Earth gives off infrared. Greenhouse gases (water vapour, CO₂, methane, nitrous oxide) absorb some of it and send part back down, slowing the energy going out. Without them Earth would average about −18 °C. Adding more upsets the balance, and Earth warms until it is restored.
The moon is the same distance from the sun as Earth, but its average surface temperature is far below freezing. Earth's thin blanket of gases makes the difference.
You know Earth balances energy in and out. This lesson: what the air does to the energy going out.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: what do greenhouse gases do?
ExploreTry it on the board and compare what happens with your prediction.
Explore: remove them, then add more
Press Remove all greenhouse gases, then Run 50 years. Where does the temperature settle?
Press Put the greenhouse gases back, drag CO₂ to 560 ppm or more, then run until energy out equals energy in.
Try it yourselfYour edits are saved in this browser.
Remove all greenhouse gases and run the years until the temperature settles.
Use the board. This ticks when you get there.
Put them back, raise CO₂ to at least 560 ppm (double the pre-industrial level) and run until Earth is in balance again.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: natural and enhanced
In your book: draw the energy-balance diagram again and add greenhouse gases absorbing and re-emitting infrared. Label natural and enhanced.
Natural greenhouse effect or enhanced greenhouse effect?
Keeps Earth about 33 °C warmer than it would be with no greenhouse gases
Extra CO₂ from burning fossil fuels
Water vapour and CO₂ that were in the air before 1800
Methane from farming and landfill adding to the air
Elaborate TypicalUse the same idea in a new context.
Elaborate: the greenhouse gases
Greenhouse gas or not?
Carbon dioxide (CO₂)
Methane (CH₄)
Water vapour
Nitrous oxide (N₂O)
Nitrogen (N₂), 78% of the airMolecules with only two identical atoms barely absorb infrared.
Oxygen (O₂)
Elaborate WiderUse the same idea in a new context.
Elaborate: methane
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Earth's surface gives off infrared. Greenhouse gases (water vapour, CO₂, methane, nitrous oxide) absorb some and send part back down: they slow energy going out, not sunlight coming in.
Natural greenhouse effect: keeps Earth about 15 °C on average instead of about −18 °C.
Enhanced greenhouse effect: extra greenhouse gases from human activity reduce the outgoing infrared, so energy in exceeds energy out and Earth warms until balance is restored.
It is not the ozone hole.
For teachers
Students explain how greenhouse gases absorb and re-emit outgoing infrared, distinguish the natural and enhanced greenhouse effect, and locate the mechanism in the energy-balance model (AC9S10U04, node 3, a hinge node). About 24 minutes.
I can explain that greenhouse gases absorb and re-emit infrared from the surface.
I can distinguish the natural and enhanced greenhouse effect.
I can explain how more greenhouse gases upset the energy balance and warm the Earth.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider.
The hinge misconceptions: confusing the greenhouse effect with the ozone hole, and thinking greenhouse gases trap incoming sunlight. The board's sunlight arrow never changes when CO₂ changes; only the infrared arrows do.
Board notes: a one-layer model. The air absorbs a fraction of the surface's infrared that depends on CO₂ (calibrated so 280 ppm gives about 15 °C, no greenhouse gases about −18 °C, and doubling CO₂ about 3 °C of warming, the middle of the measured range of climate sensitivity, which in reality comes partly from feedbacks such as extra water vapour). "Remove all greenhouse gases" removes water vapour too. The temperature settles over a few decades in the model; the real deep ocean takes centuries.
Running the lesson
Engage: count how many chose the ozone layer or trapping sunlight.
Explore: ask which arrows changed when CO₂ changed, and which didn't.
Explain: 'which flow' is the checkpoint.
The hard part (on purpose)
Placing the effect on outgoing infrared, not incoming sunlight or the ozone hole. When a student says CO₂ heats Earth because it is hot, ask where the extra energy comes from.
When students get stuck
A student says CO₂ heats the Earth because it's a hot gas. Ask: Where does the extra energy come from? Fix: From the sun, as always: CO₂ slows its escape.
A student says the greenhouse effect is bad. Ask: What was the temperature with no greenhouse gases? Fix: About −18 °C: the natural effect keeps Earth liveable; the problem is the enhanced effect.
Stretch and support
Stretch: Why does a cloudy night stay warmer than a clear night?
Support: Give the diagram with the infrared arrows unlabelled, to label absorbed, re-emitted down and escaping.
Where it leads
Next: human activity and the energy imbalance (node 4), indicators of climate change (node 5), evaluating responses (node 6). Year 9: the carbon cycle and combustion (AC9S9U03).
What each step is for
Engage: what do greenhouse gases do?: Students who think greenhouse gases trap incoming sunlight (or confuse them with the ozone hole) locate the mechanism wrongly; the board's arrows show it acts on outgoing infrared.
Explore: remove them, then add more: Removing all greenhouse gases (about −18 °C), then doubling CO₂ (a few degrees warmer), shows both the natural and the enhanced effect in one model.
Explain: natural and enhanced: Locating the mechanism on the outgoing flow, and separating natural from enhanced, is what explaining climate change requires.
Elaborate: the greenhouse gases: Naming the gases and separating them from other air pollutants prevents the ozone confusion.
Elaborate: methane: Comparing gases by their warming power applies the mechanism quantitatively.
Now you can
Explain that greenhouse gases absorb and re-emit infrared from the surface.
Distinguish the natural and enhanced greenhouse effect.
Explain how more greenhouse gases upset the energy balance and warm the Earth.