Wednesday, January 09, 2008

If you don't remember the past you will repeat it

A long time ago, Jack Barrett perpetrated ignorance about the greenhouse effect that is still relied on today by the know nothing wanna know less crowd. There were several replies, but among them, John Houghton had a brief and very useful comment explaining the origin of the Greenhouse effect. Something of current interest.

A further point which Barrett makes is to suggest that because most of the absorption by carbon dioxide of radiation from the surface occurs within 30 m of the surface, the enhanced greenhouse warming due to increase of carbon dioxide in the lower atmosphere is negligible. In fact, most of the enhanced greenhouse effect occurs not because of changed absorption of radiation from the surface (although some change does occur in the wings of the carbon dioxide band where absorption is weaker) but because as the concentration of carbon dioxide in the atmosphere increases, the average height (around 6 km) from which carbon dioxide emits radiation to space also increases. Since atmospheric temperature in the lower atmosphere falls with altitude, if nothing changes other than the amount of carbon dioxide, the amount of radiation to space is reduced. For atmospheric carbon dioxide this reduction can be accurately calculated; for doubled atmospheric concentration it is about 4 W m-2. To restore the Earth’s energy balance the temperature throughout the lower atmosphere has to increase - hence the enhanced greenhouse effect [ 1].
Thanks to Atmoz who brought the current to and fro to Eli's attention and Lubos Motl who in the course of arguing his way to this conclusion reminded us of Houghton's letter.

9 comments:

Hank Roberts said...

I'd been wondering for a long while where this "absorbed within 30 meters of the surface" notion comes from. Google turns up just a few mentions.

Somehow this is tied to the observation that once you get above the tropopause, and the water's condensed out, the radiation from the greenhouse gases is finally not going to have a very short mean free path and has a better than average chance of escaping to space.

Where there's water vapor around -- up to the distinct line at the tropopause -- the odds are going to be better than half that the infrared photon emitted, even going up, will get absorbed by the next water molecule in line.

Not obvious?

EliRabett said...

Water is down to CO2 levels by 6km which is the effective radiative level. The tropopause 12-15 km is a bit of a red herring here.

Hank Roberts said...

Hoist on my own petard; I could've looked that up.

"The tropopause lies about 18 kilometers above the Earth’s surface at the equator in the summer and 8 kilometers above the poles in winter...."
https://www.llnl.gov/str/March04/Santer.html
http://www.sciencemag.org/cgi/content/full/301/5632/479?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=Santer%2C%2BB.&searchid=1078525779498_11531&stored_search=&FIRSTINDEX=0&fdate=7/1/2003&tdate=7/31/2003

Has anyone listed mean free paths for infrared photons at different vectors/elevations/humidities/temperatures? I know a photon headed laterally has a lot of atmosphere to traverse, vs. either up into thin air or down into denser wet air.

I've always wondered if when the temperature's below freezing the water molecules get slowed down and stacked together so the mean free path for infrared increases.

But I digress.

Somewhere there must be a whole nest of the 30-meter stories and fables, perhaps they're tagged so Google doesn't index them, nice to know where that one originated.

Hans Erren said...

To restore the Earth’s energy balance the temperature throughout the lower atmosphere has to increase - hence the enhanced greenhouse effect

No that is not neccesary, this can also happen through albedo decrease, which happens in the wet season in Africa every year. Solar radiation increases: clouds form, temperature drops.
Will moonsoons become more intense or will they fail more often in an increased GHS environment? Nobody can tell: Cloud physics is very poorly parametrised in GCM's, mostly because it's based on subcell statistics.

Marion Delgado said...

Hans Erren does his best to win my troll race. Did YOU know that albedo decrease actually got rid of energy? Or that solar radiation increased in Africa during the wet season? Neither did I!! Is "Hans Erren" a climate scientist trying to win a bet by posting the most basic science errors per paragraph?

What sort of clouds form in poor Africa that they decrease albedo? Are they like the soot-filled clouds near India?

The climate trolls live in a magical world of imagination that someone should tell Disney about.

guthrie said...

talking of not learning from experience, I think an eaegle should visit Pielke junior again...

Hans Erren said...

Oops albedo increase of course. Thank you for pointing out, Marion!

abeit in a very unpleasant way.

http://en.wikipedia.org/wiki/Netiquette

Suggestion for a polite reply:
"Hans did you perhaps mean increase? Otherwise your post doesn't make sense"

Marion Delgado said...

Note for the record, and I think this has some significance, that most of the freelance trolls are like HE here. They have to make up rationalizations for science denial on their own, so they don't rise to the level of obfuscatory power the more coordinated ones do.

Compare poor John Davison over in the world of evolution denialism.

By the way, in keeping with my policy of never addressing the trolls specifically:

Trolls:

It's not always personal, although I regard perhaps the majority of you as deliberate liars, not all of you are, some of you are simply muddled and incoherent. And you're not the same as the mistaken commenters (into which category everyone must fall, probably, at some points in time).

The mistaken commenters present a fun challenge - we can either enlighten them or link them to something we found informative.

But I don't address individual blog comments - that's not worth my time, really, or pretty much anyone's time. I would much rather point out patterns and abstract points. Playing the game of point/counterpoint with lying nonsense or lunacy that's been dealt with quite well, in some cases decades ago, would be an example of what is called enabling - alcoholics solicit it all the time, for instance. It's a form of codependence - you operate within the boundaries of the delusional or unhealthy reference frame of the sick person. No thanks.

Hans Erren said...

"But I don't address individual blog comments"

No you use blanket statements like "critics of the IPCC are trolls".

You pound the table for lack of scientific arguments,