark, matt surfaces like forests and oceans have a low albedo.
Albedo =
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A patch of open ocean receives an average of 200 W m⁻² of insolation. The ocean water has an albedo of 0.06. Calculate the amount of energy absorbed by the ocean per square metre. Show your working.
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Calculate the reflected energy:
An area of land has a net radiation budget. The incoming solar radiation absorbed by the surface is 160 W m⁻². The surface radiates 390 W m⁻² upwards. The atmosphere radiates 330 W m⁻² back down to the surface (the greenhouse effect). Calculate the net radiation at the surface and state whether this leads to warming or cooling.
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Identify all energy inputs to the surface:
How it all connects
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Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- troposphere
The troposphere extends from the surface up to about 12 km. It contains 75% of the atmosphere's mass, is where all weather occurs, and its temperature decreases with altitude.
- stratosphere
Above this lies the stratosphere (from ~12 to 50 km), where temperature increases with altitude due to the absorption of UV radiation by the ozone layer.
- Atmosphere
A dynamic mixture of gases that forms a layer around the Earth, held in place by gravity. It is essential for life, regulating temperature and protecting from harmful radiation.
- Insolation
Incoming Solar Radiation. It is the main source of energy for Earth's systems.
- Albedo
The measure of the reflectivity of a surface. A high albedo (e.g., fresh snow) reflects a lot of solar radiation, while a low albedo (e.g., asphalt) absorbs a lot.
- Natural greenhouse effect
The process where certain gases in the atmosphere trap some of the outgoing long-wave radiation from the Earth, maintaining a temperature suitable for life. Without it, Earth's average temperature would be about -18°C.
Name it
Read the meaning, then pick which of this lesson’s terms it describes. Miss one and you see what your choice really means.
The process where certain gases in the atmosphere trap some of the outgoing long-wave radiation from the Earth, maintaining a temperature suitable for life. Without it, Earth's average temperature would be about -18°C.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
Composition: The atmosphere is approximately 78% nitrogen (N₂), 21% oxygen (O₂), 0.9% argon (Ar), and a small fraction of other gases, including the greenhouse gases.
Troposphere: The lowest layer, containing most of the atmosphere's water vapour, clouds, and pollution. Temperature falls with increasing altitude.
Stratosphere: The second layer, containing the vital ozone layer which protects life from harmful UV-B radiation. Temperature rises with increasing altitude.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Test Your Knowledge on Atmospheric Systems
Test Your Knowledge on Atmospheric Systems
Extra simulations & links
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Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Test Your Knowledge on Atmospheric Systems on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
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