[ExI] Greenhouse Gas

[Kelly Anderson]

Could someone please comment on this analysis? It seems to make sense to
me, and that kind of worries me. I keep thinking this is settled to some
extent in my head.... I want to believe science is not totally screwed over
by politics, I want to believe...

>>>>>>>>>>>>

The misnamed “greenhouse” effect of greenhouse gasses like CO2 is based on
the fact that they are not truly colorless. They have a “tint,” though we
can’t see it, because it’s in a part of the light spectrum that our eyes
don’t detect. GHGs are transparent in the visible part of the light
spectrum, but they absorb (block) parts of the IR spectrum.

Adding such gasses to the atmosphere has a warming effect on the lower
atmosphere, because the light arriving at the earth from the sun is much
“bluer” (shorter average wavelength) than the light emitted from the earth.
Because the earth is relatively cool, the light emitted from the earth is
mostly IR. So anything in the atmosphere that blocks IR but is transparent
to visible and UV will have a warming effect, because it lets in most of
the arriving solar radiation (that warms the earth), but blocks a much
larger percentage of the departing radiation (that cools the earth).

Even though CO2 levels are measured in parts-per-million, there’s
nevertheless already so much CO2 in the atmosphere that it blocks nearly
all of the IR that it can possibly block. So adding more CO2 doesn’t have
much effect on temperature. For CO2′s main absorption bands, we’re way, way
past the CO2 levels at which the IR is all absorbed. Only for very narrow
ranges of wavelengths at the fringes of those absorption bands, where CO2
is a very weak absorber, can adding more CO2 appreciably increase the
amount of IR blocked.

However, adding even a small amount of a different GHG (such as one of the
CFCs or HCFCs) can have a much larger warming effect, by blocking a part of
the IR spectrum for which the atmosphere would otherwise be transparent.
That’s why you may read that CFCs like Freon-12 are thousands of times more
potent as GHGs than CO2. It’s not that there’s anything fundamentally
special about Freon-12, it’s just that there’s so few Freon-12 molecules in
the atmosphere that some of their absorption bands aren’t already blocked.


Here’s an analogy. Consider moth-eaten blankets to be like GHGs, and
different positions one the blankets correspond to different parts of the
IR spectrum. The blankets have big holes in some places, but nice, dense
wool fabric in others.

Different patterns of holes in the blankets are like different GHGs. They
pass some parts of the IR spectrum, and block others. So “CO2″ blankets
have one pattern of holes, “CH4″ blankets have a different pattern of
holes, “CFC-12″ blankets have yet another pattern of holes, etc.

Now, envision an obsessively compulsive neat-freak piling on moth-eaten
blankets to try to keep warm in a chilly night. He exactly straightens and
lines up each blanket on the bed.

If he piles on a dozen identical “CO2″ blankets, with the holes all lining
up exactly, he won’t be much warmer than if he had only one or two “CO2″
blankets. But if he adds a “CH4″ blanket, with many of its moth-holes in
different places, then he’ll be a lot warmer, because some of the CO2
blanket’s holes will be blocked by the CH4 blanket, and vice-versa. And if
he adds a “CFC-12″ blanket, with some of its holes in different places than
the holes in the CO2 and CH4 blankets, he’ll be warmer yet.

Right now, we’ve got about 10 or 20 CO2 blankets piled on. Adding another 5
or 10 or 20 CO2 blankets will keep us a tiny bit warmer at the frayed edges
of the holes, but it won’t make near as much difference as adding some
other kind of blanket.

<<<<<<<<<<<

Thanks!

-Kelly
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