The greenhouse effect

Leave a car parked in the sun with the windows up, and in twenty minutes it’s an oven inside — far hotter than the air outside. Nothing was added. The same sunlight falls on the pavement and the car. But the car’s glass lets the sunlight in and won’t let the heat back out as easily. So heat builds up, and the inside settles at a new, much warmer temperature. The whole planet runs on a version of that trick. Understanding it is the difference between knowing the climate is changing and knowing why burning fossil fuels does it.

Sunlight comes in as visible light

Start with what arrives. The sun is about 5,500°C at its surface, and very hot things radiate their energy mostly as visible light — the light your eyes evolved to see. That light streams across space, passes straight through the atmosphere, and lands on the ground, the ocean, the rooftops. There it does what light does when it’s absorbed: it warms whatever it hits.

This part is simple and worth holding onto. The incoming energy is visible. The air is nearly transparent to it. On a clear day the sunlight barely notices the atmosphere is there — it sails through and heats the surface below.

The warm ground radiates heat back — but as infrared

Now the second move, and this is the one most people miss.

Everything warm radiates energy. You do; a radiator does; the warmed ground does. But the wavelength of what it radiates depends on how hot it is. The sun, blazing hot, radiates visible light. The Earth’s surface — a balmy 15°C on average, hundreds of degrees cooler than the sun — radiates at a much longer wavelength called infrared: heat radiation you can’t see but can feel as the warmth coming off a road at dusk.

So the energy that arrived as visible light leaves the surface as invisible infrared, heading back up toward space. This connects to something the course already taught: energy is conserved, and it always trickles away as heat radiating outward. The surface is doing exactly that — shedding its warmth upward as infrared.

Greenhouse gases catch the outgoing infrared

Here’s where the atmosphere stops being a bystander.

A few gases — carbon dioxide, water vapour, methane — have a particular property. They are transparent to incoming visible light (so sunlight gets through) but they absorb outgoing infrared. When a CO2 molecule catches a packet of infrared heading for space, it holds that energy for an instant, then re-emits it — in a random direction. Some goes on up. Some goes sideways. And some goes back down, toward the surface it just came from.

That’s the whole mechanism. Heat that would have escaped to space is partly intercepted and sent back. The surface receives its own warmth a second time, on top of the fresh sunlight. So it sits warmer than sunlight alone would leave it.

The honest analogy is a blanket. A blanket doesn’t make heat — it slows the heat leaving your body, so under it you settle warmer. Greenhouse gases are a blanket for the planet’s infrared. They add no energy. They slow the loss.

It’s a balance, and the gases move where it settles

Why does the warming stop somewhere instead of running away?

Because the hotter the surface gets, the more infrared it radiates. The planet keeps warming until it radiates enough heat to space to exactly match the sunlight pouring in. At that point energy out equals energy in, and the temperature holds steady. That balance point is the planet’s temperature.

Greenhouse gases don’t change how much sunlight arrives. They change the temperature at which the books balance. Thicken the blanket, and the surface has to get warmer before enough infrared can claw its way out to match the incoming sun. The balance settles higher. Not because the sun got stronger — it didn’t — but because the exit got harder.

A planet with the blanket, and one without

Put numbers on it, because the numbers tell the real story.

Work out the temperature Earth should be from sunlight alone — bare rock, no greenhouse gases catching anything — and the answer is about −18°C. A frozen planet. At that temperature the surface would radiate just enough infrared straight to space to balance the sun.

The actual average surface temperature is about +15°C. The gap — roughly 33°C of warming — is the natural greenhouse effect. Water vapour and CO2 in the air, doing exactly what we described, hold the planet 33 degrees above the deep freeze it would otherwise be. That warming is not a problem. It’s the reason there’s liquid water and life at all.

The problem is one of more. Burning fossil fuels — coal, oil, gas, the stored sunlight from earlier in the course — releases CO2 that had been locked underground for millions of years. That extra CO2 thickens the blanket a little. A thicker blanket means the balance settles a little warmer. That nudge, added year on year, is human-caused warming. Same mechanism that gives us a livable planet, pushed past where it was.

On the whole

You are sitting under that blanket right now, kept warm by gases you can’t see catching heat you can’t see, in a balance struck between a star and the cold of space. Nothing about it is exotic. It’s the same physics as the hot car, the same conservation of energy from the first lesson, the same heat-always-radiating-away from the second — just played out across a whole planet’s worth of air. None of us can feel a tenth of a degree, and none of us stands outside the balance to watch it. We’re inside it, breathing the gas that sets it. Knowing how the dial actually works — that it’s the exit, not the sun, that’s changing — is a steadier place to think from than either alarm or dismissal. The mechanism is plain. What we add to the air is the part we get to decide.