Does the greenhouse effect work like a greenhouse?

It looks like the hint is in the name, but then again I wouldn’t be writing about the greenhouse effect if it did what it says on the tin. Unfortunately for ease of understanding, but fortunately for myth-debunking columnists, the greenhouse effect does not work like a greenhouse.

The greenhouse effect starts with invisible infrared light from the sun — that’s not an oxymoron, at least not to a physicist. Isn’t scientific language fun? Anyway, the atmosphere is quite transparent to infrared, which is what makes sunlight feel warm, and once inside it heats the earth’s surface. This causes the earth itself to radiate infrared back into the sky, but — crucially — at a longer wavelength than the incoming light, a wavelength which is readily absorbed by the atmosphere, in particular by the so-called greenhouse gases.

And so greenhouse gases stop some of the energy that would otherwise radiate out into space. If you take nothing else away from the description above, note that the greenhouse effect is all about radiation.

A greenhouse, on the other hand, doesn’t work by trapping radiation, but by trapping warm air inside its walls so that it doesn’t mix with the cold air outside and dissipate. In the language of science — don’t worry, I checked ahead for more semantic shenanigans — greenhouses stay warm by suppressing convection. This is just the same way that a blanket or jacket keeps you warm.

Scientists are renowned for their use of jargon and other arcane language, but this does smack of sheer bloody-mindedness. To be fair, the original coiner of the term “greenhouse effect” probably never thought that it would one day be on everybody’s lips. But they did unwittingly set a trap for the unwary which no doubt keeps many people from understanding the terrible threat we are all facing.

And if we’re all going to bake then we should at least know why. So whenever you hear somebody declare that carbon dioxide heats the earth like a greenhouse just tell yourself — and them — that they are full of hot air.

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