Sunlight pours through the air and warms the ground. The ground glows that warmth back up as invisible heat — but gases like CO₂ act like a blanket, catching some and sending it back down. A thin blanket keeps Earth cosy instead of frozen. Add more CO₂ and the blanket thickens, less heat escapes, and everything warms up.
Most people think the greenhouse effect is purely harmful. In fact without it Earth would average about −18°C, a frozen rock; the problem is only that burning fossil fuels has thickened the blanket from 280 to over 420 ppm of CO₂.
What's actually happening
Sunlight arrives mostly as visible light, and the atmosphere barely notices it — the air is clear, so the light sails through and warms the ground. The warm ground then does what all warm things do: it glows. Not in visible light, but in infrared, the invisible heat radiation you feel from a radiator across a room. That outgoing infrared is Earth trying to shed its heat back to space, and how easily it escapes decides the planet's temperature.
Here's the catch. A handful of gases (carbon dioxide, water vapour, methane) are transparent to incoming sunlight but opaque to outgoing infrared. They absorb that escaping heat and re-radiate it in all directions, including back down. The planet still loses heat to space, just more slowly, like a blanket that doesn't stop you cooling but slows it enough to keep you warm. This natural greenhouse effect is not the villain: without it Earth would average about −18 °C, a frozen rock. It's why we have liquid oceans and a habitable 15 °C instead.
The problem is thickening the blanket. Burning fossil fuels has pushed CO₂ from a long-standing ~280 parts per million to over 420, and more trapping gas means less infrared escaping at any given temperature. To get rid of heat as fast as the Sun delivers it, the surface has to run hotter — so the whole system settles at a warmer equilibrium. The simulator shows the bargain directly: slide the CO₂ up and watch fewer red rays make it out and the thermometer climb. The physics has been understood since the 1850s; what's new is how fast we're moving the slider.
Earth lets sunlight in easily but lets heat out only grudgingly, and a few trace gases set how grudgingly — thicken the blanket and the whole planet must warm to rebalance.
- 1Put a thermometer in each of two clear jars. Leave one as plain air; drop a fizzing tablet (or a splash of vinegar on baking soda) in the other to fill it with CO₂, then seal both.
- 2Sit them side by side under a bright lamp or in direct sun and read the temperatures every few minutes.
- 3The CO₂ jar warms faster and ends hotter — Eunice Foote's 1856 experiment, repeated on a windowsill.
Common questions
Not in itself. Without it Earth would average about −18 °C, a frozen rock. The natural effect gives us liquid oceans and a habitable 15 °C. The problem is humans thickening the blanket.
More trapping gas means less infrared escapes at any given temperature, so the surface must run hotter to shed heat as fast as the sun delivers it. Burning fossil fuels has pushed CO₂ from about 280 to over 420 parts per million.
Since the 1850s. Eunice Foote filled jars with different gases in sunlight in 1856 and found the CO₂ one heated most — the first known demonstration that CO₂ traps heat.