Since the world no longer needs a good 10 cent cigar, and the White Owls Dad and Grandpa Rabett thought were good were anything but, Eli is driven to again try and provide a relatively simple explanation of the greenhouse effect.
For those of you who crave death by mathematical physics the bunnies suggest Ray Pierrehumbert's on line text book, or in a somewhat (and I warn you, only somewhat) simpler form in an arXiv article by Arthur Smith.
Eli will alter the text based on suggestions however we are going to KEEP IT SIMPLE. All entries become the property of Rabett Labs.
There are two complementary mechanisms
- The total energy emitted by the earth has to equal the total energy absorbed from the sun. The only mechanism for either of these processes is radiation to and from space
- The earth radiates in the infrared, the sun radiates at much shorter wavelengths, principally in the visible.
- It gets colder the higher you go in the troposphere and the density of molecules is lower.
- From 3 the rate of radiation from IR active (greenhouse gas, GHG) molecules (CO2, H2O, CH4) is lower the higher you go (hotter things radiate more, see Stefan-Boltzmann law, more molecules radiate more).
- In the atmosphere below 100 km there is always a local effective temperature, thus the absorbtivity of the GHGs equals its emissivity (Kirchhoff's law)
- Greenhouse gases are IR absorbers/emitters. They effectively block radiation escaping to space at wavelengths they absorb EXCEPT at high enough levels where the gas density is low and the radiation can escape directly to space without being absorbed by other GHG molecules(this is about 7 km which is still in the troposphere).
- Because radiation to space is blocked at IR wavelengths where GHGs (and clouds) absorb, the surface has to warm so that radiation can increase in unblocked areas of the spectrum and escape to space.
- This means that there will also be increased radiation in regions of the spectrum where greenhouse gases absorb which, in turn, will warm the atmosphere but cannot escape to space. Radiation absorbed in the atmosphere and scattered back to the surface, warming the surface yet further.
- As the atmosphere warms, the areas of the spectrum where the greenhouse gas molecules absorb widens as more energetic levels of the GHG are populated (The link points to on line software, which you can model this yourself)
Walking Mechanism. Increasing the amount of GHGs widens the spectral regions where they absorb and narrows the windows where radiation can escape to space. To maintain balance with incoming solar, the atmosphere and surface warm further
and Chewing Gum Mechanism. As the GHG mixing ratio increases, the effective level at which the Earth can radiate to space at the wavelengths emitted by the GHGs climbs, but because of the cooling with altitude and the lower density of molecule, to maintain the same radiation rate and balance the solar input, the new higher level at which the earth radiates has to warm, and to do this the entire troposphere and the surface have to warm.
Since the increase of CO2, methane, nitrous oxide, and other GHGs are known to be caused by humans, so must an increase in surface temperature driven by these mechanisms.
Does the greenhouse effect operate by the same mechanism as greenhouses? No (except if you think of one blocking off energy loss by closing regions of the spectrum for radiative loss, and the other blocking off energy loss by stopping convection)