The greenhouse effect is not the problem. It is the reason Earth is not a frozen rock. The problem is that we have made it stronger than it was, and every mark in this section depends on you keeping those two ideas apart.
📚 What you need to know
The Sun emits short-wave radiation, including visible light and ultraviolet. Most of it passes straight through the atmosphere.
Earth’s surface absorbs that energy and re-emits it at longer wavelengths — infrared, or thermal energy.
Greenhouse gases absorb the outgoing infrared and re-emit it in all directions, including back down. Less energy escapes to space.
Key greenhouse gases: water vapour, carbon dioxide, methane and nitrous oxides.
Earth’s average surface temperature is about 15 °C. Without the greenhouse effect it would be roughly −18 °C.
The enhanced greenhouse effect is the human-driven strengthening of this natural process, and it causes global warming.
The step-by-step process
🧩 What actually happens to the energy
The Sun emits energy as solar radiation, including visible light and UV.
This radiation enters the atmosphere. Some is reflected straight back from the surface, clouds and ice.
Most is absorbed by Earth’s surface, warming it.
The warmed surface re-emits that energy at longer wavelengths as infrared radiation.
Some of that infrared passes through the atmosphere and escapes into space.
Some is absorbed by greenhouse gases and re-emitted in all directions, so a share returns to the surface.
Less energy is lost to space than would otherwise be, so the surface stays warm.
The whole mechanism turns on one fact: greenhouse gases are nearly transparent to incoming short-wave radiation but opaque to outgoing long-wave radiation. Energy gets in easily and leaves with difficulty. Write that sentence and you have the concept.
The numbers to quote
Actual average surface temperature ≈ 15 °C
Without any greenhouse effect ≈ −18 °C
So the natural greenhouse effect is worth about 33 °C of warming
Natural versus enhanced
Nothing about the mechanism changes between the panels. Only the concentration of absorbing gas changes, and with it the proportion of outgoing infrared that gets sent back down.
The greenhouse gases
Gas
Where it comes from
What makes it notable
Water vapour
Evaporation from oceans, lakes and rivers; transpiration from plants; sublimation from ice and snow; fossil fuel combustion
The most abundant greenhouse gas. Its concentration depends on temperature, so it amplifies the effect of the others rather than driving change itself. Usually left out of mitigation because it cannot be controlled directly
Carbon dioxide
Burning coal, oil and natural gas; deforestation releasing stored carbon; industrial processes such as cement production
The main driver of the enhanced effect because of its high concentration and long lifespan in the atmosphere
Methane
Livestock digestion, landfills, natural gas extraction, rice paddies, wetlands
Traps heat far more effectively than carbon dioxide per molecule, but is present in much lower concentrations so its total effect is smaller
Nitrous oxides
Synthetic and organic fertiliser use, fossil fuel combustion, industrial processes
Extremely potent per molecule, again offset by very low concentration
Potency versus concentration: methane and nitrous oxide are much stronger per molecule than carbon dioxide, but there is far less of them. Total warming effect = potency × amount. Say both halves and you will not lose the mark.
The water vapour feedback
This is the loop worth understanding properly. A warmer atmosphere holds more water vapour. More water vapour absorbs more infrared, which warms the atmosphere further, which lets it hold still more water vapour. That is a positive feedback loop, and it is why water vapour is described as amplifying the effect of the other gases rather than starting the change.
Aerosols
Aerosols are tiny particles or droplets suspended in the air. The one to know is black carbon:
Produced by incomplete combustion of fossil fuels, wood and other biomass.
Found in emissions from diesel engines, cooking stoves and open burning of vegetation.
It absorbs sunlight directly and warms the atmosphere.
When it settles on snow and ice it darkens the surface, reducing reflectivity and speeding up melting.
WORKED EXAMPLE
Earth’s mean surface temperature is about 15 °C. Calculations suggest it would be about −18 °C with no greenhouse effect. Calculate the warming produced by the natural greenhouse effect, and explain why the enhanced greenhouse effect is still a problem. [4]
Step 1: subtract15 − (−18) = 33The natural greenhouse effect provides about 33 °C of warmingStep 2: explain why more is not better
Ecosystems, agriculture and human settlements are adapted to the current range, not to a warmer one.
Step 3: the rate matters
The change is happening over decades, not tens of thousands of years, so species and societies cannot adapt in time.
Watch the double negative in the subtraction. Losing the minus sign turns 33 into −3.
💡 Exam tip
Always use the wavelength language: short-wave in, long-wave out. It is what separates a top answer from a vague one.
Say greenhouse gases re-emit in all directions, not that they “reflect” heat back down. Reflection is a different process.
Learn the 15 °C and −18 °C figures. They come up as calculations.
Give both potency and concentration when comparing gases.
If the question says “greenhouse effect”, answer about the natural process. If it says “enhanced” or “accelerated”, answer about human activity.
⚠️ Common mix-up
Treating the greenhouse effect as bad. It is natural and necessary; the enhanced version is the issue.
Confusing it with ozone depletion. Different gases, different layer, different radiation. They are separate problems.
Saying greenhouse gases stop sunlight entering. They let short-wave radiation through; they intercept it on the way out.
Calling methane the biggest problem because it is more potent. Carbon dioxide contributes more overall because there is so much more of it.
Forgetting water vapour is a feedback, not a driver. Its concentration responds to temperature rather than setting it.
Up next: How the Atmosphere Moves (HL) — uneven heating, circulation cells, and the ozone cycle in the stratosphere.
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