Friday, December 31, 2010

On greenhouses and the Greenhouse Effect

From time to time, some people get excited about the fact that the greenhouse effect seems wrongly named. An extreme case is the paper of Gerlich and Tscheuschner, where they devote a whole section 2 (19 pages) to the question. It continues to arise; Judith Curry touched on it in a recent thread, where discussion turned to the old Wood's experiment

The standard response correctly says that none of this matters. The atmospheric  GHE is what it is, and is no less true if it turns out that the allusion to greenhouses is inaccurate. Wikipedia takes this approach, with a refreshing brevity and directness.

However, there is a little more to the story (which still says nothing about the truth of the GHE). Greenhouses do work mainly by blocking heat loss through convection. But IR flux blocking is not totally insignificant.

Wood's experiment

The description of this in the 1909 Philosophical Magazine is indeed brief. Some people give the impression that he built a greenhouse out of rock salt, but the geometry seems to be simpler and on a much smaller scale. Still, he concluded:
It seems to me very doubtful if the atmosphere is warmed to any great extent by absorbing the radiation from the ground, even under the most favourable conditions.

This final statement is little emphasised, but seems to be amply justified by the sketchiness of the note:
I do not pretend to have gone very deeply into the matter, and publish this note merely to draw attention to the fact that trapped radiation appears to play but a very small part in the actual cases with which we are familiar.

Indeed he didn't.

Modern Greenhouses

Ironically, the situation that forced Wood to use rock salt as an IR-permeable material is now reversed. In his day, greenhouses were always made of IR-blocking glass. Now they are often made of IR-permeable plastic. And, in support of Wood, they do work. They block convective heat transport, and also provide an insulating effect to limit conduction through the boundary.

However, a moderate amount of heat is still transferred by IR. Ironically, a reason why it isn't larger on balance is because of the countervailing downwelling IR, also a subject of occasional skepticism. Downwelling IR, from the atmosphere and especially clouds, is not that much less than upwelling. Glass blocks both upwelling and downwelling; the nett flux blocking is small, but if you are focussing on the fate of heat emanating from the surface, the IR fraction that is blocked us a larger fraction.

A real greenhouse issue

A covering that blocks IR actually makes a real improvement to greenhouse efficiency. It is more expensive, and people are willing to pay for it. Here are some industry observations:
NSW Ag Dept:
For example, films may be used to exclude ultra violet (UV) light for chemical free pest control or reflect long wave infra red (IR) radiation to improve heat retention at night. ... Long wave radiation (2500-40000 nm) absorbers reduce the loss of heat radiated from materials and objects (including plants) inside the greenhouse.

Or this advice
You can also buy a plastic film with an infrared inhibitor; it cuts heat loss inside the greenhouse by up to 20% on a cloudless night.

Actually, it would also cut loss on a clear day.

And here is a pamphlet which describes the practicality in some detail (for heated greenhouses, but the principle is the same).
The IR / anti-condensation treated films cost about $0.015 per square foot more than untreated films but reduce energy use by 15 to 20%.

Scale Issues

The main thing wrong with the Wood's style reasoning is scale. IR blocking is a minor effect on the scale of a greenhouse because convection is relatively much more effective. The temperature gradients are huge compared to what is maintained in the atmosphere. Wood compounded this by experimenting on a much smaller scale again.

In the atmosphere, IR transport is more important than convection, so the blocking effect of GHG's matters much more.


Friday, December 24, 2010

Merry Christmas and happy holidays to all

It isn't freezing everywhere

Thursday, December 16, 2010


I should start with an apology for light posting lately. We have a family gathering with folks from overseas, but that wasn't the obstacle. We've been hassling to get some building work finished before the occasion (now done).

This post won't say much about the actual issue of ocean acidification. Rather it's a response to something that seems to always happen when the topic arises. Someone inevitably contends that, no, the ocean can't be acidifying because it's alkaline. pH>7 and all that.

There was a long discussion here, for example. What prompted me to post was seeing it happen recently on Judith Curry's blog. It's a sterile topic - a bit like arguing about whether the greenhouse effect is well named. But there's a bit of science in it. The argument can be answered on four levels.

The practical argument

The meaning is well-known. CO2 in solution will tend to dissolve calcium carbonate, thus disrupting life-forms. It may have other biological effects.

As with the greenhouse effect, human discourse doesn't require literal exactness. We can speak of currency inflation without arguing about whether dollar bills are getting bigger. etc.

The language argument

The objection is that you can't acidify something if it doesn't become "acid". But that just isn't normal usage. If you beautify something, it doesn't have to become beautiful. We can be enriched without becoming rich.

The theoretical chemistry argument

OK, getting more substantive. The notion that acidic is identified with pH<7 invokes an old notion of acidity. Since about 1923, the more general process going on in acid-base chemistry has been recognised as sharing an electron pair, rather than anything specific with protons. When sulphur trioxide reacts with calcium oxide to produce calcium sulphate, it is easy to recognise this as an acid-base reaction, with SO3 as the acid. No hydrogen is involved.

This is pretty much the case in the ocean. The overall reaction is something like:

CO2 + CaCO3 + H2O → Ca++ + 2HCO3-

Water is a reagent, and there may be a role for protons. But it isn't clear why pH 7 should matter in any way.

The aqueous chemistry argument - buffering

pH 7 is the neutral point of a particular acid-base equilibrium - in pure water. It also applies when a strong acid neutralises a strong base. But the ocean is not pure water, and does not have strong acids or bases.

A solution with substantial concentration of a weak acid and its corresponding base is described as buffered. That is because the pH is stabilised near the neutral point (pKa) of that equilibrium. If you add a strong acid, the protons will react with the corresponding base to produce more of the weak acid. Similar if you add a strong base. The pH changes little, even though real acid-base reaction occurs.

That's why pH 7 is irrelevant here. The ocean is dominated by a 3-way buffering involving CO2, bicarbonate and carbonate ions. There is a further solubility equilibrium between Ca++, CO3-- and solid calcium carbonate (eg aragonite). Adding CO2 pushes everything in the acid direction, which reduces CO3-- concentration and tends to dissolve CaCO3. It's a bit more complicated because the solubility equilibrium is not exactly attained - it is fairly easy for CaCO3 to be supersaturated in solution. But that is the direction.