Geoff Price made me aware of a paper, by an apparently highly published physicist, that considers the infrared absorption of atmospheric carbon. It concludes that
CO2 is a very weak greenhouse gas and cannot be accepted as the main driver of climate change.
You might suggest that I should just ignore such a clearly nonsensical paper, but it is being promoted on notrickszone (or, as I like to call it, fulloftrickszone) and it’s sometimes useful to try and understand what they’ve done wrong.
Essentially what this paper concludes is that, relative to a baseline CO2 concentration of 400ppm, the change in radiative forcing, , if we change the atmospheric CO2 to a new concentration, , is
which is considerably smaller than suggested by other analyses:
In others words, this new analysis suggests that doubling atmospheric CO2 should only produce a change in forcing of 1.3 Wm-2, rather than 3.7 Wm-2.
So, what’s wrong with this new analysis? Let me try and explain using the figure on the right, which I produced using MODTRAN. The left-hand panel shows an example of a spectrum that you might measure if you were observing the Earth from space. The right-hand panel is the associated atmospheric temperature profile.
The coloured curves in the left-hand panel are example blackbody spectra at different temperatures. What this shows is that in some wavelength ranges, the spectrum we would observe comes from regions that are quite warm, and in other wavlength ranges, from regions that are quite cool. This is because, in some wavelength bands, the surface can emit directly to space, while in others, its coming from within the atmosphere (in some cases, even from the stratosphere, but I’ll mostly ignore that). Since the temperature drops with altitude (in the troposphere, at least) the emission from within the atmosphere is coming from a region that is colder than the emission coming directly from the surface.
If we then increase atmospheric CO2, while leaving everything else unchanged, that will act to block some of the outgoing flux. What essentially happens is that some of the flux will end up coming from higher in the atmosphere that it did when atmospheric CO2 was lower. Since the temperature drops with altitude (in the troposphere) this means that it will now be coming from regions that are cooler and that, hence, emit less. Therefore, the outgoing flux goes down and the system will have to warm to return to energy balance. As already pointed out, doubling atmospheric CO2 is estimated to reduce the outgoing flux by about 3.7Wm-2.
So, what is wrong with this more recent analysis? I think the answer is on page 5, where it says
we consider an isothermal atmosphere of T = 288 K.
Well, if the atmosphere is isothermal (constant temperature – 288K) then it doesn’t matter where the emission is coming from; it will always look like a 288K blackbody. It could all be coming from the surface, some from the surface and some from within the atmosphere, or all from within the atmosphere; it will make no difference. Similarly, if you change the atmospheric CO2 concentration, then you may change where the emission is coming from, but you won’t change the outgoing spectrum; it will still look like a 288K blackbody.
Therefore, I don’t even really know what the paper has actually calculated, but it almost certainly isn’t what the author thinks it is, and it isn’t a representation of the change in forcing due to a change in atmospheric CO2. Unless I’m missing something, estimating the change in forcing due to a change in atmospheric CO2 requires taking the temperature profile of the atmosphere into account. That this paper did no such thing would seem to immediately mean that what it presents is clearly not representative of the change in forcing due to a change in atmospheric CO2. Essentially, its no great surprise that it gets a result that is inconsistent with other analyses, since it doesn’t even seem to be doing an appropriate calculation.