By JR on Thursday, March 15, 2012
Skeptics are divided into two camps on the greenhouse effect. The larger camp say it does exist but is of trivial importance for policy and the smaller camp says it does not exist at all.
Being just a humble social scientist with minimal background in physics, it is of no importance for me to take a stand on the question but I have recently been trying to get the issues clear in my head in pursuit of a most unlikely goal: Expressing the theory entirely in plain words so that those of us who are uncomfortable with algebra and technical terms from physics can get a handle on the whole thing.
I think the biggest barrier to understsanding for me has been the old law of the conservation of energy (Yes: I know about Einstein). This law does, I suspect, feel intuitively wrong for almost all of us. It states that energy is neither created nor destroyed. It just bounces around at different times in different places.
Most of us, I think, experience energy as something that gets used up -- as when the gas bottle on our BBQ runs out. But in fact energy is only changed into another form, which may or may not be of any benefit to us.
Lubos Motl, my favourite Pilsener, has been very kind in trying to show me how that works in the atmosphere. This is what I get from it:
When radiation from the sun hits the earth, it warms the earth. But a warm body also gives off heat so the earth warms its atmosphere by convection (contact) but also by radiation. It's not only the sun but also the earth that gives off energy in the form of radiation.
But since the earth is much cooler than the sun it gives off energy in largely different wavelengths. The energy that bounces off the earth is largely in the infrared (IR). But different surfaces respond differently to IR. IR passes right through some gases but gets soaked up by others. Some gases, principally water vapour, soak it up and therefore become hotter. So they become hot bodies too. And what do hot bodies do? They radiate energy in all directions, some of which is downwards toward the earth. And that is the storied Greenhouse effect: The heat that is bounced back towards the earth.
And CO2 is a minor partner in that process. Like water vapour, it too absorbs and subsequently emits IR radiation. Because it is such a tiny fraction of the "greenhouse" process, however, variations in its levels have negligible heating effect on the earth. It's levels of water vapour that matter.
So that's it! That's my attempt to explain greenhouse theory in words only. Partly in response to my desire for a really simple explanation of Greenhouse theory, Roy Spencer has put up his own, much more sophisticated explanation.
And don't blame Lubos for any errors above. Blame my shaky understanding.