IB Physics HLClimate & the Greenhouse EffectPaper 1 & 2Global Warming~10 min read
The Greenhouse Effect
You’ve met the greenhouse gases — now let’s watch them do their job. The greenhouse effect is how a thin blanket of gas keeps Earth warm and cosy instead of frozen solid. It’s a natural, helpful process. The problem starts when we pile on extra gas and the blanket gets too thick. That’s the enhanced greenhouse effect, and it’s what drives global warming.
📘 What you need to know
Sunlight arrives mostly as short-wave radiation (visible + UV); the atmosphere lets the visible light through to warm the ground
The warm Earth re-radiates energy as long-wave infrared
Greenhouse gases absorb this infrared and send some back down, so heat builds up and Earth stays habitable
They work because their molecules have a natural frequency in the infrared — infrared makes them resonate and heat up
UV, being higher frequency, is energetic enough to break bonds; infrared only makes molecules vibrate
The enhanced greenhouse effect = humans adding extra greenhouse gases, so less heat escapes
CO2 has risen by more than 100 ppm to about 420 ppm (2020); average temperatures are up over 1 °C since pre-industrial times
How the greenhouse effect works
Follow the energy on its journey. Sunlight comes in mostly as visible light and ultraviolet. The atmosphere is nearly see-through to visible light, so it passes straight down and is soaked up by the ground — warming the surface.
A warm surface can’t hold onto that energy, so it re-radiates it. But Earth is far cooler than the Sun, so it glows at much longer wavelengths: infrared. Now the greenhouse gases go to work. They absorb this outgoing infrared and re-emit it in all directions — so a good share is sent back down to the surface. Some still escapes to space, but the rest stays trapped in the surface–atmosphere system. The more greenhouse gas there is, the more infrared gets held back, and the warmer Earth becomes.
The one-line version: Short-wave sunlight comes in and warms the ground; long-wave infrared tries to leave; greenhouse gases catch it and send some back — so heat piles up.
Why is sunlight “short-wave” but Earth’s glow “long-wave”? There’s a tidy rule — Wien’s law — that says the hotter an object is, the shorter the wavelength it peaks at. Let’s use it to see the difference.
Wien’s displacement lawλmax = (2.90 × 10−3) ÷ T
WE 1
The Sun’s surface is about 5800 K and Earth’s surface is about 288 K. Find the peak wavelength each one radiates at, and say which part of the spectrum that is. (Wien’s constant = 2.90 × 10−3 m K.)
Step 1 — the Sun (5800 K)λ = (2.90×10⁻³) ÷ 5800λ ≈ 500 nm (visible)Step 2 — the Earth (288 K)λ = (2.90×10⁻³) ÷ 288λ ≈ 10 μm (infrared)The hot Sun glows in visible light (short-wave); the cool Earth glows in infrared (long-wave). That’s exactly why the gases let sunlight in but trap Earth’s outgoing heat.
Why greenhouse gases in particular?
Not every gas traps heat — it comes down to how a molecule responds to different radiation. Think of pushing a child on a swing: push at the right rhythm and they swing higher; push at the wrong rhythm and nothing much happens.
Ultraviolet is high-frequency and very energetic. It carries enough punch to break the bonds inside a molecule.
Infrared is lower-frequency. It doesn’t break anything — instead it makes the atoms vibrate.
Greenhouse-gas molecules have a natural frequency that sits in the infrared. So when infrared hits them, it’s the “right rhythm” — they resonate, wobble harder, and heat up. Then they re-emit that infrared back towards Earth.
UV is high-frequency and energetic, so it can snap molecular bonds. Infrared has a lower frequency that matches a greenhouse-gas molecule’s natural rhythm — so the molecule resonates, vibrates harder, and heats up instead.
The enhanced greenhouse effect
Here’s the key idea to keep straight: the natural greenhouse effect is a good thing — without it Earth would be far too cold. The trouble is the enhanced greenhouse effect, where human activity pumps extra greenhouse gases into the air. A thicker gas blanket lets even less heat escape, so the planet warms up.
The pink band is the greenhouse-gas layer — thin on the left, thicker on the right. Orange = incoming sunlight, green = heat escaping to space, red = heat trapped and sent back. With a thicker, human-boosted layer, fewer green arrows escape and more red arrows stay — so the planet heats up.
The numbers back this up. Carbon dioxide has climbed by more than 100 ppm to reach about 420 ppm (2020), and average global temperatures have risen over 1 °C since pre-industrial times.
WE 2
Carbon dioxide has risen by about 100 ppm to reach 420 ppm. Estimate the percentage increase compared with its earlier level.
Step 1 — find the earlier level
earlier = 420 − 100 = 320 ppm
Step 2 — percentage increase = rise ÷ earlier × 100= (100 ÷ 320) × 100≈ 31%A roughly one-third jump in CO2 — a big change for a gas that’s such a strong infrared absorber.
Most of this extra gas comes from things people do. You don’t need to memorise every source, but it helps to see the pattern:
Greenhouse gas
Main human sources
Formula
Carbon dioxide
Burning fossil fuels, burning wood, deforestation
CO2
Methane
Decaying organic matter — landfill, manure, crops
CH4
Nitrous oxide
Artificial fertilisers, burning fossil fuels
N2O
Exam-favourite catch: the enhanced greenhouse effect increases the global average temperature and is caused by human activity. If a multiple-choice option says “decreasing” or “natural causes”, it’s wrong. Increase + human activity — that’s the pair to remember.
Sunlight in short-wave
warms surface
Earth emits infrared
gases absorb & re-emit down
Heat accumulates
extra gas = less escapes
Warming
🛠️ Explaining the greenhouse effect in an answer
In: short-wave sunlight passes through the atmosphere and warms the surface.
Out: the warm Earth re-radiates energy as long-wave infrared.
Trap: greenhouse gases absorb the infrared and re-emit some of it back down.
Why them: their natural frequency is in the infrared, so they resonate and heat up.
Enhanced: more greenhouse gas → less heat escapes → temperature rises (human-caused).
💡 Top tips
Natural = good, enhanced = the problem. Don’t muddle the two.
Short-wave in, long-wave out — sunlight enters, infrared is what gets trapped.
Resonance is the reason: infrared matches the molecule’s natural frequency.
UV breaks bonds; infrared just makes molecules vibrate — a classic compare-and-contrast.
Enhanced effect = increase + human activity — the exam’s favourite answer.
⚠ Common mistakes
Saying greenhouse gases stop sunlight coming in — they trap the infrared going out
Calling the greenhouse effect “bad” — the natural one keeps Earth habitable; only the enhanced one is the issue
Saying the enhanced effect cools the planet, or that it’s from natural causes — it’s warming, from human activity
Claiming UV is what greenhouse gases absorb — it’s infrared (UV would break bonds, not resonate)
Mixing up short-wave (incoming) and long-wave (outgoing) radiation
Quick recap: Short-wave sunlight warms the ground; Earth re-radiates long-wave infrared; greenhouse gases resonate with that infrared, absorb it, and send some back — keeping Earth warm. Adding extra gas (the enhanced effect) traps more heat, so global temperatures rise. It’s an increase, and it’s human-caused.
You can now tell the whole story: sunlight in, infrared out, gases trapping the difference — and how humans have tipped that balance. Next we make it quantitative in Energy Balance Problems, where we treat Earth and its atmosphere as bodies swapping radiation, and actually calculate how the temperature settles (and shifts).
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