IB Chemistry HLTopic 4 — Energy from FuelsPaper 1 & 2Core idea~11 min read
CO2 Levels and the Greenhouse Effect
The greenhouse effect is not the problem — without it the Earth would be frozen solid. The problem is that we have been thickening the blanket. This page covers the measurements, the mechanism, and the molecular reason why carbon dioxide traps heat while the nitrogen making up most of the air does nothing at all.
📘 What you need to know
Atmospheric CO2 is rising because of combustion of fossil fuels, plus deforestation, cement production and livestock farming.
Continuous measurements have been made at observatories such as Mauna Loa since 1958; methane records begin in 1984.
The graph shows a rising trend with an annual wobble caused by seasonal changes in the rate of photosynthesis.
The main greenhouse gases are CO2, CH4, N2O and water vapour.
Mechanism: the Sun’s shortwave radiation reaches the surface, is absorbed, and is re-emitted as longwave infrared. Greenhouse gases absorb that infrared and re-radiate it in all directions, so some returns to the surface.
A gas is a greenhouse gas only if a molecular vibration changes its dipole moment. This is why N2 and O2 are not.
The natural greenhouse effect keeps Earth habitable. Adding more greenhouse gas gives the enhanced greenhouse effect, which causes global warming.
Where the extra carbon dioxide comes from
The dominant source is straightforward: burning fossil fuels releases carbon that had been locked underground for hundreds of millions of years. Electricity generation and transport account for most of it. But three other human activities matter:
Deforestation — clearing land removes trees that were absorbing CO2, and burning the cleared material releases more.
Cement production — making cement thermally decomposes limestone, and that reaction releases CO2 whether or not any fuel is burnt to drive it.
Livestock farming — cattle release large amounts of methane, a much more powerful greenhouse gas per molecule than CO2.
Notice that cement is a double hit: CaCO3 → CaO + CO2 is endothermic, so you burn fuel to drive it and the reaction itself emits carbon dioxide.
The measurements
Since 1958, observatories have taken continuous quantitative readings of atmospheric gases. Mauna Loa in Hawaii is the most famous, chosen because it sits far from cities and industry, so the air it samples is well mixed and representative.
The annual wobble is the whole northern hemisphere breathing. In spring and summer, plants photosynthesise faster than they respire, pulling CO2 out of the air; in autumn and winter it goes back.
Why the wobble matters in an exam. If asked to explain the annual fluctuation, the answer is seasonal changes in the rate of photosynthesis, because photosynthesis removes CO2 from the atmosphere. If asked about the long-term rise, the answer is combustion of fossil fuels. Two different questions, two different answers.
How the greenhouse effect works
Follow the energy in four steps. Each one is a mark.
🧩 The mechanism, step by step
The Sun emits mainly shortwave radiation, which passes through the atmosphere and reaches the surface.
The surface absorbs it and warms, then re-emits the energy at longer wavelengths — as infrared.
Some infrared escapes to space, but greenhouse gases absorb a proportion of it.
Those excited molecules re-radiate in all directions, so a share comes back down. The surface stays warmer than it otherwise would.
The natural version of this keeps the Earth’s average temperature around 15 °C instead of roughly −18 °C. It is the enhancement of the effect that is the problem, not the effect itself.
Why nitrogen and oxygen do nothing
Nitrogen and oxygen make up about 99% of the atmosphere, yet neither contributes any warming. Carbon dioxide is 0.04% and contributes a great deal. That looks strange until you look at how infrared absorption actually works.
Infrared radiation is absorbed when it makes a molecule vibrate — stretch or bend. But a molecule can only absorb the photon if that vibration changes its dipole moment, because it is the oscillating dipole that couples to the oscillating electric field of the radiation.
N2 and O2 are diatomic molecules made of two identical atoms. Stretching them does not create any separation of charge, so there is no dipole change and no absorption.
CO2, H2O and CH4 contain bonds between different atoms with different electronegativities. When they bend or stretch asymmetrically, the dipole moment changes, and infrared is absorbed strongly.
Carbon dioxide is symmetrical overall, so it has no permanent dipole — but its bending and asymmetric stretching vibrations create a temporary one, and that is enough.
