We humans kick up a lot of dust and burn a lot of stuff. A concept developed in the 1970’s was the “human volcano”. There is a lot of truth in this analogy. Back then, despite dead wrong rants, the sober scientific consensus was that it was unclear whether the net effect of the human volcano was to warm or cool the planet.
“At the present rate of nitrogen build-up, it’s only a matter of time before light will be filtered out of the atmosphere and none of our land will be usable.”
The quote above from Kenneth Watt, a self-appointed and widely read guru, in 1970. He was worried about cooling. Nobody worries much about “nitrogen build-up” these days, nor cooling; we have new witches to burn.
Al Gore likened the CO2 greenhouse effect to a blanket, keeping the planet warm by reducing energy loss to space. This might work if the blanket was a “space blanket”with a coating to reflect the dim light radiated from the surface back down. If you have ever used a space blanket to stay warm, you will know that it helps, but you would much rather have a down blanket. This is because conduction and convection are very important as well as radiation, and the down blanket reduces conduction and convection better than a space blanket.
Somewhat more sophisticated came the argument that rather than a blanket, the greenhouse effect relies on pushing the “net radiative altitude” higher. Since the troposphere cools with altitude, and radiative intensity varies to the fourth power of temperature, pushing this radiative altitude higher would reduce radiation to space. We will show that significant CO2 radiation to space comes from the stratosphere where temperature increases and radiation to space increases to the fourth power of increasing temperature with increasing concentration.
According to MODTRAN, as seen from the altitude of polar orbiting satellites at 70 km, the fundamental bend of CO2 (and its rotations) radiates at a temperature of 220K. This temperature corresponds to an altitude of 12-13 km at the tropical lapse rate. The atmosphere continues to decline in temperature in the tropics to 17 km, where the lapse suffers a relapse and the atmosphere begins increasing in temperature with altitude.
If you set MODTRAN to 410 ppm CO2, and set all the other greenhouse gasses to zero; you can vary the altitude to see where the CO2 radiance is coming from.
Above you can see that the total IR upward flux looking down from the tropopause at 17 km is 397.21 W/m2. You can also see that without the other absorbing gasses, the planet radiates at the Planck curve for surface temperature except for the deviations caused by CO2. The lowest part of the CO2 deviation is seen radiating at the temperature of the 17 km altitude, about 195K, as seen from the blue lapse curve to the right.
When we jump up to 70 km we see that the upward IR flux increases to 400.35 W/m2. This means that 3.14 W/m2 of the CO2 radiance polar orbiting satellites at 70 km see is coming from the stratosphere. As you can see from the blue lapse curve to the right, temperature increases with altitude in the stratosphere. The bottom of the CO2 deviation conforms to and radiates at the 220K Planck curve, a temperature 25 degrees higher than the bottom of the deviation at 17 km. In the tropical atmosphere, 220K matches an altitude of 24 km in the stratosphere.
When we drop down to 24km the upward flux is 398.15, meaning that 2.2 W/m2 of the CO2 radiance seen from 70km takes place above 24 km.
We decided to continue the exercise above and record the MODTRAN upward radiance at one meter and thereafter at 1 km increments to 70 km. It can be seen that CO2 radiance increases in the lower stratosphere and then levels out beginning at 43 km. Seemingly, 43 km is the last inflection point in CO2 radiation to space.
We began this post with examples of muddled thinking about the impacts of human absorptive gasses in the atmosphere. We end the post having presented data indicating that we need a far more nuanced approach.