gizmo_2655 wrote on Aug 6^th, 2010 at 10:02pm:


Bah - it's exactly the same. A greenhouse effect is a greenhouse effect regardless of what causes it, and the magnitude of the Greenhouse Effect  has varied significantly through prehistory. No question about that whatsoever. The irradiance of the sun has also varied with time. There is no question that there has been natural variation over time. Nobody is arguing otherwise.

Let's get this straight too. There is no argument that it has been warmer or that there was more CO2 in the atmosphere in the dim and distant past. That's correct, but it's a red herring.

Now let's get down to talking about the basics.

gizmo_2655 wrote on Aug 6^th, 2010 at 9:53pm:


Pretty close.  Do you remember the The Stefan–Boltzmann law, which states that the total energy radiated per unit surface area of a black body per unit time (black-body irradiance) J,  is directly proportional to the fourth power of the black body's absolute temperature T:

   J = σ T^4.

Where σ (sigma) is the Stefan–Boltzmann constant and is equal to 5.67E-08 J s^-1 m^-2  K^-4

We can use this theory as a first approximation even though neither the Sun nor the Earth are black bodies.

By using that equation, Stefan was able to calculate the temperature of the Sun with a reasonable degree of accuracy. He arrived at a temperature of about 5700 K, compared to today's estimate of the temperature of the Solar surface is about 5780 K.

Apart from that, we can predict the wavelength of light emitted by the Sun and the Earth from their respective temperatures.

Now the energy of black body radiation varies with temperature and wavelength. As the temperature of the black body increases, two distinct behaviours are observed. One effect is that the peak of the distribution shifts to shorter wavelengths. This is known as Wien's displacement law. There are various forms of this, and it's a derivative of Planck's Law of Black Body radiation.  

λmax = 2.897E-03 /T

Where λmax is the peak wavelength in meters or the wavelength at which we get maximum spectral emissive power. T is the temperature in Kelvins.

Here's a more mathematical description:

http://planetphysics.org/encyclopedia/WienDisplacementLaw.html

I should probably speak about Planck's law of blackbody radiation too.

Here's a reference:

http://astronomyonline.org/Science/PlancksLaw.asp

If I need to explain the relationship between energy and wavelength any further, just let me know.

On the diagram, you can see where terrestrial radiation sits in relation to solar radiation and all the rest. So in terms of the Greenhouse effect, this is where all the action is.

We're talking about roughly 15% of the incident solar energy being re-radiated in the form of long wave infrared radiation. It's what happens to that long wave radiation coming from the earth that largely determines the change in Energy Balance for the Earth.  

How are we going so far?

mozzaok wrote on Aug 7^th, 2010 at 11:10am:


Understanding that we know just enough to start asking questions, is almost always the hallmark of an intelligent person!

perceptions_now wrote on Aug 7^th, 2010 at 11:57am:


Thank you perceptions.....

As Dr Who once said "Never be too sure of anything, it's sign of limited intelligence"

muso wrote on Aug 7^th, 2010 at 12:14pm:


But it is a useful demarcation as far as holding conversation goes though....
The biggest problem with this sort of discussion is differences in definitions and context.

For example, when I say 'Greenhouse Effect' I'm referring to the natural 'insulation' system that regulates the Earth's temperature to a liviable range..

Where as it seems you are referring to the apparent increases in Greenhouse gases..

Same term, different meanings....

muso wrote on Aug 7^th, 2010 at 1:32pm:


Kind of....although ALL the atmospheric gases (including nitrogen and oxygen do contribute)....after all the whole atmosphere is part of the Greenhouse Effect.

Also read replies 9 and 10 on the sticky thread:

http://www.ozpolitic.com/forum/YaBB.pl?num=1247904929

It will save me cutting and pasting the information again.