The Earth and its atmosphere · System
Carbon dioxide, humans and climate
A gas making up four hundredths of one per cent of the air decides the temperature of the planet. Two centuries of burning things has moved that number by half again.
Start here
The Earth absorbs a certain amount of energy from the Sun and radiates the same amount back out to space. Work out the temperature that balance needs and the answer is about −18 °C.
The average surface temperature of the Earth is about 15 °C. Thirty-three degrees have to come from somewhere, and it is not the Sun — the Sun is already in the sum.
Where do the extra thirty-three degrees come from?
From the atmosphere slowing the escape of heat. Certain gases let sunlight through on the way in but absorb the infrared radiated back out from the warmed ground — so energy leaves more slowly than it arrives, and the surface settles at a higher temperature. This is the greenhouse effect, and without it the planet would be frozen. The question is not whether it happens. It is what happens when you increase the gases responsible.
Carbon dioxide, methane and water vapour are greenhouse gases. They are transparent to visible light and opaque to the infrared radiation the Earth gives off — and that difference is the entire mechanism.
Since about 1750, human activity has raised the carbon dioxide in the air from around 280 parts per million to over 420 — mostly by burning fossil fuels, with clearing forests and making cement accounting for the rest. That is not a subtle change to a small number. It is an increase of about half.
Watched first · four steps
How the effect actually works
0 of 4 revealed
Most descriptions of this get one step wrong. Reveal them one at a time and watch what the gases do and do not do.
Sunlight passes straight through the atmosphere and warms the ground.
Coming in · visible light · passes straight through
Visible light is not absorbed by carbon dioxide or methane at all — those gases are transparent to it. Nothing about the incoming energy is affected, which is why “the gases trap the Sun’s rays” is the wrong picture.
The warmed ground radiates energy back out — as infrared, not as light.
Going out · infrared · emitted by the warmed surface
Anything warm radiates, and the wavelength depends on the temperature. The Sun is thousands of degrees and radiates visible light; the ground is around 15 °C and radiates infrared. Same energy, different kind of wave — and that difference is the whole mechanism.
Greenhouse gases absorb infrared and radiate it again in all directions.
Going out · infrared · absorbed and re-emitted
Carbon dioxide and methane are opaque to infrared even though they are transparent to light. They absorb the outgoing radiation and re-emit it — some upwards, some back down towards the surface. Nitrogen and oxygen, 99 per cent of the air, do neither.
Energy leaves more slowly than it arrives, so the surface settles at a higher temperature.
In and out
Nothing is permanently trapped — the energy does eventually escape, and in the long run as much leaves as arrives. What changes is the temperature at which that balance is reached. Add more of the absorbing gases and the balance point moves up.
One thing the name gets wrong.
A greenhouse is warm mostly because the glass stops warm air from getting out of it. Nothing in the atmosphere does that. There is no lid, air moves freely, and no gas is holding anything in place.
What the two share is only this: something makes it harder for the energy to leave than to arrive. The glass does it by stopping air; the gases do it by absorbing infrared. The name is the one we are stuck with. The mechanism is the one worth knowing.
Key fact
Greenhouse gases let sunlight through and absorb the infrared the warmed surface radiates back out. More of them means energy leaves more slowly, and the surface settles at a higher temperature.
There is more water vapour in the air than any other greenhouse gas, and people sometimes offer that as an argument. It is not one. How much water vapour the air holds is decided by the temperature: warm the air and it holds more, cool it and the extra falls out as rain within days. It follows the temperature rather than setting it. Carbon dioxide does not — it stays in the air for a very long time whatever the weather does, which is why the gas that has changed the climate is the one people added.
Your turn · four pieces of evidence
What does each one actually show?
0 of 4 judged
Decide what each one can and cannot establish on its own.
Evidence 1 · direct measurement
Carbon dioxide has been measured continuously at an observatory in Hawaii since 1958. It has risen every single year, from 315 to over 420 parts per million, with a small saw-tooth wobble each year.
On its own, this establishes that the amount really is changing, and by how much.
It shows the rise is real and steady, measured in one clean location far from any city. What it does not show on its own is the cause. The yearly wobble is the northern hemisphere breathing: plants take carbon dioxide in through the summer and give it back through the winter. So the line falls for part of every year and rises across every decade — and a fall you can point at is not the same thing as a trend.
Evidence 2 · ice cores
Bubbles of air trapped in Antarctic ice give a direct sample of the atmosphere going back 800,000 years. Across all of it, carbon dioxide stayed between 180 and 300 parts per million.
On its own, this establishes how the present compares with the whole past record.
The first two are both true and the first is the one that matters. Carbon dioxide has always varied naturally — the ice cores prove it, and they show it moving in step with the ice ages. What they also show is that the present level is well outside the whole 800,000-year range, and that it arrived in two centuries rather than in ten thousand years.
