IB Chemistry SLTopic 4 — Energy from FuelsPaper 1 & 2Core idea~13 min read
CO2 Levels and the Greenhouse Effect
Nitrogen and oxygen make up 99% of the air and do almost nothing to the planet’s temperature. Carbon dioxide is a rounding error by comparison — and it matters enormously. The difference comes down to how the molecules vibrate.
📚 What you need to know
Atmospheric CO2 has been measured continuously at Mauna Loa since 1958, rising from about 315 ppm to over 420 ppm.
The annual wobble is seasonal photosynthesis; the long-term climb is burning fossil fuels and deforestation.
Incoming short-wave radiation passes through the atmosphere; the warmed Earth re-emits it as long-wave infrared.
Greenhouse gases absorb that infrared and re-radiate it in all directions, including back down, warming the surface.
A gas absorbs infrared only if a vibration changes its dipole moment — which is why N2 and O2 do not.
The natural greenhouse effect makes Earth habitable; the enhanced greenhouse effect from human emissions causes global warming.
What is actually rising
One of the most important datasets in science, and one of the simplest: the same instrument on the same mountain, month after month, since 1958.
Two things are happening on that graph at once, and exam questions almost always ask you to separate them.
The saw-tooth is seasonal. Most of the world’s land plants are in the northern hemisphere, so when they grow in spring and summer, photosynthesis pulls CO2 out of the air and the concentration dips. In autumn and winter, respiration and decay put it back.
The upward trend underneath is the part that does not reverse. It comes from carbon that was locked underground for hundreds of millions of years and is now being returned to the atmosphere in a couple of centuries.
Where the extra carbon dioxide comes from:
Combustion of fossil fuels for electricity, heating and transport — by far the largest source
Deforestation, which both releases stored carbon and removes the trees that would have absorbed more
Cement production and other industry
Livestock farming and landfill, which release methane, a different but even more powerful greenhouse gas
WORKED EXAMPLE
Atmospheric CO2 was about 315 ppm in 1958 and is now over 425 ppm. Calculate the percentage increase. Suggest, with a reason, which month of the year the concentration usually peaks.
Percentage increase425 − 315 = 110 ppm(110 ÷ 315) × 100 = 34.9an increase of about 35%The peak monthNorthern hemisphere plants have been dormant all winter, so CO₂ has been accumulating. The peak comes just BEFORE the growing season takes hold — around May — and the minimum around September, after a summer of photosynthesis.
Careful with ppm. 425 ppm is 0.0425% of the atmosphere — a tiny fraction, which is exactly why people find the effect surprising. The concentration is small; the absorption is not.
How the greenhouse effect works
The atmosphere is transparent on the way in and partly opaque on the way out. That asymmetry is the whole effect.
Follow the energy. Radiation arriving from the Sun is short-wave — mostly visible and ultraviolet — and passes almost unhindered through the atmosphere to the surface. The Earth absorbs it, warms, and re-emits energy. But a surface at about 15 °C radiates at much longer wavelengths, in the infrared.
Infrared is precisely what greenhouse gases absorb. They then re-radiate it in all directions, and the portion sent back downwards warms the surface further. The energy is not trapped forever — it is delayed, and a delay is enough to raise the equilibrium temperature.
Get the two wavelengths the right way round. Sunlight in is short-wave; Earth’s emission out is long-wave infrared. Say it that way and the mark is yours; say “heat comes in and cannot get out” and it is not.
This is entirely natural and entirely necessary. Without any greenhouse effect Earth’s average surface temperature would be roughly –18 °C instead of about +15 °C, and the planet would be frozen. What has changed is the concentration of these gases, which increases the fraction of outgoing infrared that gets absorbed. That extra warming is the enhanced greenhouse effect, and it is what “global warming” refers to.
Why only some gases count
Here is the question that separates a good answer from a vague one: the air is 78% nitrogen and 21% oxygen, so why is neither of them a greenhouse gas?
A molecule can only absorb an infrared photon if the vibration it excites changes the molecule’s dipole moment. N2 and O2 are made of two identical atoms, so the bond is perfectly non-polar; stretching it does not create a dipole, and the infrared passes straight through.
CO2 is symmetrical and has no permanent dipole — but bend it, and one appears. That fleeting dipole is enough.
The main greenhouse gases and where they come from:
Gas
Main sources
Why it absorbs infrared
Carbon dioxide, CO2
Combustion of fossil fuels, deforestation, respiration
Bending and asymmetric stretching create a dipole
Methane, CH4
Livestock, landfill, rice paddies, gas leaks
C–H bonds are polar and vibrate asymmetrically
Water vapour, H2O
Evaporation, and a product of all combustion
Bent and permanently polar
Nitrogen oxides, NOx
High-temperature combustion in engines
Polar bonds between different atoms
N2 and O2
Most of the atmosphere
Do not absorb — identical atoms, no dipole change
WORKED EXAMPLE
From the list N2, CO2, O2, CH4, Ar, identify the greenhouse gases and explain your choice.
Step 1 — apply the testDoes any vibration of the molecule change its dipole moment?Step 2 — work throughN₂ ✗ identical atoms, no dipoleO₂ ✗ identical atoms, no dipoleAr ✗ a single atom cannot vibrate at allCO₂ ✓ bending creates a dipoleCH₄ ✓ polar C–H bonds vibrating asymmetricallyCO₂ and CH₄Argon is worth remembering as the clean case: no bonds, no vibrations, no absorption.
Consequences
The enhanced greenhouse effect raises average global temperature, and the knock-on effects are what climate science actually measures: melting land ice and thermal expansion of seawater raising sea levels, shifting rainfall patterns, more frequent extreme weather, ocean acidification as CO2 dissolves, and disruption to agriculture and ecosystems. The chemistry you have just done — carbon in, carbon dioxide out, infrared absorbed — is the whole mechanism in three steps.
💡 Exam tip
Use the words short-wave in, long-wave infrared out. Vague answers about “heat being trapped” score poorly.
Say re-radiated in all directions, not “reflected”. Reflection is the wrong process.
Explaining why N2 is not a greenhouse gas? The mark is for no change in dipole moment, not just “it is non-polar”.
Distinguish the natural greenhouse effect (essential, keeps Earth about 33 °C warmer) from the enhanced one (caused by human emissions).
On a CO2 graph, always account for both the oscillation and the trend. They have different causes.
⚠️ Common mix-up
Confusing the greenhouse effect with the ozone hole. Different gases, different radiation, different problem.
Saying greenhouse gases stop the Sun’s rays getting in. They do the opposite — incoming radiation passes through.
Claiming the greenhouse effect is itself bad. Without it the planet would be uninhabitable; it is the enhancement that is the problem.
Assuming a gas absorbs infrared just because it is a gas. The dipole test decides it.
Reading the seasonal dip as CO2 levels falling. The trend is upward every single year.
Up next: Biofuels — fuels grown rather than dug up, and the honest accounting behind the claim that they are carbon neutral.
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