Greenhouse gas
Formula
Main sources
Carbon dioxide
CO2
Combustion of fossil fuels and wood, respiration, cement production, decomposition of carbonate rocks
Methane
CH4
Livestock, landfill sites, rice paddies, coal mining, natural gas leaks
Nitrous oxide and other nitrogen oxides
N2O and NOx
Fertilisers, high-temperature combustion in engines, impurities in fossil fuels
Water vapour
H2O
Evaporation from oceans, combustion of all hydrocarbon fuels
The enhanced greenhouse effect
Human activity raises the concentration of greenhouse gases. More infrared is absorbed and re-radiated, less escapes to space, and the Earth’s average surface temperature rises. That is the enhanced greenhouse effect, and the resulting warming is what drives climate change: rising sea levels from thermal expansion and melting ice, shifting rainfall patterns and more frequent extreme weather.
WORKED EXAMPLE
Atmospheric CO2 was about 315 ppm in 1958 and about 425 ppm in 2025. Calculate the percentage increase, and express the 2025 value as a percentage of the atmosphere.
Step 1: Find the increase425 − 315 = 110 ppmStep 2: Divide by the original value(110 ÷ 315) × 100 = 34.9%Step 3: ppm means parts per million425 ÷ 1 000 000 × 100 = 0.0425%a 34.9% rise, to 0.0425% of the atmospherea tiny fraction of the air doing a very large amount of work
WORKED EXAMPLE
The graph shows CO2 currently rising at about 2.5 ppm per year. (a) Estimate the year the concentration would reach 500 ppm if this rate continued. (b) Explain why the estimate is likely to be too late rather than too early.
(a) How far to go, at what rate?500 − 425 = 75 ppm75 ÷ 2.5 = 30 yearsaround the year 2055(b) Look at the shape of the curvethe gradient has increased every decade, so the rate is unlikely to stay at 2.5an extrapolation assuming a constant rate will overestimate the time taken
WORKED EXAMPLE
Explain why methane is a greenhouse gas but oxygen is not, even though oxygen is far more abundant.
Step 1: What absorption requiresinfrared is absorbed only if a molecular vibration changes the dipole momentStep 2: OxygenO₂ has two identical atoms, so stretching produces no charge separationno dipole change → no absorptionStep 3: MethaneC−H bonds are polar, and bending vibrations change the overall dipoleCH₄ absorbs and re-radiates infrared; O₂ cannot, whatever its abundance
💡 Exam tip
Use the words shortwave for incoming and longwave infrared for outgoing. Saying “heat” for both loses the distinction the marks depend on.
Say greenhouse gases re-radiate in all directions, not that they “reflect” or “trap” radiation. Reflection is the wrong mechanism.
Distinguish the natural greenhouse effect (essential for life) from the enhanced one (caused by human activity).
For the annual wobble, the answer is seasonal variation in photosynthesis. For the long-term rise, it is fossil fuel combustion.
If asked why CO2 absorbs infrared, mention bond vibrations that change the dipole moment.
Quote data from a graph with units and read values off carefully — these are easy accuracy marks.
⚠ Common mix-up
Confusing the greenhouse effect with the hole in the ozone layer. They are unrelated: different gases, different part of the spectrum, different problem.
Saying greenhouse gases stop radiation entering. Incoming shortwave radiation passes through; it is the outgoing infrared that gets absorbed.
Calling the greenhouse effect a bad thing. Without it the Earth would average roughly −18 °C. It is the enhancement that is the problem.
Assuming abundance decides importance. Nitrogen is 78% of the air and irrelevant here; CO2 is 0.04% and central.
Writing “carbon” when you mean “carbon dioxide”. They are different substances and examiners mark accordingly.
Saying CO2 has a dipole moment. It does not have a permanent one — its vibrations create a temporary one.
Forgetting methane. Per molecule it is a far stronger greenhouse gas than CO2, which is why livestock and gas leaks matter.
Up next: Biofuels — the idea that you can burn a fuel without adding any net carbon to the atmosphere, how close that gets to being true, and the chemistry of making biodiesel and biogas.
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