The Amateur Climatologist

Lubos Motl has a new climate post mostly about radiative saturation of the CO2 band and climate sensitivity. He gets most of the main scientific points right (the next CO2 molecule costs us slightly less than the last one, the magnitude of the first order effect) but manages to draw some slightly odd conclusions. The chorus line of his worshipfull cult is rather more out to lunch, of course.

Let me pick out a clinker or two:

Most of the absorbed infrared rays are instantly transformed to kinetic energy of the atmosphere and this energy is not re-emitted.

I take issue with the last two words. Most of the greenhouse effect takes place in the troposphere, which is sandwiched between the warmer stratosphere above and the (usually) warmer ground below. The troposphere receives energy in four forms: solar radiation, infrared radiation (IR) from above and below, convected heat from below, and heat from the condensation of moisture convected up from the surface. Essentially all the energy it exports is in the form of infrared radiation, so it's always radiating more IR than it receives.

At least ideally - when you neglect the Doppler width of the lines and other effects

This is a minor point, but pressure (collisional) broadening is much more important than Doppler in the troposphere and (at least the lower) stratosphere.

The big problems come when he reports his results. He correctly states that in terms of effects of CO2 we are already halfway to the radiative forcing at a doubled CO2 from pre-twentieth century levels. One problem with this is that the thermal effect of the forcing is gradual - the ocean has a very high thermal inertia, so we won't see the full effects of the current CO2 levels, even if we stop emitting today, for another two or three decades. The other problem is that CO2 levels are unlikely to peak at 560. There might be another doubling by the end of the century.

How about saturation? Well, there is room for a few more doublings, since atmospheric CO2 is less than 0.04 %. More worrisome is the very likely prospect of positive feedback - adding heat to the oceans and atmosphere adds more H2O, the most important greenhouse gas, to the atmosphere. Also a bit bothersome is the fact that Venus, which has about 2^20 times as much CO2 as Earth, is a lot hotter than that factor (ln 2^20) of 20 suggests. Its closer proximity to the Sun explains only a little of the difference.

Lumo adds:

It's just like when you want your bedroom to be white. You paint it once, twice, thrice. But when you're painting it for the sixteenth time, you may start to realize that the improvement after the sixteenth round is no longer that impressive.

A more apt comparison might be wrapping your heater in 16 layers of paper towels rather than one. Those who want insight into how all this works might check out MODTRAN. I have some notes on what it means here.