The greenhouse effect is not pollution, not a problem, and not something we invented. It is a natural process that keeps this planet about 33 °C warmer than it would otherwise be. The trouble only starts when we make it stronger. Getting that distinction right is worth marks in almost every question on this topic.
📚 What you need to know
Greenhouse gases (GHGs) absorb heat radiated from the Earth’s surface. The key ones are water vapour, carbon dioxide, methane and nitrous oxide.
Sunlight passes through the atmosphere as short-wave radiation. The surface absorbs it and re-emits it as longer-wave infrared — and that is what greenhouse gases absorb.
Absorbed heat is re-emitted in all directions, so some returns to the surface instead of escaping to space. The Earth stays warm.
Without it, the average surface temperature would be about −18 °C instead of about 15 °C.
The enhanced greenhouse effect is the strengthening of this natural process by human emissions — the cause of global warming.
Aerosols such as black carbon also affect the balance: they absorb sunlight, warm the air and darken snow and ice.
The gases that do the work
Greenhouse gas
Where it comes from
What makes it matter
Water vapour
Evaporation from oceans, lakes and rivers; transpiration from plants; sublimation from ice and snow; a product of burning fossil fuels
The most abundant greenhouse gas. Its concentration depends on temperature, so it amplifies whatever the other gases do rather than driving change itself
Carbon dioxide
Burning coal, oil and natural gas; deforestation, which releases carbon stored in trees; industrial processes such as cement production
The main driver of the enhanced effect, because it is released in huge quantities and stays in the atmosphere for a very long time
Methane
Livestock digestion, landfill, rice paddies, natural gas leaks, wetlands
Traps heat far more effectively than carbon dioxide — over twenty times as much per molecule over a century — but is present in much smaller amounts
Nitrous oxide
Synthetic and organic fertilisers, fossil fuel combustion, some industrial processes
Around 300 times the warming effect of carbon dioxide per molecule, again at a far lower concentration
Potency versus quantity. Methane and nitrous oxide are individually far stronger, yet carbon dioxide causes more warming overall, because there is so much more of it and it lasts so long. If a question asks why carbon dioxide gets the attention, that is the answer.
Water vapour is usually left out of climate policy, and students find that odd. The reason is that we cannot control it directly: its level is set by temperature. Warm the planet with carbon dioxide and the extra water vapour follows automatically — which is why it is treated as a feedback, not a cause.
Aerosols
Aerosols are tiny solid particles or liquid droplets suspended in the air. Black carbon, produced by incomplete combustion of fossil fuels, wood and other biomass, comes from diesel engines, cooking stoves and open burning. It matters twice over: it absorbs sunlight and warms the air around it, and when it settles on snow and ice it darkens the surface, so less sunlight is reflected and melting speeds up.
The mechanism, step by step
Follow the energy. Every mark in this section comes from getting the order and the wavelengths right.
🧩 The energy path
The Sun emits short-wave radiation, including visible light and ultraviolet.
Most of it passes straight through the atmosphere, because greenhouse gases do not absorb these wavelengths.
Some is reflected by clouds, ice and bright surfaces. The rest is absorbed by the Earth’s surface, which warms up.
The warm surface re-emits that energy at longer wavelengths — infrared, felt as heat.
Greenhouse gases absorb this infrared radiation and re-emit it in all directions, including back down.
Some heat still escapes to space, but less than would without the gases — so the surface stays warmer.
Notice that nothing is “trapped” forever. Heat still leaves; it just takes a longer, more roundabout route out.
The blanket comparison is a good one: a blanket does not make heat, it slows the rate at which your heat escapes. Greenhouse gases do the same for the planet.
Natural versus enhanced
Without any greenhouse gases, the infrared leaving the surface would go straight out to space and the average temperature would sit near −18 °C — oceans frozen, and no life as we know it. The natural greenhouse effect lifts that to about 15 °C. Adding more greenhouse gases lifts it further still.
The middle panel is what makes Earth habitable. The right-hand panel is what the rest of this topic is about.
The water vapour feedback
Here is the loop that makes carbon dioxide more dangerous than its own absorption suggests. A warmer atmosphere can hold more water vapour. Water vapour is itself a greenhouse gas, so it causes more warming, so the air holds more water vapour again. That is positive feedback: the original change is amplified rather than damped down.
The loop to remember
more CO2 → warmer air → more evaporation → more water vapour → more warming → back to the start
Worked examples
WORKED EXAMPLE
Explain how the greenhouse effect keeps the Earth’s surface warm. [4]
Step 1: energy arrives
The Sun emits short-wave radiation, which passes through the atmosphere largely unabsorbed.
Step 2: the wavelength changes
The surface absorbs it, warms, and re-emits the energy as longer-wave infrared radiation.
Step 3: the gases act
Greenhouse gases such as carbon dioxide and water vapour absorb infrared and re-emit it in all directions, including back towards the surface.
Step 4: the resultLess energy escapes to space than arrives directly at the surface, so the surface settles at a higher temperature — about 15 °C rather than about −18 °C.Short-wave in, long-wave out, some sent back down
WORKED EXAMPLE
Methane traps over twenty times more heat per molecule than carbon dioxide. Explain why carbon dioxide is still considered the more important greenhouse gas. [3]
Accept the fact
Per molecule, methane really is far more effective at absorbing infrared radiation.
Bring in concentrationCarbon dioxide is present at roughly 0.04% of the atmosphere; methane is hundreds of times rarer.Bring in lifetimeCarbon dioxide remains in the atmosphere far longer, so emissions accumulate over decades and centuries rather than fading.Potency × quantity × lifetime = total effect
💡 Exam tip
Use the words short-wave and long-wave (or infrared). The change in wavelength at the surface is the marked point.
Say greenhouse gases re-emit in all directions, not that they “reflect” or “bounce” heat back. Re-emission is the correct mechanism.
Quote 15 °C and −18 °C. Two numbers, and they prove the effect is necessary.
Always distinguish the natural effect from the enhanced effect, even when the question does not.
For “why does that gas matter” questions, weigh potency against concentration and lifetime.
⚠ Common mix-up
The greenhouse effect is not bad. Without it the planet would be frozen. Global warming comes from enhancing it.
It is not the ozone hole. Ozone depletion is a separate problem, in a different layer, involving different gases and ultraviolet rather than infrared.
Greenhouse gases do not block sunlight coming in. They are transparent to short-wave radiation; they act on the outgoing infrared.
Heat is not permanently trapped. It is delayed. The Earth still radiates to space, just more slowly.
Water vapour is not the cause of climate change. It is a powerful feedback that responds to temperature, which is why it is treated separately.
Aerosols do not all warm. Black carbon warms, but many other aerosols reflect sunlight and cool. Say which one you mean.
Up next: Atmospheric Processes That Drive Climate — how uneven heating, circulation and the water cycle turn that energy balance into the climate of a place.
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