Lesson focus: How can gases change Earth’s energy balanceWhether the Earth takes in the same amount of energy from the Sun as it gives out.?
Explain the greenhouse effect and connect it to changes in atmospheric composition.
Build on earlier learning
Carbon cycleConnect atmospheric carbon changes to their effects on energy transfer.
You will be successful when you can…
- Distinguish sunlight and infrared radiation.
- Describe greenhouse-gas absorption and emission.
- Explain temporary energy imbalance.
- Connect fossil carbon to warming.
Words that carry the science
- infrared radiation
- Electromagnetic radiation emitted strongly by Earth because of its temperature.
- greenhouse gas
- Atmospheric gas that absorbs and emits infrared radiation at particular wavelengths.
- energy balance
- State in which incoming and outgoing energy rates are equal.
- emission
- Release of electromagnetic radiation.
- absorption
- Transfer of radiation energy into matter.
Explain warming through infrared energy transfer and restored balance
01Sunlight enters, infrared leaves
Earth absorbs mainly shortwave solar energy and emits infrared radiationInvisible rays given off by warm objects that carry heat energy, like the heat you feel from a fire.. Some outgoing infrared escapes directly to space. In simple steps: sunlight warms the Earth’s surface. The warm surface gives off infrared radiationInvisible rays given off by warm objects that carry heat energy, like the heat you feel from a fire. (heat radiation).
02Greenhouse gases absorb and emit
Water vapour, carbon dioxide, methane and other gases absorb particular infrared wavelengths and emit infrared in all directions, including upward and downward. Carbon dioxideA colourless gas that we breathe out and that is made by burning fuels; it traps heat in the atmosphere. is a greenhouse gasA gas in the air, such as carbon dioxide or methane, that absorbs infrared (heat) given off by the Earth's surface and keeps the Earth warm.: it absorbs some of this infrared and sends some back down to the surface. With more carbon dioxide, less heat escapes to space, so energy builds up and the Earth gets warmer.
03A changed atmosphere changes balance
More greenhouse gas can initially reduce outgoing energy for the same surface temperature. Energy accumulates and the surface/troposphere warm until outgoing energy again matches incoming energy.
Carbon dioxide and global temperature
Since 1958, carbon dioxide in the air has been measured at Mauna Loa in Hawaii. Plot it against the average global temperature: as carbon dioxide has risen, so has the temperature. The greenhouse effect explains why.
Controls
Approximate, rounded values (NOAA Mauna Loa carbon dioxide; NASA GISTEMP temperature). A correlation alone does not prove cause; the greenhouse effect gives the mechanism.
Use a simple energy budget
Incoming absorbed energy is 240 units; outgoing energy falls temporarily from 240 to 238 units.
- Initial balance: 240 in and 240 out.
- After change: 240 in and 238 out.
- Net gain = 2 energy units per time.
- Warming raises outgoing infrared until balance is restored at a higher temperature.
How to make the reasoning visible: Persistent warming requires an energy imbalance, not energy appearing from nowhere.
Radiation pathway check
Label each arrow by radiation type, direction and interaction.
| Arrow/event | Radiation | Direction | Outcome |
|---|---|---|---|
| Sun to surface | Mostly shortwave | Down | Absorbed/reflected |
| Surface emission | Infrared | Up | Energy leaves surface |
| Atmospheric absorption | Infrared | Into gases | Molecules gain energy |
| Atmospheric emission | Infrared | Up + down | Some escapes/some returns |
| Space escape | Infrared | Up | Earth-system energy loss |
Why is “greenhouse gases block all heat” wrong?
How does warming help restore balance?
Use the table. What happens to some of the infrared radiation that greenhouse gases absorb?
Check your understanding
Answer from memory, then check against the suggested answer.
What is infrared radiation?
What is greenhouse gas?
What is energy balance?
Name one greenhouse gas.
What type of radiation does the Earth give out?
Connect fossil-fuel combustion to warming.
Why does downward infrared not violate conservation of energy?
Why does burning more fossil fuels add to global warming?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Answer the parts to explain how more carbon dioxide in the air can make the Earth warmer.
Sunlight warms the Earth’s surface. What does the warm surface give off?
(1)What does carbon dioxide do to some of that infrared?
(1)With more carbon dioxide, what happens to the heat escaping to space?
(1)So what builds up?
(1)So what happens to the temperature?
(1)Total for this question: 5 marks
Command word: Follow the heat step by step. Carbon dioxide does not block all heat.