I’m scheduled to give a lecture on “The Science of Climate Change” to an adult-education class next month. My topic changed at the last minute because of circumstances beyond the control of those of us involved. I decided to take a chance and force myself to learn something.
I’ve followed the climate change debate loosely, starting a decade or more ago with some of the early modeling by colleagues. I was using similar computer models for another problem, and I started out skeptical because I knew the limitations of those models. But the models have improved mightily, and the evidence seems pretty firm that the climate is changing rapidly and the reason for it is all the carbon dioxide we’ve been pouring into the air for a couple of centuries now.
But, having started skeptical, and following my usual principle of preparing for questions my audience might ask, I’ve been looking at what the skeptics have to say as well as the IPCC website, RealClimate, and other explanations of the science.
I’ve said before that I think the science is not being explained well, and in working through the material along with a discussion with skeptics at an undisclosed interweb bulletin board, I think I see one of the stumbling blocks. It’s not clear to me whether the people who have seized on this are genuinely confused or are just trying to muddy the waters. I think there are probably some of both.
This particular stumbling block comes in several guises. The basic claim is that the explanation of carbon dioxide’s role as a greenhouse gas as given by the IPCC and others contravenes the second law of thermodynamics, or perhaps the first. The argument seems to be that there cannot be a layer of gas high in the sky that radiates energy back to earth because the temperature of that layer of gas would be lower than the earth’s surface temperature. I have not tried to catalogue all the variants. There are at least two levels to refuting this argument.
First, the scientists of the IPCC have learned their thermodynamics and will spot basic flaws. Those of us who have taken thermodynamics courses know how the sins of perpetual motion machines against thermodynamics can be subtly hidden. The sheer number of scientists looking at this material assures that such errors will not slip through.
The other level is a bit more fun. I didn’t realize how much the greenhouse metaphor had made its way into my own thinking until I started looking at these “second-law” arguments. It turns out I had two parallel lines of thought: a mental picture and the mechanisms of heat transfer and molecular dynamics.
The standard explanation of the greenhouse effect is that the glass in a greenhouse is transparent to visible light, which strikes the soil and other objects in the greenhouse and is converted to heat, or infrared radiation (I could say light here or radiation after visible, but I’m using the common idiom), to which the glass is opaque. So the light comes in, and the heat doesn’t get out. The glass is separate from the interior of the greenhouse, a shell over it. In the same way, it is said, carbon dioxide is transparent to visible light and absorbs infrared radiation, thus heating things up.
But it’s not quite “in the same way.” The carbon dioxide isn’t separate from the rest of the atmosphere, it’s thoroughly mixed in. Gases mix rapidly and completely. There isn’t a shell of greenhouse gas in physical analogy to that shell of glass on a greenhouse. Unfortunately (and this is where my mental picture got suckered in), most of the diagrams explaining the greenhouse effect look like this one from My Climate Change:
Problem with Plovers has an explicit layer of greenhouse gases!
Even the Department of Energy does it.
This one, from the University of Arizona, is a little better, but it’s still got a layer. In fairness, some of those layers represent the top of the atmosphere, but if you’re thinking about the glass in a greenhouse, those layers might seem to be the equivalent of that glass.
So the well-mixed carbon dioxide absorbs that infrared, maybe two feet from the ground, maybe ten thousand feet from the ground. After it absorbs that energy, it’s warmer than the surrounding molecules are, so transferring that energy to them is right in line with the laws of thermodynamics. No problem.