Why greenhouse gas warming doesn’t break the second law of thermodynamics
This is generating many comments, see below for an update!
Behind the scenes some skeptics are suggesting that CO2 can’t warm us because the atmosphere is colder than the planet, and it would break the 2nd Law of Thermodynamics (see Postma*, for example, p 6 – 7). I disagree. The 2nd Law of Thermodynamics applies to net flows of heat, not to each individual photon, and it does not prevent some heat flowing from a cooler body to a warm one.
Imagine three blocks of metal side by side. They are 11°C, 10°C, and 9°C. Think about what happens to the photons coming off the atoms in the middle of the medium temperature block between the other two. If heat never flows from cooler blocks to warmer blocks, all those photons have to go “right“, and not ever go “left”, because they “know” that way is towards a cooler block? (How would they?!)
The photons go both ways (actually every way, in 3D). There are more coming from the 11°C block to the 10°C block, sure, but the the 10°C block is sending ‘em back to the 11°C block too. So heat is flowing from cold to hot. It happens all the time. Net heat is flowing always hot to cold. But some heat is going the other way, every day, everywhere, bar possibly a black hole.
People are being caught by semantics. Technically, strictly, greenhouse gases don’t “warm” the planet (as in, they don’t supply additional heat energy), but they slow the cooling, which for all pragmatic purposes leaves the planet warmer that it would have been without them. It’s a bit like saying a blanket doesn’t warm you in bed. Sure, it’s got no internal heat source, and it won’t add any heat energy that you didn’t already have, but you sure feel cold without one. – Jo (JoNova)