The greenhouse effect
Sunlight comes in, heat goes out, and a few gases slow the way out. Learn how that keeps Earth liveable, and why adding more of them warms it.
ENERGY BALANCE
Energy in, energy out
The Sun sends energy to Earth, mostly as visible light. Earth can only get rid of energy one way: by radiating heat back to space as invisible infrared. The planet's temperature settles where energy out matches energy in. Change either side and the temperature moves until the two match again.
A pot on a low flame stops getting hotter once it loses heat as fast as the flame adds it. Put a lid on, and it loses heat more slowly, so it warms up to a new, higher level before it balances again.
How the greenhouse effect works
- 1 · Sunlight arrives
About 30% is reflected straight back to space by clouds, ice and bright surfaces. The rest passes through the air and is absorbed, warming the land and the sea.
- 2 · The ground glows in infrared
Everything warm gives off infrared. You can't see it, but you feel it as the heat coming off a sunny wall in the evening.
- 3 · Some gases catch it
A few gases in the air, the greenhouse gases, absorb much of that infrared and send it out again in every direction, including back down. So the surface is warmed by the Sun and by the atmosphere.
- 4 · A new balance
Add more of these gases and less heat escapes for a while. The planet warms until it again sends out as much energy as it receives.
Check yourself
Parisa's classmate says: "CO₂ is only about 0.04% of the air, and nitrogen is 78%. A gas that rare can't change the climate." What is the flaw in his argument?
- Nothing: the gases that make up most of the air are the ones that control temperature
- CO₂ is heavier than air, so it sinks and forms a thick warm layer near the ground
- CO₂ comes out of exhausts hot, and that heat adds up across the planet
- What matters is absorbing infrared: CO₂ does, nitrogen and oxygen barely do
Show the answer
What matters is absorbing infrared: CO₂ does, nitrogen and oxygen barely do
Right. What matters is how strongly a gas absorbs infrared, not how common it is. Nitrogen and oxygen are almost transparent to it; CO₂ is not.
Greenhouse gas
A gas that absorbs and re-emits infrared radiation, slowing the escape of heat to space. The main ones are water vapour, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and ozone. Nitrogen and oxygen, which make up about 99% of dry air, are not greenhouse gases.
CO₂ from burning coal, oil and gas; methane from livestock, landfills and gas leaks; nitrous oxide mostly from fertilised farmland.
Check yourself
Water vapour is the largest single contributor to the natural greenhouse effect, so the CO₂ we add can't really matter.
Show the answer
False
False. Water vapour is indeed the biggest contributor, but it doesn't set the temperature on its own: it rains out within about a week or two, and how much the air holds depends on how warm it is. CO₂ stays for centuries, so it sets the temperature, and the water vapour then follows and amplifies the warming. That's a feedback, which you'll meet again in lesson 8.
A thicker blanket on a winter night
Your body makes heat at a steady rate. Under a thin blanket, you lose it fast and feel cool. Swap it for a thicker one and you don't make more heat, you just lose it more slowly, so you end up warmer. Adding greenhouse gases thickens Earth's blanket. Where the analogy breaks: a blanket works by trapping warm air, while greenhouse gases absorb and re-emit radiation. (Real greenhouses, by the way, work mostly like the blanket: glass stops warm air from blowing away.)
The two big ones: CO₂ and methane
Carbon dioxide (CO₂)
Weaker per tonne, but emitted in huge amounts. A large share stays in the air for centuries, some of it for thousands of years. The biggest driver of warming so far.
Methane (CH₄)
Much stronger per tonne: roughly 80 times CO₂ over 20 years, roughly 30 times over 100 years. But it breaks down in about a decade. The second-biggest driver so far.
Check yourself
Does it add greenhouse gases to the air?
- A petrol car driving across town
- A solar panel working on a sunny day
- A leak in a natural gas pipe
- A wind turbine spinning
- Heavily fertilised wheat fields
- A sailboat crossing a lake
Show the answer
Adds greenhouse gases: A petrol car driving across town, A leak in a natural gas pipe, Heavily fertilised wheat fields
Adds none while running: A solar panel working on a sunny day, A wind turbine spinning, A sailboat crossing a lake
Check yourself
Hamid says the planet is warming because the hole in the ozone layer lets more sunshine in. Which reply best corrects him?
- He's right, and that's exactly why sunscreen and sunglasses matter more every single year
- Warming comes from heat struggling to get out; the ozone hole is about ultraviolet
- The ozone hole is caused by CO₂, so in the end it's the same problem with a different name
- There is no ozone hole; it was a measuring mistake
Show the answer
Warming comes from heat struggling to get out; the ozone hole is about ultraviolet
Yes. Think of the two sides of the energy balance. The warming is on the out side: greenhouse gases make it harder for infrared to leave.
Check yourself
Match each gas to what makes it different
Show the answer
- Nitrogen → Most of the air, but lets infrared pass
- Carbon dioxide → The biggest driver so far; lingers for centuries
- Methane → Much stronger per tonne; gone in about a decade
- Water vapour → Amount follows temperature; rains out within days
Lesson recap
- Earth's temperature settles where heat going out as infrared matches sunlight coming in.
- Greenhouse gases absorb that infrared and send some back down; naturally, they keep Earth about 33 °C warmer than it would be.
- What matters is how a gas treats infrared, not how common it is: CO₂ matters, nitrogen doesn't.
- CO₂ lingers for centuries; methane is stronger but short-lived; water vapour follows temperature and amplifies it.
- The ozone hole is a separate problem about ultraviolet light.