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Friday, November 4, 2011

The Greenhouse Analogy

The Greenhouse Analogy

When I discovered that thermal conduction was underestimated as an impact of CO2 I was not particularly surprised. Having worked in Heating and Air Conditioning for many years, I am used to looking for system inefficiencies and ways to take advantage of natural heat sources and sinks. What was surprising is how much the atmosphere does behave as a Greenhouse.

The radiant effect is of course the main concept that rules the greenhouse roost as far as climate science is concerned. Builders of real greenhouses know that ground source energy, reduced convective loss and insulation are just as important as the radiant effect in trapping heat.

Dr. Roy Spencer, is one of many that has built and experiment to measure the impact of Down Welling Long wave Radiation. In his experiment, Ambient temperature and dewpoint remained more stable than the radiant temperature he was measuring. The relatively steady conduction of surface temperature to the surface air, stabilized the local ambient conditions. This would indicate that surface/air conductivity is not negligable with respect to DWLR since in effect, DWLR is dependent on the surface emission of thernal flux.

Convection in the atmosphere is somewhat limited by water vapor and clouds. That sounds counter intuitive, since clouds and rain are the most obvious convective events. Warm moist air rises from the surface as it is heated, most often by solar energy at the surface. What is not so obvious is that sunlight absorbed by the water vapor and clouds in the atmosphere tends to stabilize the air above the latent shift layer of the atmosphere. This layer is known as the Atmospheric Boundary Layer (ABL) or Planetary Boundary Layer (PBL).
While the atmosphere below the ABL is subject to considerable turbulence from local thermals, the drag of the surface reduces wind speeds. The area below the ABL is a quasi-laminar flow region in many cases. In other words, there is less convection than there would be without the latent shift of energy from the surface to the APL.

The latent shift also regulates the radiant impact of solar energy by shifting the ratio of solar absorbed at the surface versus in the atmosphere. Water in the form of liquid and ice above the APL has a more open radiant window to cool than it would below where the outgoing radiation would interact with water and water vapor above.

As in a greenhouse, the surface to air boundary is responsible for most of the energy that can be retained when solar absorption is not optimum. Since water vapor has a limited half-life in the atmosphere, more water vapor implies more precipitation which enhances the thermal transfer from the surface both via latent and conductive processes.

Apparently, Carbon Dioxide also improves the surface/air thermal conduction both as CO2 in the air and as Carbonic Acid in the oceans, ice and in moisture laden air. The small conductive impact is completely over looked, yet has the potential to greatly impact surface cooling with improved thermal transfer.

Because the conductive impact is at the surface and the true radiant impacts well above the surface, conductive change has 30 to 40 percent more impact per unit change than radiant impacts which have to compete with water vapor in order to transfer energy to the surface.

The Greenhouse Analogy is much better than I would have ever thought.

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