IB Physics HLClimate & the Greenhouse EffectPaper 1 & 2Infrared Absorption~10 min read
Greenhouse Gases
Sunlight streams in, warms the ground, and the Earth sends that energy back out as invisible infrared. Most of it never makes it to space — a small club of gases soaks it up on the way out. Meet the greenhouse gases: water vapour, carbon dioxide, methane and nitrous oxide. This page covers where they come from, why the atmosphere waves sunlight through but grabs infrared, and exactly which wavelengths each gas absorbs.
📘 What you need to know
The four main greenhouse gases, in decreasing order of contribution: water vapour (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O)
Every greenhouse gas has both natural and human-made origins
A greenhouse gas absorbs the long-wave radiation re-emitted by the Earth’s surface, trapping it in the atmosphere so it isn’t lost to space
Only about 25% of incoming (short-wave) solar radiation is absorbed by the atmosphere — but about 80% of the outgoing (long-wave) radiation is
Carbon dioxide and water vapour have the biggest impact; ozone, methane and nitrous oxides play smaller roles
A gas’s significance depends on its concentration and on how strongly it absorbs specific wavelengths
The main greenhouse gases
Four gases do most of the greenhouse work. Listed from biggest contributor to smallest, they are water vapour, carbon dioxide, methane and nitrous oxide. Each one turns up in the atmosphere completely naturally — no humans required:
Greenhouse gas
Natural sources
Formula
Water vapour
Evaporation from the oceans, seas and plants
H2O
Carbon dioxide
Volcanic eruptions, wildfires and respiration
CO2
Methane
Decomposition in oceans and soils — and famously, termites
CH4
Nitrous oxide
Soils and the oceans
N2O
The order to memorise: H2O > CO2 > CH4 > N2O. Bar heights are schematic — it’s the ranking that matters, not exact shares.
Human activity adds extra amounts of each of these too — the specific human sources get their own moment when we build the enhanced greenhouse effect on the next page. For now, the exam-ready fact is simply that every greenhouse gas has both natural and man-made origins.
Two freebies here. First: most students assume carbon dioxide is the number one greenhouse gas — it isn’t. Water vapour tops the table, and CO2 comes second. Second: you do not need to memorise the specific sources of each gas. All the examiner wants is that each one has both natural and human-made origins.
WE 1
Which option lists the main greenhouse gases in order of decreasing contribution to the greenhouse effect?
A. carbon dioxide → water vapour → methane → nitrous oxide
B. water vapour → carbon dioxide → methane → nitrous oxide
C. water vapour → methane → carbon dioxide → nitrous oxide
D. carbon dioxide → methane → nitrous oxide → water vapour
Step 1 — recall the ranking
water vapour → carbon dioxide → methane → nitrous oxide
Step 2 — match it to the optionsAnswer: BWater vapour on top — the one everyone forgets. A and D are the classic traps.
What makes a gas a “greenhouse” gas?
When radiation from the Sun hits the Earth, the surface warms up and then re-radiates that energy back out as long-wave radiation. A greenhouse gas is any gas that absorbs this re-radiated radiation, keeping the energy in the atmosphere instead of letting it escape to space.
Greenhouse gas — definition
A gas that absorbs the long-wave radiation re-emitted by the Earth’s surface, trapping it in the Earth’s atmosphere so it is not lost to space
The name is no accident. These gases do the same job as the glass in a garden greenhouse: sunlight streams in easily, but the warmth struggles to get back out.
Within the family, carbon dioxide and water vapour have the most significant impact on the greenhouse effect. Oxygen and ozone (O2 and O3), methane and nitrous oxides also absorb, but their overall effect is smaller.
Short-wave in, long-wave out
Here’s the asymmetry that powers the whole effect. Incoming solar radiation is mostly short wavelength (ultraviolet and visible light), and the atmosphere is nearly transparent to it — only around 25% is absorbed on the way in. Incoming UV, for example, is grabbed by ozone. But the radiation the Earth re-emits is long wavelength (infrared), and the greenhouse gases absorb around 80% of it on the way back out.
The atmosphere is picky: it lets most short-wave sunlight through (1 in 4 arrows absorbed) but traps most of the outgoing infrared (4 in 5 arrows absorbed). Schematic — reflection (albedo) is left out so we can focus on absorption.
That trapped 80% is what keeps the Earth at a habitable temperature. But it also makes the system sensitive: if the chemical make-up of the atmosphere changes, the balance shifts with it, and the Earth’s mean surface temperature fluctuates.
Short-wave in UV + visible
only ~25% absorbed
Earth’s surface absorbs & warms
re-emitted as long-wave IR
Greenhouse gases absorb ~80%
WE 2
The Earth’s surface re-emits long-wave radiation with an average intensity of 350 W m−2. Greenhouse gases absorb 80% of this outgoing radiation. Calculate the intensity absorbed by the atmosphere and the intensity that escapes to space.
Step 1 — absorbed intensity is 80% of the outgoing radiationabsorbed = 0.80 × 350 = 280 W m⁻²Step 2 — whatever isn’t absorbed escapesescaping = 350 − 28070 W m⁻² escapes to spaceCompare the way in: the atmosphere barely touches incoming sunlight (~25%) but swallows most of the outgoing infrared. That lopsidedness is the greenhouse effect in numbers.
