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ArgoNavis
2007-Jan-28, 05:50 AM
The Greenhouse effect involves heat emitted by the Earth following warming by the Sun being trapped and heating the atmosphere.

The greenhouse gases capable of this are water vapour, CO2 and methane.

What is the mechanism for water vapour, CO2 and methane being opaque to IR radiaition that the Earth emits.

korjik
2007-Jan-28, 06:09 AM
IR photons are absorbed by the chemical bonds then reradiated in a random direction.

ArgoNavis
2007-Jan-28, 06:29 AM
IR photons are absorbed by the chemical bonds then reradiated in a random direction.

Does this have something to do with the size of the molecule?

is the molecule not dissassociated?

korjik
2007-Jan-28, 07:22 AM
IR dosent have enough energy to dissassociate any of the gasses you mentioned. You need to get up into the UV range for that.

Basically an IR photon is absorbed as either a vibrational state in the chemical bonds, or a rotational state by spinning the whole molecule.

dtilque
2007-Jan-28, 08:16 AM
Does this have something to do with the size of the molecule?Not the size specifically. As I understand it, any gaseous molecule made up of at least 3 atoms will be a greenhouse gas. CFCs, for example, are also greenhouse gasses. They have at least half a dozen atoms.

As korjik says, the energy from a photon is absorbed into either vibration of the bonds or rotation of the molecule. Molecules with 3+ atoms have more rotational states than those with only 2 atoms. So they can absorb a larger fraction of the photons that hit them. That's what makes them greenhouse gasses.

ArgoNavis
2007-Jan-28, 08:57 AM
thanks everyone

trinitree88
2007-Jan-28, 11:18 PM
Argo..If you burn the leaves from your trees, you make carbon dioxide. If they are composted, you get lots more methane, with some carbon dioxide. Since methane is ~ 25 times better as a greenhouse gas, the banning of leaf burning in many states ordered by the gov. has contributed immensely to the increase of, not decrease of, global warming.....just the opposite of what was intended. I'm sure our leaders will rectify this.:shifty: pete

Spaceman Spiff
2007-Jan-28, 11:28 PM
Argo..If you burn the leaves from your trees, you make carbon dioxide. If they are composted, you get lots more methane, with some carbon dioxide. Since methane is ~ 25 times better as a greenhouse gas, the banning of leaf burning in many states ordered by the gov. has contributed immensely to the increase of, not decrease of, global warming.....just the opposite of what was intended. I'm sure our leaders will rectify this.:shifty: pete

Except that you need to account for the present relative absorptivities of these two gases to the total infrared absorptivity (= 1 - transmissivity) of our present atmosphere. What you state (in bold) is at least roughly correct for the present state of affairs of our atmosphere's IR absorptivity - molecule for molecule. But the methane abundance is far below that of carbon dioxide, at least for now. :)

As a purely hypothetical example (I don't have any hard numbers at my finger tips): if we doubled the carbon dioxide optical depth from say 1 to 2 units, that would easily outweigh quadrupling the methane optical depth from 0.001 to 0.004 units. And unless it is small, absorptivity certainly is not a linear (or otherwise simple) function of abundance (and that conclusion is also wavelength dependent).

[notes: optical depth 'tau' = opacity (cm^2/g) * density of gas (g/cm^3) * physical depth (cm); absorptivity goes something like 1 - exp(-tau); these are wavelength dependent quantities.]

Another consideration is the number of CO_2 vs. CH_4 molecules released per unit mass of leaves for a typical combustion and decomposition process (don't have that off the top of my head).

While I am not suggesting we should ignore methane in the problem of global climate change, I am skeptical that composting leaves in the U.S. has resulted in a significant net increase in the IR opacity of earth's atmosphere. Remember, too - mother nature composts leaves all by herself. :)

Somebody in climate science has probably worked out the roles and impact of the methane budget on Earth's energy balance.