Black body radiation, CO2 and thermal infrared

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Black body radiation, CO2 and thermal infrared

Postby HBS Guy » 18 Jan 2018, 08:22

A black body is a body that absorbs and emits all energy falling on it. Gamma rays, XRays, UV, visible light, infrared, radio waves etc.


Black-body radiation is the thermal electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body). It has a specific spectrum and intensity that depends only on the body's temperature, which is assumed for the sake of calculations and theory to be uniform and constant.[1][2][3][4]

The thermal radiation spontaneously emitted by many ordinary objects can be approximated as black-body radiation. A perfectly insulated enclosure that is in thermal equilibrium internally contains black-body radiation and will emit it through a hole made in its wall, provided the hole is small enough to have negligible effect upon the equilibrium.

A black-body at room temperature appears black, as most of the energy it radiates is infra-red and cannot be perceived by the human eye. Because the human eye cannot perceive color at very low light intensities, a black body, viewed in the dark at the lowest just faintly visible temperature, subjectively appears grey (but only because the human eye is sensitive only to black and white at very low intensities – in reality, the frequency of the light in the visible range would still be red, although the intensity would be too low to discern as red), even though its objective physical spectrum peaks in the infrared range.[5] When it becomes a little hotter, it appears dull red. As its temperature increases further it eventually becomes blue-white.

The Earth is not a perfect black body but then again perfect black bodies do not exist. It exists within almost a perfect vacuum so energy pretty much only arrives or leaves via radiation so it can be considered a black body.

Even the sun receives and reradiates radiation so can be considered in that respect a blackbody.

There is a spectrum of radiation emitted from a black body and for the earth that is infrared: this is why the nightside of the earth is black: it is radiating the heat it soaked up during the day but it is radiating it in the IR part of the spectrum which we can’t see. Hotter bodies than the earth appear red, really hot black bodies appear blue-white. This is all classic physics. You can read up about this further if you like:


But let us look at the earth, it has some main elements that make it up: the surface both solid and seas, icesheets and the atmosphere including clouds.

Clouds and icesheets, snow cover etc reflect light back out to space, oceans can store enormous amounts of heat and so on. As I said, the Earth is not a perfect black body, they do not exist.

As I said the spectra of the black body radiation emitted by the earth is in the infrared. This is a problem: certain gas molecules with three or more atoms absorb infrared. We all know them: CO2, H2O, CH4, N2O, O3 and HFCs etc.

These greenhouse gases have a space between two of the atoms that fits a specific IR wavelength: the photon is absorbed making the molecule more energetic: it spins/vibrates faster but it cannot hold onto the photon which moves onto another dipole and leaves the molecule. The molecule could be pointing in another direction and the photon moves from a different dipole so it could be directed downwards as likely as upwards. And the photon could be absorbed/emitted by more than one GHG molecule.

This is why temperatures have moved 1.2°C since preindustrial times: all that CO2 emitted warms the earth which sees overall more water vapor in the atmosphere keeping only a few spots in the IR spectrum not blocked by GHGs.

It is STUPID to keep emitting CO2! Collective suicide!

If it wasn’t for CO2 the average temperature would be -25°C or so. I am frequently told that by the more idiot deniers. So, the Greenhouse Effect works, they are saying but it doesn’t work is the unspoken corollary :rofl

I will add to this a lot of the scientific aspects of AGW. The black body radiation of the earth is not the only thing causing changes to temperatures on earth, the subject of another thread. Probably not for a while.
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