Sunday, August 03, 2008

A question for phycisists

In this load of scaremongering bullshit, some nef arsehole named Andrew Simms says the following...
Greenhouse gases trap incoming solar radiation, warming the atmosphere.

I was thinking, "that's not really correct. Greenhouse gases trap the heat reflected from the earth, surely." So, maybe it is just a badly constructed sentence. However, it got me thinking...

And I realised that I didn't know the answers (or not in detail) to the following questions, so I thought that I would open them up to the floor (as it were).
  1. If greenhouse gases trap reflected heat from the Earth, do they also trap the incoming heat in the atmosphere, thus preventing it reaching the earth?

  2. If not, why not? Is it because only the reflected heat contains the correct wavelengths?

  3. If the greenhouse gases trap the incoming heat, is that why we expect to see a warming of the troposphere?

So, over to you, dear readers...

UPDATE: if you want to know why the nef report is total arsebuckets, why not try this...


Roger Thornhill said...

Maybe it is an issue of wavelength. The wavelengths coming to the earth are different from those radiated from the warmed surface perhaps.

Not only that, the angle of incidence might come into it. IR from the Sahara sands would be radiated in an even pattern at all angles from the surface I would think, so much of it hitting the atmosphere at far shallower angles than that of the incoming solar radiation ever would.

Shallower angles could mean more reflection?

Different wavelengths could mean more reflection?

Just a thought.

Steve said...

Roger has it right. The incident radiation is black body at 6000K, peaking at about 500nm, in the middle of the band where the atmosphere is transparent (insert anthropic principle here).

The re-radiation is 300K black body with a peak at 10 microns (01/20 the temperature -> 20x the wavelength, and CO2 absorbs (and thus scatters by re-radiation) strongly in the 12-18 micron range.

By scattering the outbound IR, half of what is absorbed gets sent back down again, and has to try again. This is the converse of the effect of scattering in the optical range by e.g. sulphate hazes, which prevent some energy getting in in the first place.

ntk said...

Further to Steve's comments, this image might be useful.

Oxbridge Prat said...

ntk's image is indeed helpful. You can see that the atmosphere absorbs the ultraviolet and a few bits of near infrared from the incoming solar radiation, but lets the great majority through. By contrast a large fraction of the outgoing far infrared is absorbed. The importance of CO2 is primarily that it helps fill in the hole around 15 microns in the water absorption.

The physics of the greenhouse effect is far more complex than the descriptions normally given in the popular press, which are wild oversimplifications. Of course you shouldn't necessarily assume that just because somebody uses a wild oversimplification in talking to journalists (and then is, likely as not, misquoted by them) they don't understand what they're talking about.

Unfortunately the real physics is too complex to fit in a comments section! You might try reading A Tutorial on the Basic Physics of Climate Change, which is the companion article to Monckton in the APS Physics & Society issue which has caused so much recent excitement, though you'll find some of it tricky (it assumes 2nd year undergraduate Physics at various points). This does make a few approximations, but far fewer than usual, and the basic ideas are, as far as I can tell, correct.

Frederick Davies said...

1 & 2. No, they trap re-emitted radiation from the surface or the Earth and lower atmosphere. As shown in the comments above, the re-emitted radiation has a different wavelength distribution as the one coming from the Sun. This change in wavelengths makes it possible for greenhouse gases (mainly water vapour) to absorb it and re-irradiate it (scatter).

3. The discussion above has been based on radiative physics alone, but there are other effects at work. In the tropics, the greenhouse gases are so thick (remember the water vapour?) that most of the radiation coming from the surface and lower atmosphere gets absorbed before it gets to escape, so other mechanisms are necessary for the heat to get out; one of these is convection (heat being transported together with a mass of hot air when it ascends). If you put the whole thing into a computer and try to find a solution, you get that the warming caused by changes in solar irradiation and that caused by increased greenhouse effects produce different solutions: in the second case, you get faster warming in the lower troposphere above the tropics than on the surface. The relevance of this is that it isn't happening.

Stevo said...

The word "trap" is wrong. This misunderstanding of the greenhouse effect is based on a bad analogy from a misunderstanding of the physics of greenhouses. Look up Professor Woods' experiment done in 1909.

The simplest way to explain the greenhouse effect is to start with the lapse rate - the fall of temperature with increasing height. As air rises by convection, the pressure drops, the gas expands, and so the temperature falls. (about 6C/km.) The temperature of the Earth is controlled by the temperature at the 'top' of the atmosphere (actually, for CO2 this is only about 6-10 km up) where it radiates to space, which must maintain a fixed temperature to balance the incoming radiation, and by the lapse rate which fixes the rate of change down to the ground.

Because there is more CO2, the CO2 part of the atmosphere is thicker, the top where it radiates to space about 150 m higher for every doubling of CO2 (it's logarithmic because pressure varies exponentially with height), and therefore the fixed lapse rate down to the surface gives a 1 C higher surface temperature. The 40% rise/century in CO2 levels we've seen ought to give about half that.

This CO2 induced warming is then supposedly tripled by various guessed and made-up "feedbacks", giving the 3C everyone is panicking about.

Hope that helps.

Stevo said...

Oh, yeah. And I missed answering your actual questions.

1. Water vapour does a bit, blocking about as much energy from getting in as CO2 'traps' in IR getting out. CO2 is pretty much transparent to the incoming visible light.

2. Moot. But radiation bouncing around in the atmosphere more-or-less cancels out (from any layer, as much goes up as down), it's only the fluxes with ground/ outer space at the top and bottom that really matter.

The main significance wavelengths have is that air is fairly transparent to visible light, so the air is heated from the bottom, and it is thick and opaque enough in the lower layers to IR that heat cannot escape fast enough by radiation, triggering convection. The heat flows are about equal (25W/m^2 to both greenhouse IR and convection, 75 W/m^2 to evaporation), but convection controls the rate. The precise radiation balance usually only ever matters on windless nights when convection stops.

3. I assume you're referring to the tropical upper troposphere hotspot, that's has failed to be observed. That's because the temperature rise evaporates water which carries heat away from the surface - like sweating - and dumps it into the upper troposphere. All the modellers have posited big water vapour increases because water vapour is also the strongest greenhouse gas and this multiplies the pathetic effects of CO2 into something scary.

The official version of all this is given in Soden and Held of 2006 ( and the Woods thing I mentioned can be found at