the chemistry of the greenhouse effect

Just a quick mention of an interesting study that I saw today. Three researchers from NASA and Purdue have a paper in the Journal of Physical Chemistry called Identifying the Molecular Origin of Global Warming [LINK]. It was brought to my attention by a news story on NewScientist.com [LINK]. Now based on that title, what would you expect from this paper? The news article doesn't completely make sense to me, perhaps because the writer tries to get so many barely-related things into such a tiny piece. It had me intrigued though, so I went and read the paper.

Before we get to the conclusions of the paper, let's consider what we know about global warming, specifically the greenhouse effect. We know that the most important greenhouse gas in terms of the human impact on climate is carbon dioxide. The way that the greenhouse effect works is that the molecules of carbon dioxide absorb infrared radiation emitted from the Earth's surface. Those molecules then emit infrared radiation at a slightly colder temperature, and they radiate in all directions. This has the effect of storing energy in the system, first by keeping it in molecules (before they radiate it away), and second by radiating back to the atmosphere and surface. So what does this paper have to offer?

The study uses calculations of molecular properties to investigate the greenhouse warming potential of different gases. The GWP of a gas is essentially a measure of how strongly a gas absorbs in the infrared, and how much warming it could cause over a given time in the atmosphere (typically 100 years). The way I've seen it presented, carbon dioxide is given a value of 1, and other gases are then shown compared to carbon dioxide. This study shows that there are a couple of families of molecules that have very large GWP, and presents an argument for how it comes about. The gist seems to be that molecules that have carbon-fluorine or carbon-chlorine bonds are particularly good greenhouse gases. These happen to include chlorofluorocarbons (CFCs), perfluorocarbons (PFC) and hydrofluorocarbons (HFCs). The warming potential increases strongly with the number of bonds between the F or Cl and the carbon atom. This seems to be because the vibrational modes of the molecules, which are quite pronounced for these bonds, plus the stretch length of these bonds mean that the infrared interaction is within the atmospheric window. If this is all sounding vague and Greek-ish, it's because I don't completely understand all the terminology in the paper. The point is just that F, and to a lesser degree Cl and H, have strong bonds with C in these molecules, and their vibrational modes are in the atmospheric window, meaning they can absorb strongly in the correct band to make a difference to climate.

The major problem with the paper is that these gases are quite rare in the atmosphere, and it is hard to make the case that they are making a significant difference to the climate. The counter to such an argument is that this paper shows the physical mechanism at the molecular level that is responsible for some gases being very good greenhouse gases. It means that you can basically know from the outset whether some gas, perhaps an industrial product of some sort, will be a strong greenhouse gas. It also can serve as an early warning against over-using these gases, since large increases in their production could have consequences for the climate system.

The secondary problem with this paper is the way it seems to be interpreted in that news story (and thus likely others). It's presented almost like it's the first time we've understood what is going on with the greenhouse effect. Of course, that is rubbish, as we've had a good handle on the basics for over a century, and detailed studies of CO2 for decades. The authors don't try play down their results either, which you can tell just from the title. This is not a world-shattering study; it is a nice piece of chemistry that has some application to climate science.

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