Tuesday, May 27, 2008

Escape from the greenhouse

What is a greenhouse? Is the Earth's climate a greenhouse? Why are infrared (IR)-opaque gases misnamed "greenhouse" gases?

What is a greenhouse? A greenhouse is an environment artificially controlled to maintain equable conditions of temperature and humidity suitable for growing plants in colder and highly variable climates. It's a controlled "climate box."

There is one source of heat in the Earth's lower atmosphere (visible and ultraviolet radiation incoming from the Sun) and three means by which the radiation, converted to infrared, can escape upward. In one case, it remains IR radiation and escapes as such. In the other two cases, convection and evaporation, the IR radiation is converted to a form of heat in matter (air and water vapor) before it moves upward. The first mechanism is, by itself, easy to understand and straightforward to control. The other two are much harder to either control or understand.

What a greenhouse does is to put a lid on the escape of heat through convection and evaporation. These two upward heat flows are trapped and turned back downward. OTOH, a greenhouse allows radiation in and radiation out unimpeded or only mildly controlled. Because radiation flow is easy to control, conditions in the greenhouse - temperature and humidity - can be regulated with a fair degree of accuracy. That's the point of a greenhouse. The walls of a greenhouse also put the kibosh on winds, shutting off another source of climate variability.

Is the Earth's climate a greenhouse? No. A greenhouse is close to a "pure radiative heat transport" situation. The Earth's climate is strongly influenced by the other two heat flow mechanisms and can't be considered a greenhouse, even as an approximation. Greenhouses are built because the Earth's climate conditions are not equable, especially in temperate regions that experience large daily and seasonal swings of temperature and humidity. The closest natural situation on Earth to a greenhouse is the tropics, and even there conditions vary a lot over the year. Upward heat convection and evaporation are, if anything, stronger in the tropics than elsewhere.*

But there's a deeper point. As concentrations of IR-opaque gases rise, they put a larger obstacle in the way of heat escaping from the surface as radiation, but they do nothing directly to affect the other heat flows (convection and evaporation). IR-opaque gases don't make the Earth's climate more "greenhouse-y" in fact. To do that would require strong limits on the other forms of heat transport, just as a real greenhouse does. But the IR-opaque gases modify the radiative heat flow - the opposite of a greenhouse.

Why the "greenhouse" effect and "greenhouse" gases? A posting last year discussed the origins of this misguided metaphor in both popular and scientific misunderstandings about heat transport from a century or more ago. In 1909, English scientist R. W. Wood proved that greenhouses don't "trap" radiation - quite the contrary.

Unfortunately, the bad metaphor stuck in decades of popular books and scientific texts on climate. Climate and weather books often flag the faulty double metaphor (greenhouses don't "trap" radiation, and the Earth's climate isn't a greenhouse anyway). But most scientists have given up on trying to fix it. Some books use other metaphors as catchphrases and mneumonics, like "atmosphere effect," for what is in fact a complex series of heat flow constrictions and diversions. Last year, I used the fairly exact analogy of a constricted garden hose.

The "greenhouse" and "greenhouse gas" language is fallacious through and through. Now that they have contributed to the rise of the "global warming" hysteria, these runaway bad metaphors have done far more damage than anyone could have imagined 50 or 100 years ago. The related bad metaphor of "heat trapping," rarely stated in explicit form, also lurks in the background and adds to the confusion.

A lesson from greenhouses about control and predictability. Armed with a correct understanding of greenhouses and why Earth's climate isn't one, we can see that greenhouses exemplify a very important point about control and prediction of climate.

Greenhouses have a steady climate inside because they're "radiation boxes." Radiation transport is the simplest part of the climate problem and, by itself, the easiest to predict. That's why greenhouses work: they rely on "radiation in-radiation out" only. Part of the trick of greenhouses is that they also shut off (or strictly confine) the other, "wilder" parts of climate, convection-turbulence and evaporation-condensation. If these forms of heat flow were allowed to roam wild and free, the temperature and humidity in the greenhouse could be not controlled or predicted. That would destroy its purpose and make the greenhouse no different from the general lack of predictability and control in the atmosphere - the real weather we face every day.

To paraphrase Foster Morrison again, the degree of isolation controls the degree of predictability. That's especially the case when climate has two parts wildness (chaotic-turbulent convection and evaporation-condensation) to one part easy (radiation). A greenhouse isolates a small piece of the atmosphere from the larger wildness outside and allows that piece to be heated and cooled by a steady and thoroughly nonchaotic flow of radiation.

The Earth's climate as a radiation box. It might be objected that viewed from the outside, the Earth's atmosphere is a radiation box. After all, there's no air or water vapor in outer space, so radiation is the whole game. Radiation flows in, and only radiation flows out. That's correct, but it doesn't make the Earth's atmosphere a greenhouse.

The ultimate reason is one of relative scales. In the Earth's atmosphere, the scale of convective heat transport is tens or hundreds of meters; the scale of evaporation and condensation (as clouds), a kilometer or so. The latter is six to 12 times smaller than the height of the lower atmosphere, the former 20 to 100 times smaller. There is no sense in which the Earth's atmosphere as a whole can be viewed as a single greenhouse - it's too big. It can fit many, many greenhouse-sized boxes. But none of these imaginary boxes would be closed; they would have to be open to air and water flows and thus not greenhouses. While they would have the right size, they would not function as greenhouses, which work because they isolate a small piece of atmosphere from the rest.

Without being closed to air and water flows, such imaginary would-be greenhouses couldn't act as greenhouses, with all their steadiness and predictability. And, because of its size, neither can the atmosphere as a whole.

POSTSCRIPT: Freeman Dyson, one of the last representatives still alive from the heroic mid-century era of physics, writes about "global warming," carbon dioxide, and plants in the New York Review of Books.
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* Thus the troposphere-upper atmosphere boundary is highest in the tropics, because of that strong upward "push." Upward convection and evaporation are weakest in the polar regions, and that same boundary is low over the poles, sometimes (during the polar winter) almost touching the surface ("sky falling to the ground").

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