söndag 25 juli 2010

Chilling Greenhouse Gases?

In the earlier post CO2 Cooling we pondered the question if more GreenHouse Gases GHG (like water vapor) will warm or cool the Earth surface? 

Let's consider a simplistic (but possibly relevant) argument: Let's agree that without any atmosphere an isotherm Earth would have a blackbody temperature of 255 K as given by 
Stefan-Boltzmann-Planck's Law. What would the temperature be with a fully opaque atmosphere effectively extending an isotherm Earth to include the atmosphere. Right: Again 255 K.  

The real atmosphere is something in between fully transparent and fully opaque with 
an Earth surface temperature of 288 K. Apparently we cannot say a priori that making the atmosphere more opaque by adding GHG will warm the Earth surface: Adding too much would cause cooling.  

Simplistic, yes, but anyway relevant?

5 kommentarer:

  1. This would be true if GHG:s absorbed equally for all frequencies, but, of course, they don't. Adding more CO2 isn't going to make the atmosphere opaque to visible light from the sun.

  2. Yes of course, but an isothermal atmosphere has a temp of 255 K, and at what radiation characteristics does it become isothermal, if only radiation
    is considered?

  3. It becomes isothermal if all incoming sunlight is absorbed at a high altitude. This is not going to happen if you add more CO2. Venus is a good example for how temperature keeps increasing if you add more GHG:s.

  4. The atmosphere of Venus is almost opaque and the temperature gradient
    does not depend on GHG, but gravitation.

  5. It is a good idea to build (first) some simplistic models. They can be used to understand the underlying basic physics.

    - Sun heats the earth (as the UV radiation passes through the GH gases).
    - Heat from earth goes up, but the GH gases act as insulation layer.
    - Heat conduction laws give an easy way to calculate, what is the temperature on earth surface.

    I think the climate scientists have confused a lot by talking about the "bacradiation" - which totally goes against physical laws. More correct is to talk about GH gases being in insulation layer, slowing down the heat transfer (and causing temperature gradient).