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# How do greenhouse gases warm the Earth?

Year 10 Earth and space · Earth's energy balance

<!-- Speaker note: 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 (Engage and Explore 10 min · Explain 8 min · Evaluate 3 min · Typical adds 1 min, Wider about 2 min). -->

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## Today

**We are learning to** 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.

- 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.

<!-- Speaker note: 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. -->

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## Why it matters

> 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.

<!-- Speaker note: You know Earth balances energy in and out. This lesson: what the air does to the energy going out. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**How do greenhouse gases warm the Earth?**

- They absorb infrared given off by Earth's surface and send some back down, slowing the energy going out
- They trap incoming sunlight
- They make the sun hotter
- They let more sunlight in through a hole in the ozone layer

<!-- Speaker note: What happens: They absorb outgoing infrared from the surface and send part of it back down, so energy leaves more slowly. Sunlight passes through them easily. The ozone hole is a different problem. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose the ozone layer or trapping sunlight. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fearth-10%2Fgreenhouse%2Fstep-2&code=widget%3A+climate%0Amode%3A+greenhouse%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-10/greenhouse/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask which arrows changed when CO₂ changed, and which didn't. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* They absorb outgoing infrared from the surface and send part of it back down, so energy leaves more slowly. Sunlight passes through them easily. The ozone hole is a different problem.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Which flow

**Greenhouse gases slow which flow of energy?**

- Heat from Earth's core
- Wind
- Infrared going out from Earth to space
- Sunlight coming in

* Answer: Infrared going out from Earth to space

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: 'which flow' is the checkpoint. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Natural or enhanced

**Natural greenhouse effect or enhanced greenhouse effect?**

Groups: Natural · Enhanced

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

* Answer: Keeps Earth about 33 °C warmer than it would be with no greenhouse gases → Natural; Extra CO₂ from burning fossil fuels → Enhanced; Water vapour and CO₂ that were in the air before 1800 → Natural; Methane from farming and landfill adding to the air → Enhanced

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Rise and level off

**When you raised CO₂, why did the temperature rise and then level off?**

- The sun got hotter
- CO₂ is hot
- The ozone layer got thinner
- Less infrared escaped, so energy in exceeded energy out; the warmer surface then gave off more until out equalled in again

* Answer: Less infrared escaped, so energy in exceeded energy out; the warmer surface then gave off more until out equalled in again

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: the greenhouse gases

**Greenhouse gas or not?**

Groups: Greenhouse gas · Not a greenhouse gas

Carbon dioxide (CO₂) · Methane (CH₄) · Water vapour · Nitrous oxide (N₂O) · Nitrogen (N₂), 78% of the air · Oxygen (O₂)

* Answer: Carbon dioxide (CO₂) → Greenhouse gas; Methane (CH₄) → Greenhouse gas; Water vapour → Greenhouse gas; Nitrous oxide (N₂O) → Greenhouse gas; Nitrogen (N₂), 78% of the air → Not a greenhouse gas; Oxygen (O₂) → Not a greenhouse gas

<!-- Speaker note: What this step is for: Naming the gases and separating them from other air pollutants prevents the ozone confusion. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: methane

**There is about 200 times less methane than CO₂ in the air, yet it causes about a quarter as much extra warming. Why?**

- Kilogram for kilogram, methane absorbs far more infrared than CO₂ (it traps about 80 times as much heat over 20 years)
- Methane lasts forever
- Methane lets more sunlight in
- Methane is hotter

* Answer: Kilogram for kilogram, methane absorbs far more infrared than CO₂ (it traps about 80 times as much heat over 20 years)

<!-- Speaker note: What this step is for: Comparing gases by their warming power applies the mechanism quantitatively. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Explain how adding CO₂ warms the Earth.**

* CO₂ absorbs infrared from the surface and sends some back down, so less energy escapes; energy in exceeds energy out, and Earth warms until it gives off enough to balance again.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this 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). -->