The discussion at WUWT about a non-radiating atmosphere got me thinking about where DOES our atmosphere radiate? I mean, we have all this talk about “CO2 backradiation” and “IR Windows” and such. Can we figure out just where the heat energy really does the “leaving”? And after that, does it point to a potential mechanism? And is that mechanism limited by the “CO2 Absorption”?
The two WUWT threads:
First off, what does the earth look like from deep space. Does it have a lot of complex texture and a lot of notches and windows? Or does it look like your basic ball of dirt and water?
We present two observations each spanning 1 day, taken at gibbous phases of 57° and 77°, respectively. As expected, the time-averaged spectrum of Earth is blue at short wavelengths due to Rayleigh scattering, and gray redward of 600 nm due to reflective clouds. The rotation of the planet leads to diurnal albedo variations of 15%-30%, with the largest relative changes occurring at the reddest wavelengths. To characterize these variations in an unbiased manner, we carry out a principal component analysis of the multi-band light curves; this analysis reveals that 98% of the diurnal color changes of Earth are due to only two dominant eigencolors. We use the time variations of these two eigencolors to construct longitudinal maps of the Earth, treating it as a non-uniform Lambert sphere. We find that the spectral and spatial distributions of the eigencolors correspond to cloud-free continents and oceans despite the fact that our observations were taken on days with typical cloud cover. We also find that the near-infrared wavebands are particularly useful in distinguishing between land and water. Based on this experiment, we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope.
So from deep space they can make out land and water. The clouds do not confound the image much, despite their large impact on albedo. Notice not much mention made of CO2 absorption bands. When we look at their data, there is some reduction in spectrum but mostly relative to the 300-400 nm area that is normally bright; and not relative to the broad swath up to 1000 nm area to speak of:
Compare that with the ‘CO2 notch’ at 15000 nm that we are told will hold in all the IR and burn the planet to a cinder:
That graph is from a satellite looking down at Niger at noon per:
Figure 7-8 Terrestrial radiation spectrum measured from a satellite over northern Africa (Niger valley) at noon. Blackbody curves for different temperatures are included for comparison. The plot shows radiances as a function of wavenumber (n = 1/l). The radiance is the radiation energy measured by the satellite through a viewing cone normalized to unit solid angle (steradian, abbreviated sr). Radiance and fn are related by a geometric factor. Major atmospheric absorbers are identified. Adapted from Hanel, R.A., et al., J. Geophys. Res., 77, 2629-2641, 1972.
There are big notches for CO2 and Ozone. I note in passing that the Global Warming Hysterics never like to talk about natural variation of ozone with solar cycle changes and how that might matter… vis our present loss of ozone as the sun has gone all sleepy in the UV bands and the stratosphere has gotten cold. Also note that at the 8000-9000 nm band and in the 10000 – 13000 nm band we can see the surface happily dumping loads of heat at 320 K ( about 47 C ) so the CO2 isn’t stopping that hot surface from radiating like crazy… Below 15000 nm, at 18000 nm, we again have increased radiation in longer IR wavelengths.
To me, this just says that at high noon the CO2 is absorbing one heck of a lot of incoming IR and transferring that excess heat via conduction to the other gases in the air. That is, it is stopping some of the IR at 15000 nm from reaching the ground more than it is ‘re-radiating’ it as ‘downwelling IR’. Basically, the solar levels of sunlight are so high that all the OTHER wavelengths look high in comparison AT NOON as they bounce off the dirt, water, and clouds. But as we see from space, the CO2 absorbs some, but does not re-radiate as hot gas.
So, seen from deep space, we look mostly like land and water with a more or less average amount of water vapor in the air, and little to no indication of any odd ‘notches’… What happened to the CO2 effect?
So I was looking at this article, that is mostly just a defense of the AGW thesis vs the “atmospheric pressure did it” thesis:
This post is prompted by recent posts by Steve Goddard on WUWT about the GHE and the lapse rate on Venus. They muddle the effects, in a way that is quite often seen in the blogosphere. The meme is that surface warming is due to the lapse rate and not to the GHE. Often on WUWT this comes down to even more simplified assertions that warming is due to atmospheric pressure.
