IB Physics SLTopic B.2 — Climate & the Greenhouse EffectPaper 1 & 2Greenhouse Gases · Absorption Spectra~6 min read
Greenhouse Gases
Not every gas in the atmosphere interacts with radiation the same way. A handful of them — the greenhouse gases — are unusually good at absorbing the infrared radiation Earth radiates back out, and that one property is what keeps our planet habitable.
📘 What you need to know
The four main greenhouse gases are water vapour, carbon dioxide, methane and nitrous oxide — each has both natural and human-made sources.
A greenhouse gas absorbs the long-wave radiation Earth re-emits and re-radiates it in all directions, rather than letting it escape straight to space.
Water vapour and carbon dioxide have by far the biggest impact on the greenhouse effect; ozone, methane and nitrous oxide contribute far less.
Only about 25% of incoming (mostly short-wave) solar radiation is absorbed by the atmosphere on its way in, but roughly 80% of outgoing (long-wave) radiation is absorbed on its way back out — that asymmetry is the whole reason the atmosphere warms up.
Each gas only absorbs strongly at specific wavelength bands, set by the structure of its molecules.
The Main Greenhouse Gases
Ranked roughly by how much they contribute to the greenhouse effect:
🌫️ The line-up
Water vapour (H₂O) — enters the atmosphere through evaporation from oceans, seas and plants. The most significant greenhouse gas by contribution.
Carbon dioxide (CO₂) — released by volcanic eruptions, wildfires and respiration, alongside large-scale human emissions. Its rising atmospheric concentration is what makes it such a major driver of warming.
Methane (CH₄) — produced as oceans and soils decompose organic matter, with termites among the natural sources too.
Nitrous oxide (N₂O) — emitted naturally from soils and oceans.
You don’t need to memorise every source in detail — the key idea examiners look for is simply that each greenhouse gas has both a natural origin and a human-made one.
Why These Gases Trap Heat
When solar radiation strikes Earth, most of the visible and ultraviolet light passes straight through the atmosphere and warms the surface. That surface then re-radiates energy as long-wave infrared radiation. A greenhouse gas is defined by what happens next: it absorbs that outgoing infrared radiation and prevents it from escaping straight to space, in much the same way glass traps heat inside a greenhouse — which is exactly where the name comes from.
The imbalance is striking: only around a quarter of incoming solar radiation is absorbed by the atmosphere on the way in, but around four-fifths of the outgoing infrared radiation is absorbed on the way back out. That’s the mechanism that keeps Earth’s surface warmer than it would otherwise be.
Absorption Windows — Which Gas Absorbs What
Every molecule only absorbs radiation at wavelengths that match its own internal structure, so different greenhouse gases specialise in different parts of the spectrum:
Ozone (O₃) absorbs close to all of the Sun’s incoming ultraviolet radiation, plus a narrow band of outgoing infrared. Despite this, it’s a minor greenhouse contributor overall — it exists in far lower concentrations in the atmosphere than CO₂ or water vapour.
Carbon dioxide (CO₂) absorbs infrared strongly across a wide range, with an especially strong band around 15 μm. Its ever-increasing atmospheric concentration is exactly why it’s treated as such a significant contributor.
Water vapour (H₂O) is the single best absorber of infrared radiation of the main greenhouse gases, covering an even broader range of wavelengths — and its atmospheric concentration rises as the air itself warms.
Schematic, not to scale. Incoming solar radiation peaks in the visible; Earth’s outgoing radiation peaks in the infrared. Ozone, carbon dioxide and water vapour each absorb strongly in different parts of that outgoing band.
Quick recap: greenhouse gases absorb and re-emit outgoing infrared radiation — they don’t reflect it. Water vapour and CO₂ dominate the effect; ozone, methane and nitrous oxide matter far less, mainly because they’re present in much smaller concentrations.
WE 1
Ozone absorbs almost 100% of incoming ultraviolet radiation, yet it is not considered a major contributor to the greenhouse effect. Explain why.
Key idea
Absorption strength alone doesn’t determine a gas’s overall impact.
Reasoning
Ozone is found in much lower concentrations in the atmosphere than carbon dioxide or water vapour.
Low concentration → small overall contribution, despite strong absorptionA gas needs both a strong absorption ability and a significant atmospheric concentration to be a major greenhouse contributor.
WE 2
Earth’s surface emits 390 W m⁻² of long-wave radiation. If the atmosphere absorbs 80% of this radiation before it can escape, calculate the intensity that reaches space directly.
Step 1 — Find the fraction that escapes
If 80% is absorbed, 20% escapes directly.
Step 2 — Calculateintensity escaping = 0.20 × 390= 78 W m⁻²The remaining 312 W m⁻² is absorbed by the atmosphere and re-emitted in all directions, including back toward the surface.
💡 Top tips
Don’t try to memorise exact absorption percentages for each gas — focus on the order of significance: water vapour and CO₂ first, then ozone, methane and nitrous oxide.
In “explain” answers, always say greenhouse gases absorb and re-emit radiation, never that they “reflect” it — examiners specifically penalise “reflect”.
Keep the ozone layer’s UV-shielding role separate from its (much smaller) role in the greenhouse effect — they’re related but distinct ideas.
⚠ Common mistakes
Describing greenhouse gases as “reflecting” heat back to Earth — the correct mechanism is absorption followed by re-emission in all directions.
Assuming the strongest absorber is automatically the biggest contributor — atmospheric concentration matters just as much as absorption strength.
Mixing up short-wave (incoming solar) and long-wave (outgoing terrestrial) radiation — greenhouse gases mainly interact with the long-wave radiation leaving Earth.
Up next: The Greenhouse Effect — where we put these gases to work and build the resonance model that explains exactly how they warm the atmosphere.
Want this to actually stick before the exam?
Book a free session and we’ll work through greenhouse gases and energy-balance problems until they’re second nature.