Which gas absorbs which wavelengths?
Not every greenhouse gas grabs the same radiation — each one has its own absorption bands, the stretches of wavelength it can soak up. Here’s the map:
Each gas has its own absorption bands. Notice the yellow column: the atmosphere is mostly transparent to visible light, which is why sunshine reaches the ground at all.
Ozone (O3)
Ozone absorbs close to 100% of the Sun’s incoming ultraviolet rays. It also strongly absorbs outgoing infrared in a narrow band between 9 μm and 10 μm. Despite that talent, it is not a significant contributor to the greenhouse effect — there simply isn’t much of it, as it’s found in much smaller concentrations than the other gases.
You’ve probably heard of the “hole in the ozone layer”. Park it — that’s a different story entirely. Ozone’s headline job is soaking up harmful incoming UV, so ozone depletion is about losing our sunscreen, not about the greenhouse effect. Keep the two separate in exam answers. (Bonus fact: ozone depletion has improved massively thanks to emission controls — humans can get it right sometimes!)
Carbon dioxide (CO2)
Carbon dioxide is a good absorber of infrared radiation with wavelengths between 1.5 and 30 μm, and it absorbs especially strongly at 15 μm. Its concentration in the atmosphere is increasing, which makes it one of the most significant contributors to the greenhouse effect.
Water vapour (H2O)
Water vapour is the best absorber of infrared radiation of the lot, with a huge band stretching from 0.8 to 35 μm. And here’s a twist worth noticing: the warmer the air becomes, the more water vapour it holds — so its concentration rises as temperatures rise.
The atmosphere as a whole
Add up all the gases and the picture is clear: most of the ultraviolet, infrared and microwave radiation is absorbed by the atmosphere. But the atmosphere is mostly transparent to visible light — the gases neither absorb nor emit much of it. That transparent window is why sunlight makes it down to warm the surface in the first place.
So what decides how much a particular gas actually matters? Two things working together: how much of it there is, and how well it absorbs.
What decides a gas’s impact
relative significance of a gas = its concentration in the atmosphere + how strongly it absorbs specific wavelengths
Ozone absorbs brilliantly but is too dilute to matter much. Carbon dioxide absorbs well and keeps getting more plentiful — that combination is exactly what makes it such a big player.
WE 3
Infrared radiation of wavelength 15 μm leaves the Earth’s surface. (a) Identify the greenhouse gas that absorbs this wavelength most strongly. (b) Explain why this gas is one of the most significant contributors to the greenhouse effect.
(a) Match the wavelength to a band
15 μm sits inside the 1.5–30 μm band — and it’s the very wavelength CO₂ absorbs most strongly
carbon dioxide(b) Significance = concentration + absorption
CO₂ absorbs strongly across 1.5–30 μm, and its concentration in the atmosphere is increasing — so it traps a growing share of the outgoing infrared
Name both halves of the argument: strong absorption AND rising concentration. One without the other only gets half the marks.
🛠️ Tackling greenhouse-gas questions
The big four, in order: H2O → CO2 → CH4 → N2O (decreasing contribution).
Origins? Every greenhouse gas has both natural and human-made sources.
Direction matters: short-wave in (~25% absorbed), long-wave out (~80% absorbed).
Match gas to band: O3 → UV plus 9–10 μm; CO2 → 1.5–30 μm (strongest at 15 μm); H2O → 0.8–35 μm.
Judging significance? Concentration plus absorption strength — always argue both.
💡 Top tips
Water vapour is the biggest contributor, not CO2 — a favourite multiple-choice trap.
“The atmosphere is transparent to visible light but absorbs infrared” — this one sentence unlocks most explain questions in this topic.
Quote the pair of numbers: ~25% absorbed on the way in, ~80% on the way out.
Ozone = UV protection, a separate story from the greenhouse effect — don’t mix the two in answers.
“Long-wave” and “infrared” mean the same radiation here — the stuff Earth re-emits.
Quick recap: The main greenhouse gases — water vapour, carbon dioxide, methane and nitrous oxide, in decreasing order — absorb the long-wave infrared the Earth re-emits. The atmosphere absorbs only ~25% of incoming short-wave sunlight but ~80% of the outgoing infrared, which keeps Earth habitable. A gas’s importance comes from its concentration plus how strongly it absorbs its particular wavelengths.
⚠ Common mistakes
Ranking CO2 above water vapour as the top contributor
Dragging the ozone hole into greenhouse-effect answers — they’re separate issues
Claiming the atmosphere absorbs most incoming sunlight (it’s mostly transparent to visible light)
Saying greenhouse gases block radiation from the Sun — they trap the long-wave radiation leaving Earth
Forgetting the natural sources — every greenhouse gas has both natural and man-made origins
That’s the cast list sorted: four main gases, natural origins, and each with its own slice of the infrared spectrum. But why does CO2 grab 15 μm specifically? Because molecules have natural frequencies they love to vibrate at — and that resonance story is exactly where we go next. In The Greenhouse Effect we build the full mechanism, from resonating molecules to the enhanced warming humans are adding, before crunching complete energy-balance problems.
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