UPDATE: Discussion below has convinced me that I had this wrong as I first looked at it. (Not that it matters, it mostly had just caused me to find the ‘net flux’ map below that is what I think really matters). So I’m rewriting this section a bit to look at the 15,000 nm notch as largely irrelevant rather than the original stimulus to look for the net flux graph.
Where I ran into this graph. Stare at it for a minute or two. Look particularly at the measured IR Temperature in the strongest “CO2 Notch” at 15000 nm ( 15 micrometres). (The absorption lines below 6000 are off the chart and the 9500 nm area is too little absorption to be of interest). WHY is it lower when looking down from 20 km and higher when looking upward from the ground?
IF the CO2 in the air both absorbs and emits at the same spectra, if it really IS keeping all the IR in, ought not we see the same IR when we look “down” at that CO2 (since it emits in all directions) as when we look up at it? ( In the 9500 nm area, we do come close, with both up and down looking to be about 240 K -/+ 10 K ).
UPDATE: At this point, where I had been all interested in asking “Where is the radiation coming from?”, I’m going to instead make the point, as comments showed, that the 15,000 nm notch is actually blocking. Frankly, it strengthens the case that CO2 is not relevant, IMHO. SO, OK, the ‘look down’ shows CO2 blocking at high altitude (colder air temps) while the look up shows it blocking at low altitude (hotter air temps). So what happens? The CO2 just radiates around the energy until such time as the CO2 whacks into some other molecule, that then radiates the heat out in one of those OTHER bands that isn’t blocked, OR, drives convection of the troposphere to move water vapor to altitude and dump heat to the stratosphere (where below we will see the ‘net flux’ is fairly smooth).
What we see is that the atmosphere is relatively cold and NOT emitting well in the CO2 15000 nm notch. Not much “downwelling” going on from a poorly emitting gas… It is shown as about 225 K ( subtract 273 so about -48 C ) and you just don’t get a lot of heat off of something at that temperature.
When we look up, we see about 267 K, or just a couple of degrees below zero C. How can this be? (Not a lot of heat sources at a polar ice sheet and not a lot of warm air rising…)
UPDATE: It can be by virtue of the fact that the graph was from summer and relative warmth of 0 C existed near ground level.
How can the “magic gas” radiate more down than it does up?
So the CO2 is blocking IR. Sufficiently that there is no effective communication between those high and low CO2 molecules (or they would approximate as they exchange energy). That implies the heat from the CO2 is leaving via conduction to other gases and surfaces that then radiate, evaporate, or convect.
Look at the height at which the ‘look down’ was done. 20 km. Golly, that is way up there… but not high enough. What we have here is a simple demonstration that the CO2 in the lower atmosphere acts to prevent IR radiation in the lower troposphere (IMHO via conduction to the rest of the air of any absorbed energy), but does nothing to stop radiation at the top of the stratosphere / bottom of the stratopause.
When we look at the temperature profile of the air, we see a cold lower level, and a nice ‘about zero’ layer a bit above 40 km (and above the ‘max ozone’). What this says to me is that we have a very simple existence proof that the CO2 in the troposphere is rather irrelevant. Convection takes the heat right past it, up to that stratosphere level, where it is then radiated at high altitude.
As the stratospheric temperatures go, so goes the earth… and convection sends the heat to the stratosphere / tropopause. The heat then gets globally circulated and mixed as it is transported to the poles and dumped.
If you would understand the earth’s IR emissions you simply must look at the stratosphere and how heat is transported to it, and by it. Heat accumulating at the equator, being transported via the stratosphere, and dumped to space at the poles.
A graph embedded in that article from:
Showing net energy flux. Heat GAIN at the equatorial regions, heat loss at the poles. There has to be transport in between those areas. Air travels TO the equator from the poles in the troposphere, it’s going to be relatively hot equatorial air that rises to the stratosphere and moves to the poles (where it then sinks and causes a lot of very interesting weather effects, including the polar vortex.)
Notice how lacking in any ‘texture’ is this graph. Little evidence for low altitude artifacts. Little impact from storms, land forms, etc. The net flux is largely leaving in a well mixed layer. What might that be? What layer is well mixed circumferentially, but more slowly mixed pole to pole?
has a nice animation of it.
