I was listening to Dennis Prager on the radio while out at lunch a few days ago. For those of you unfamiliar with him, Prager usually falls somewhere towards the rather sparsely populated “level-headed and reasonable” end of the conservative talk show host spectrum. Usually. This time, though, on the subject of global warming, he said this (I’m paraphrasing from memory here):

We can’t even seed clouds to make it rain, and yet we think we can raise the global temperature?

Analogies are, generally speaking, a pretty weak form of argument. The point being made with an analogy is only as good as the analogy itself. Mathematically speaking (or systems theoretically speaking, for that matter), we can regard an analogy as an isomorphic mapping between the salient features in the case being examined and the case to which an analogy is being made. A stronger analogy will have more numerous and directly relevant mappings; a weaker analogy, fewer and/or less relevant mappings.

Prager’s analogy appears to have only a single salient factor in common between the two cases: they both have to do with humans changing the environment. And in entirely different ways, too (precipitation versus global average temperature), so it’s not a particularly relevant mapping. Leaking a bunch of oil into the Gulf has to do with humans changing the environment too, but Prager didn’t choose that analogy.

The science is pretty straightforward on this: if you add a bunch of CO2 to the atmosphere, the atmosphere will grow warmer over time. CO2 is fairly transparent in the wavelengths of incoming sunlight (i.e., it lets most of the sunlight through), but is more opaque in the longer infrared wavelengths of heat released by the Earth. Thus, it traps heat in the atmosphere. The effect is a diminishing one in that each amount of CO2 added to the atmosphere produces less warming than the previous such amount did. For example: if raising the CO2 in the atmosphere from 200 parts per million (ppm) to 400 ppm produces a global average temperature increase of X, then to get another increase of X you would have to go from 400 ppm to 800 ppm. And for another increase of X you’d have to go from 800 ppm to 1600 ppm. And so on.

This doesn’t mean that all the warming we’ve had since the 1850’s is due to man-made CO2. Since the last minima of the “Little Ice Age” in 1850 the Earth has been warming naturally, and humans have only been dumping significant amounts of CO2 into the atmosphere since about 1950. In fact one model suggests that only a small fraction of the global temperature increase to date is due to man-made CO2.

Where were we?

Oh yes. Prager’s analogy sucked.