Monday, June 16, 2008

Twice more

Having read the many comments about Rabbet Lab's simple explanation of man made climate warming, Eli makes a few deletions and additions to reach the final product.

  1. The total energy emitted by the earth has to equal the total energy absorbed from the sun. All this energy comes and goes as light.
  2. The earth radiates in the infrared, the sun radiates at much shorter wavelengths, principally in the visible.
  3. It gets colder the higher you go in the troposphere and the density of molecules is lower.
  4. 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).
  5. In the atmosphere below 100 km the ability of GHG molecules to absorb radiation equals their ability to emit radiation (Kirchhoff's law)
  6. 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 (about 7 km which is still in the troposphere).
  7. 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.
  8. 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 warms the surface yet further.
  9. 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).
There are two complementary mechanisms

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.
  1. Humans have increased the concentrations of carbon dioxide, methane, nitrous oxides and other GHGs in the atmosphere. Increasing the concentration of CO2 by a factor of two will increase global temperature by a bit more than 1 C (~1.8 F). This is a forcing.
  2. This increase in global temperature warm the oceans and thus increases atmospheric water vapor (also a GHG) enough that the total rise in global temperature will be ~3 C (best estimate range is btw 2 and 5 C for 2x CO2). This is a feedback.
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)


Brian said...

Do the 100km in #5 and 7km in #6 conflict with each other? I'm missing something.

EliRabett said...

They are two different things. Below 100km Kirkhoff's law holds. The level at which emission upwards in the IR leaks into space is 7 km. Need to think about differentiating that.

neil craig said...

#1 is wrong. The core of the Earth is hot essentially due to radioactivity & produces a significant, though not enormous in global terms, amount of heat, as anyone who has visited a dep mine can attest. This obviously adds to our IR signature.

EliRabett said...

Neil, neil, neil, is there anything you won't fall for?

Anonymous said...

Dear Elirabett

In my opinion it is always worth examining and reviewing one´s assumptions before making any pronouncements.

I sense the unspoken caveat "other things being equal" in everything you say.

With best regards


EliRabett said...

as in all other things being equal the sun will rise in the morning.