Stratospheric Circulation Simulation
Almost all air enters the stratosphere over the tropics. A slow, mean vertical circulation, called the Brewer-Dobson circulation, lofts air over the tropics from the troposphere into the stratosphere. Air lofted into the stratosphere then moves either to the north or the south where it drops back down to the troposphere, completing the circulation by moving back towards the tropics.
A second, faster, horizontal circulation is active in the stratosphere. This stratospheric circulation moves from east to west around the equator and changes directions to west-to-east towards the poles. The net result is that particles transported out of the tropics may cover the globe in only two months time.
So if you want to know how the earth sends IR to space and what controls our temperatures; look to the stratosphere. At ground level we’re just in the side effect realm. It is only AFTER all the interesting processes have happened that the descended polar air gives us our weather, an artifact that the AGW Global Warming Hysterics call “climate”.
What controls the stratosphere and upper altitude ozone is what controls the heat radiation to space. AFTER that, the decent of that cooled air to ground level, where it is finally below the largely irrelevant CO2 “blanket”, does nothing much of interest to global energy balance; until it has once again returned to the tropics to be lofted back to the stratosphere.
Looking at surface temperatures in the temperate zone is looking at an artifact of stratospheric changes and the CO2 overhead in the troposphere is entirely irrelevant to the process.
The question has been raised if CO2 would not entirely absorb all the infrared in short order, such that it would not be possible to actually detect (or ‘see’ it, as above) the IR of a layer of air some distance away. That CO2 makes the air effectively opaque to IR in the bands where it absorbs. There are two points that make that assertion a bit, er, irrelevant to the ‘seeing’.
First off is the very narrow actual absorption ‘bands’ of CO2. It is in VERY narrow spikes with wide areas of transmission between them. There’s plenty of opportunity for IR to be emitted in bands just a bit each way (via broadening or via other species) and for that IR to propagate to the sensors that formed the broad spectrum plotted above.
This graph shows the percent transmission of IR in various bands. Notice just how narrow the ‘dips’ labeled CO2 really are. Most of the dips are from water, H2O. Notice that between 2 and 5 micrometres there are plenty of peaks with about 80% or more transmission. Also, remember that we saw in the chart above that past 15 the absorption again drops ( transmission increases in the LW IR band).
The other “minor” point is the existence proof of things that use IR to see, right through that CO2 laden air. From the Wiki:
3-8 µm In guided missile technology the 3-5 µm portion of this band is the atmospheric window in which the homing heads of passive IR ‘heat seeking’ missiles are designed to work, homing on to the Infrared signature of the target aircraft, typically the jet engine exhaust plume
8–15 µm This is the “thermal imaging” region, in which sensors can obtain a completely passive picture of the outside world based on thermal emissions only and requiring no external light or thermal source such as the sun, moon or infrared illuminator. Forward-looking infrared (FLIR) systems use this area of the spectrum. This region is also called the “thermal infrared.”
While I’m sure all sorts of folks can come up with all sorts of hypothetical, theoretical reasons, and perhaps even models to show that the IR simply MUST be absorbed and can not leave, I think I’ll go with the folks who make missiles and have their lives depending on seeing through that IR band. Well, that, and those gaping holes of 80% transmission… The Abrams Tank has a thermal imaging system and was shooting other tanks at a couple of thousand meters, so I’m pretty sure the thermal band is ‘transparent enough’ for an imager at 20,000 km looking down to see the thermal image of the air below it, and one looking up to see more than a few metres of ground source air. Another analogy: I get sunburns, despite the abysmal UV transparency over at that low nm end of the scale… The Ozone does not absorb it all, nor does the CO2 absorb all the IR.
So while the 15,000 nm notch may well be blocked, there are plenty of adjacent areas where other IR can leave and gas collisions can move the heat to other molecules for radiation, or for convection to where it can be radiated.
The heat leaves, it leaves at altitude, and that is where the stratosphere is warmest and the IR photons have the least obstructed path to space. Most of the heat is delivered to altitude via convection / condensation, and CO2 is largely unimportant.