Thursday, May 03, 2007

Climate greenhouse: The power of bad metaphors

It's surely obvious by now that the full range of heat flow and temperature modifications in the lower atmosphere due to water and convection should add up to what is commonly called the "greenhouse effect" - and they do, more or less. But the metaphor and the implications latent in it badly distort the reasons for why the Earth is as warm as it is and lead to endless misunderstandings and exaggerations.

How a greenhouse works. A greenhouse is a hothouse for plants, specially equipped with a glass roof to let sunlight in. It's typically well-watered too. However, contrary to legend, it does not trap significant infrared (IR) radiation. The temperature enhancement in a greenhouse is partly due to the watering, but mainly due to the trapping of convective heat currents that rise but then can't escape through the roof. This was proved in 1909 by R. W. Wood's experiment with two greenhouses, one with a glass roof, the other with a rock-salt roof. The former absorbs heat radiation, the latter does not. There was only a small difference in temperature.

The lower atmosphere is not a greenhouse. It should also be obvious from the previous weeks' postings that the Earth's climate does not work this way. Convection plays an important but still secondary role in heat transport - its main role is to warm the cloud bottoms, but also to efficiently get rid of heat by transporting it through to the cloudtops, where it's radiated away into space. Because temperature stops falling with altitude at the tropopause, there is a greenhouse-like "lid" on convective heat flow. But at that altitude, virtually all heat transport is by radiation anyway, in air that's very dry and transparent to IR radiation. The heat gets out.

The lower atmosphere does not trap heat. The real damage that the misleading "greenhouse" metaphor is not only the wrong explicit analogy - the metaphor carries with it another, totally wrong implication that heat is somehow "trapped" in the lower atmosphere, like locking someone in a closet. You hear this constantly: carbon dioxide (CO2) - like water vapor, although that's mentioned far less often - is a "heat-trapping" gas, which is nonsense. Occasionally, someone will take stab in the right direction by calling them "heat-absorbing" gases, which is at least half-true. But the other half is that molecules good at absorbing IR radiation are also good at emitting it.* What these molecules really are is exceptionally good at passing the heat along, like an efficient bucket brigade. That's why they steepen the temperature slope (lapse rate). And so here on this blog, molecules of this type - like water, carbon dioxide, and methane (CH4) - will be henceforth referred to as "IR-active," a shorthand meaning they are very efficient at absorbing and emitting heat radiation. Very soon, we'll take a closer look at how steepening the lapse rate affects climate.

Heat is not trapped in the lower atmosphere, and the energy flows in and out are never far from balance. Visible light flows in and is used in certain ways, is transformed into longer-wavelength radiation or temporarily converted to convection, catalyzes the release of latent heat from water, then flows out. The temperature distribution is a result of this interlocked network of heat flows.

Don't settle for bad metaphors. Textbooks often do flag the faulty "greenhouse" metaphor and at least attempt to substitute something less objectionable, like "atmosphere effect." Even that, improvement that it is, doesn't convey the real complexity of the Earth's enhanced temperatures. The enhancement, as the recent postings have explained, is the net result of a whole bundle of effects that both raise and lower temperature. With the exception of clear-air convection, all of them are associated with water.** If we want to give an accurate name to this collection of heat-flow modifications, the best bet is something like "The Water-Convection Effect." Each piece of it should get its own metaphor - like "steambath effect" (evaporation-condensation cycle), "shiny blanket effect" (clouds), "boiling effect" (convection), and "tropospheric lid" (tropopause).

Proper points of comparison. Finally, two further distortions are dragged along with the "greenhouse." The first is the misleading comparison of the actual surface of 288 oK to the fictitious 250 oK surface with infinitely thin but still reflective clouds - the Earth's surface with only the solar radiation coming in that doesn't get reflected, but missing the latent heat of evaporation-condensation. While useful as an accounting device, the comparison is physically meaningless. The right comparisons are:
  • Dry atmosphere: T(surface) = 279 oK = 6 oC = 43 oF
  • Wet atmosphere with evaporation, but no clouds: T(surface) = 304 oK = 31 oC = 89 oF
  • Wet atmosphere, with evaporation and thin, reflective clouds, but no convection: T(surface) = 269 oK = -4 oC = 25 oF, T(clouds) = 247 oK = -26 oC = -15 oF
  • Wet atmosphere, with all the effects of water included, plus convection (our climate): T(surface) = 288 oK = 15 oC = 59 oF, T(cloudbottoms) = 275 oK = 2 oC = 36 oF, T(cloudtops) = 250 oK = -23 oC = -9 oF
Water vapor is overwhelmingly the most important "greenhouse" gas. The second misbegotten notion is a direct result of the hysteria surrounding carbon dioxide, a minor constituent of Earth's atmosphere - everyone forgets, water is the main star when it comes to temperature enhancement, withe the key supporting role played by convection. Carbon dioxide is like the bit-part actor who gets a couple lines.***

Where did the "greenhouse" come from? The "greenhouse" idea regarding IR-active gases has been around in some form for over a century. But the modern version was hatched in the 1950s with attempts to understand the intense heat of Venus. An upcoming posting about Venus will delve into this history a bit. We'll see then why Venus' atmosphere has little to tell us about Earth's, they're so radically different - but we'll also see that Venus' atmosphere does hold one important lesson rarely noted. We'll also meet Mars' more Earth-like atmosphere and learn a different but equally important lesson.
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* Kirchhoff's law of radiation again.

** Convection can exist without clouds, but clouds as we know them cannot exist without convection.

*** Methane is then like the silent character in the back who looks menacing, but never actually does anything.

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