Wednesday, August 20, 2008

Greenhouse Misnomer

It's a nuisance that greenhouses don't work by the greenhouse effect. Some seem to want to make it out to be some sort of catastrophe or indictment of science instead. But it is annoying nevertheless.

The greenhouse effect's existence was discovered by Fourier in 1827. Namely, the surface of the earth is too warm unless something else were going on -- as we found in our simplest climate model. Digressing a second, Fourier realized this even though it was more than 30 years before anyone knew that there were greenhouse gases. Tyndall published his experiments, that showed water vapor (H2O) and carbon dioxide (CO2) among others to be greenhouse gases in 1861. Conservation of energy is a very powerful law -- you can reach the conclusion even if you don't know how, exactly, it comes about.

In any case, the idea was that greenhouses let the sun's rays in and didn't let the air in the greenhouse radiate energy back out. The definition of 'greenhouse gas' became this -- that they were transparent to solar energy (completely or nearly so) but absorbed earthly radiation (in the infrared, rather than the visible and near-infrared of the sun).

Unfortunately for purists, greenhouses don't work that way even though 'greenhouse gases' do. The way greenhouses work is to physically trap air near the surface of the earth. The sun's rays heat the surface (by radiation) and the surface heats the air in the greenhouse (by conduction). Since the greenhouse is closed, the warm air can't rise very far and the greenhouse stays warm.

Ok, those are two word pictures. Both are nice sets of words. But ... can we do an experiment that will show us which one is right? We can, and it was published in 1909. The experiment built two different greenhouses (small, but enough to test the idea). In one, the walls were the usual glass, which is transparent to sunlight but tends to absorb 'earthlight'. In the other, the walls were halite (rock salt) which is transparent to both sunlight and 'earthlight'. If the infrared properties of the walls matter, then the two greenhouses will reach different temperatures. If it is only important that there be a wall, the halite-walled greenhouse will get just as warm as the glass-walled. (Give or take a little for how well the two conduct heat.) The result was that the halite made just as effective a greenhouse as the glass. (Wouldn't work so well as a practical matter. Why? But just as effective for what was tested.)

So greenhouses don't work by the greenhouse effect. Planetary atmospheres, on the other hand, do. It's a nuisance that the words don't really apply thoroughly. But this is English. It's not like this is the first time there has been a definition that didn't make rigorous sense. We're talking in a language where flammable and inflammable mean the same thing after all.

Project: Build your own demonstration of the greenhouse effect versus greenhouses. See
http://www.wmconnolley.org.uk/sci/wood_rw.1909.html for the original experiment and a discussion of the significance.

5 comments:

Ian said...

Halite is transparent to infrared - so that's why cooking food on a bed of salt is so effective...

Thanks, nice post.

Anonymous said...

Very useful article. I already understood that the lack of convection (trapped air) was the main forcing mechanism for a real greenhouse, but your article clarified this for me. An unfortunate use of the term greenhouse, indeed. It's very unfortunate that GW deniers pounce on this point, but so few have any scientific knowledge anyway.

Penguindreams said...

Interesting, Ian.

Two other comments that came in on the 20th have been hanging in queue. The problem is, neither supported their arguments. They also disagree with each other (as well as me) -- so how would anyone decide between the 2 (or 3)?

So what to do? I didn't want simply to reject them (by the way, Alastair, I'll hit 'reject' now -- do you get a message that it was rejected?) but they also require work from me. Things that require me to work will be delayed. Save time and support your arguments -- those go through even when telling me I'm wrong (c.f. tamino telling me about my error on the effect of eccentricity on the earth-sun distance).

A trimmed Alastair I've got some good news for you. Greenhouses do work the same way as the greenhouse effect. In fact it is the greenhouse effect which heats greenhouses.

The effect was actually discovered by Horace de Saussure, and reported by Fourier, in the course of inventing his helio-thermometer.


A problem being the one pointed out by William Connolley at Stoat, who was able to track the full description of Saussure -- it didn't really test the effect.

'fresh' wrote Ah dude, The Wood experiment shows that conduction is more important than radiation for the transfer of heat. If blocking radiation was important to keep the temperature of the greenhouse warm, the glass greenhouse would be warmer.

Take another look at Wood's full experiment. It was the physical barrier that was important, not the radiative characteristics of it.


Alastair: I also didn't know from your comment whether you were the Alastair I know from elsewhen. It isn't an uncommon name. I see from your comment at Stoat that you are. You'll recall from our previous discussions that one of the things I was often asking you for was support for your arguments. I'm glad to see that you've started up your own blog at http://complexclimate.blogspot.com/ to present your ideas more directly. I think some of the ideas you and I discussed previously would have been a better intro, but hopefully you'll get to them or your newer and better ideas.

Belette said...

Just to be picky... I don't think F discovered it in 1827. Its the one everyone cites, but its pretty well a reprint of an 1824 article; see http://www.wmconnolley.org.uk/sci/fourier_1827/

Did I say: good to see you blogging, and doing it so well? Well if not, I say it now. Now all we need is for you to descend into the snake-pit of wikipedia...

Penguindreams said...

Thanks for the good link and good word. It is right to say that he discovered it in 1824 or a bit earlier, with the 1824 being the first public knowledge of it. On the other hand, the paper I actually read (thanks to William sending me a copy) was the 1827 paper, also cited by Arrhenius and many others.

As to the snake-pit ... look for the flying pigs; they'll be your warning sign of that happening. Since I almost ran the Flying Pig Marathon a few years ago, this is less impossible than one might think. I'm planning on running a marathon for my next round-number age, and Pig is at a convenient time of year. Friends have done it and enjoyed.