Evidence 3 · the chemical fingerprint
Carbon from fossil fuels has a slightly different mix of carbon isotopes — atoms of carbon that weigh slightly different amounts — from carbon released by volcanoes or by the oceans. The extra carbon dioxide in the air carries the fossil fingerprint.
On its own, this establishes where the extra carbon dioxide came from.
It identifies the source, which is the one thing the Hawaii graph could not do. Plants take up the lighter isotope slightly more readily, so carbon that was once plant material — coal, oil and gas — is measurably lighter. The extra carbon dioxide in the air matches that signature, which rules out volcanoes and the oceans as the source.
Evidence 4 · the laboratory
Shine infrared through a tube of carbon dioxide and measure what comes out of the other end. The gas absorbs strongly at particular wavelengths. This was first measured in the 1850s.
On its own, this establishes why carbon dioxide should warm anything at all.
It supplies the mechanism, which the other three lack. Eunice Foote and John Tyndall showed independently in the 1850s that carbon dioxide absorbs heat radiation — long before anyone was arguing about climate. Without this, the match between rising carbon dioxide and rising temperature would be suggestive; with it, there is a physical reason for it.
No single measurement proves it. Four independent ones agreeing is a different matter.
A rising curve on its own could be natural. Ice cores on their own only tell you about the past. The fingerprint on its own says where the carbon came from and not what it does. Measuring what carbon dioxide absorbs is a fact about a gas in a tube.
Put together they make one account with nothing left over: the gas absorbs infrared, the amount in the air is rising, the extra carbon is fossil in origin, and the present level is beyond anything in 800,000 years. And one of them is doing a different job from the other three — the laboratory is the only one that gives a reason, and without a reason the other three would be three things happening at the same time.
That much is established. How much warmer it gets, how fast, and where it lands hardest are not measurements — they depend on how much more is burned, and that has not been decided yet.
Key fact
The greenhouse effect is natural and necessary: without it the surface would average about −18 °C instead of about 15 °C. What human activity has changed is its strength, by raising carbon dioxide from about 280 to over 420 parts per million since 1750.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“The planet is warming because of the hole in the ozone layer letting extra heat in.”
Both are real problems with the atmosphere. Commit before you read on.
Two entirely separate problems, and mixing them up is the most common error in this topic. The ozone layer is high in the atmosphere and blocks ultraviolet radiation; it was thinned by CFCs from aerosols and fridges, the risk was skin cancer, and it is recovering because those chemicals were banned by treaty in 1987.
The greenhouse effect happens lower down, involves carbon dioxide and methane rather than CFCs, and absorbs infrared on the way out rather than admitting ultraviolet on the way in. Different gases, different height, different radiation, different consequence — and largely different solutions. The ozone problem is the one humanity actually fixed, which is why it is worth keeping the two apart.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which of these is the correct description of the greenhouse effect?
Rung 2 · The one that catches people
Is the greenhouse effect a bad thing?
Rung 3 · Explain
Explain how the greenhouse effect keeps the Earth warm, and why increasing carbon dioxide raises the temperature further. Be precise about what the gases do and do not absorb.
Rung 4 · Take it somewhere new
Someone argues that carbon dioxide levels have gone up and down naturally for hundreds of thousands of years, so the current rise is nothing unusual. Using the evidence in this lesson, write a reply that takes the argument seriously.
Key note
Greenhouse gases such as carbon dioxide and methane let sunlight through but absorb the infrared radiation the Earth emits, keeping the surface about 33 °C warmer than it would otherwise be. That effect is natural and the planet would be frozen without it; what has changed is its strength. Burning fossil fuels has raised carbon dioxide from about 280 to over 420 parts per million since 1750, and several independent lines of evidence — direct measurements, ice cores, the chemical fingerprint of the extra carbon, and laboratory work on the gas itself — agree on the cause.
Going further
Ice cores are the best archive on the planet. Snow falling on Antarctica traps air between its flakes; the snow is buried, compressed to ice, and the trapped air becomes a sealed bubble. Drill down and each bubble is a sample of the atmosphere from the year it was buried. Near the top the years can be counted like tree rings; deeper down the ice has been squeezed too thin to count and the dating comes from models of how ice flows. The deepest cores reach back 800,000 years, and across all of that time carbon dioxide moved between roughly 180 and 300 parts per million. It has never, in that entire record, been where it is now.
Some of the consequences are slower than people expect and some are faster. Sea level rises partly because ice on land melts and partly because water expands as it warms — and the ocean is deep, so it takes centuries to warm through. Carbon dioxide already released stays in the atmosphere for a very long time. Methane is a far stronger greenhouse gas per molecule but breaks down within about a decade, which makes it the fastest thing to act on and the reason livestock, landfill and gas leaks get attention out of proportion to their tonnage.
Before this lesson
Connects to
At GCSE this becomes
- Radiative balance and infrared absorption, evaluating climate models and their uncertainties, and carbon footprints of products and processes.
Where to next
Ask Mr Badmus AI
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