IB Physics SLTopic B.2 — Climate & the Greenhouse EffectPaper 1 & 2Resonance Model · Enhanced Effect~7 min read
The Greenhouse Effect
You now know which gases do the absorbing and roughly where. This page joins the dots: how a molecule actually captures infrared radiation, and why pumping more greenhouse gases into the atmosphere doesn’t just add a little warmth — it changes the whole energy balance.
📘 What you need to know
Incoming UV and visible radiation passes through the atmosphere largely unabsorbed and warms Earth’s surface directly.
The surface then re-radiates that energy as long-wave infrared radiation, mostly at night.
Greenhouse gases absorb some of this outgoing infrared and re-emit it in all directions — including back down towards the surface — which is why heat energy accumulates rather than escaping cleanly to space.
This happens because greenhouse gas molecules have a natural vibrational frequency that falls in the infrared range, so absorbing infrared light makes them resonate and heat up.
The enhanced greenhouse effect is what happens when human activity raises greenhouse gas concentrations beyond their natural levels, trapping even more outgoing radiation and pushing average global temperatures upward.
The Resonance Model of Warming
Picture the energy flow as a nightly cycle. During the day, ultraviolet and visible radiation from the Sun streams through the atmosphere almost untouched and is absorbed by the ground, oceans and vegetation. At night, that stored energy leaves the surface again as infrared radiation. Some of this infrared escapes straight through the atmosphere into space — but greenhouse gas molecules intercept the rest.
Because their natural frequency sits in the infrared region, these molecules don’t just absorb that energy passively — they begin to resonate, vibrating more vigorously and heating up. A hot, vibrating molecule doesn’t hold onto that energy forever: it re-emits infrared radiation of its own, scattered in every direction, including straight back down towards the surface it came from.
The more greenhouse gas there is in the atmosphere, the more of this returning radiation gets sent back down — so heat energy builds up in the surface–atmosphere system instead of draining away into space.
Why Infrared and Not Ultraviolet?
The reason greenhouse gases target infrared specifically comes down to molecular energy levels. High-frequency ultraviolet light carries enough energy to actually break chemical bonds within a molecule. Infrared light is far gentler — it doesn’t have enough energy to break bonds, but it does have exactly the right energy to set atoms within a molecule vibrating. Since a greenhouse gas’s natural vibrational frequency matches incoming infrared, that’s the wavelength range it absorbs and re-emits most efficiently.
A thicker layer of greenhouse gases absorbs and re-emits more outgoing infrared radiation back toward the surface, so less escapes to space and the surface warms further.
The Enhanced Greenhouse Effect
The greenhouse effect itself is not a problem — it’s the reason Earth is warm enough to support life at all. The concern is the enhanced greenhouse effect: human activity has been steadily increasing the concentration of greenhouse gases in the atmosphere, most notably carbon dioxide, which has climbed from around 280 parts per million before industrialisation to over 420 parts per million today. With more greenhouse gas molecules available to absorb and re-emit outgoing infrared radiation, less long-wave heat escapes the atmosphere overall, and average global temperatures have already risen by more than 1°C since pre-industrial times.
Quick recap: greenhouse gases absorb outgoing infrared because their natural frequency matches it, then re-emit it in all directions via resonance. More greenhouse gas concentration means more radiation sent back to the surface — that’s the enhanced greenhouse effect.
WE 1
Explain, in terms of molecular behaviour, why greenhouse gases absorb infrared radiation but not ultraviolet radiation.
Ultraviolet
UV photons carry enough energy to break chemical bonds within a molecule, rather than being absorbed and re-emitted.
Infrared
IR photons don’t carry enough energy to break bonds, but they do match the natural vibrational frequency of greenhouse gas molecules.
Matching frequency → resonance → absorption and re-emission of IRThis is why greenhouse gases are selective: they respond strongly to infrared, not ultraviolet.
WE 2
Atmospheric CO₂ concentration has risen from about 280 ppm before industrialisation to around 420 ppm today. Calculate the percentage increase.
Step 1 — Find the changechange = 420 − 280 = 140 ppmStep 2 — Express as a percentage of the original% increase = (140 ÷ 280) × 100= 50%A 50% rise in CO₂ concentration is a major driver of the enhanced greenhouse effect.
💡 Top tips
In “explain” answers, mention both parts of the mechanism: resonance (why the gas absorbs infrared) and re-emission in all directions (why some radiation heads back to the surface).
Always tie the word “enhanced” to human activity increasing greenhouse gas concentration — that’s the specific link examiners want, not just “more heat”.
Keep the natural greenhouse effect and the enhanced greenhouse effect clearly separate in your answer — one keeps Earth habitable, the other is the current warming concern.
⚠ Common mistakes
Writing that the greenhouse effect is inherently harmful — the natural greenhouse effect is essential; it’s the enhanced version that’s the problem.
Describing molecules as “reflecting” infrared radiation instead of absorbing it and re-emitting it via resonance.
Muddling UV bond-breaking with IR-induced vibration — they’re different mechanisms operating at different wavelengths.
Up next: Energy Balance Problems — where we bring albedo, emissivity, the solar constant and the greenhouse effect together into full climate models and calculate exactly how much surface temperatures shift.
Want this to actually stick before the exam?
Book a free session and we’ll work through the greenhouse effect and energy-balance models until they’re second nature.
