Re: Greenhouse Misconception

[Leigh Palmer <palmer@SFU.CA>]

At 5:12 PM -0700 7/28/99, Rick Tarara wrote:

> Anything I know about this I've learned from this list as I too was a
> believer in greenhouses work by the greenhouse effect.  However, the
> convincing evidence (for me) is that if one makes a greenhouse using a glass
> or plastic that IS transparent to infrared, it works just about as well as
> one with glass.  Promote convection in a greenhouse or car and the
> temperatures will be much lower.  The GH effect IS present in a traditional
> greenhouse but the amount of convection is much more critical to the
> temperatures achieved.
>
> Rick

I didn't enter the fray on this topic because Rick did. I am pleased,
of course. Perhaps some of you might like to have an explanation from
an authority in the field, Richard Lindzen, Alfred P. Sloan Professor
of Meteorology, Department of Earth, Atmospheric and Planetary Sciences
at MIT. Dr. Lindzen tells us about:

<Quote follows>

"The Greenhouse Effect.

"The crude idea in the common popular presentation of the greenhouse
effect is that the atmosphere is transparent to sunlight (apart from
the very significant reflectivity of both clouds and the surface),
which heats the Earth's surface. The surface offsets that heating by
radiating in the infrared. The infrared radiation increases with
increasing surface temperature, and the temperature adjusts until
balance is achieved. If the atmosphere were also transparent to
infrared radiation, the infrared radiation produced by an average
surface temperature of minus eighteen degrees centigrade would balance
the incoming solar radiation (less that amount reflected back to space
by clouds). The atmosphere is not transparent in the infrared, however.
So the Earth must heat up somewhat more to deliver the same flux of
infrared radiation to space. That is what is called the greenhouse
effect.

"The fact that the Earth's average surface temperature is fifteen
degrees centigrade rather than minus eighteen degrees centigrade is
attributed to that effect. The main absorbers of infrared in the
atmosphere are water vapor and clouds. Even if all other greenhouse
gases (such as carbon dioxide and methane) were to disappear, we would
still be left with over 98 percent of the current greenhouse effect.
Nevertheless, it is presumed that increases in carbon dioxide and other
minor greenhouse gases will lead to significant increases in
temperature. As we have seen, carbon dioxide is increasing. So are
other minor greenhouse gases. A widely held but questionable contention
is that those increases will continue along the path they have followed
for the past century.

"The simple picture of the greenhouse mechanism is seriously
oversimplified. Many of us were taught in elementary school that heat
is transported by radiation, convection, and conduction. The above
representation only refers to radiative transfer. As it turns out, if
there were only radiative heat transfer, the greenhouse effect would
warm the Earth to about seventy-seven degrees centigrade rather than to
fifteen degrees centigrade. In fact, the greenhouse effect is only
about 25 percent of what it would be in a pure radiative situation. The
reason for this is the presence of convection (heat transport by air
motions), which bypasses much of the radiative absorption.

"What is really going on is schematically illustrated in Figure 1
[sorry, I can't do that - Leigh]. The surface of the Earth is cooled in
large measure by air currents (in various forms including deep clouds)
that carry heat upward and poleward. One consequence of this picture is
that it is the greenhouse gases well above the Earth's surface that are
of primary importance in determining the temperature of the Earth. That
is especially important for water vapor, whose density decreases by
about a factor of 1,000 between the surface and ten kilometers above
the surface. Another consequence is that one cannot even calculate the
temperature of the Earth without models that accurately reproduce the
motions of the atmosphere. Indeed, present models have large errors
here--on the order of 50 percent. Not surprisingly, those models are
unable to calculate correctly either the present average temperature of
the Earth or the temperature ranges from the equator to the poles.
Rather, the models are adjusted or "tuned" to get those quantities
approximately right."

<End quote>

I hope this excerpt from a longer paper will be of some value. If you
want to read more, the paper I took this from is "Some Coolness
Concerning Global Warming". It appeared in the Bulletin of the American
Meteorological Society in 1990 (I haven't got the exact citation handy).

Warning: Political diatribe follows.

Next time you are told that only one or two percent of scientists feel
the currently popular ideas about global warming are incorrect, ask the
question "What are the specialties of the 98 or 99 percent?" You will
find, I think, that they are liberally inclined scientists of many
stripes - *other* than those relevant to climate science! There is
nothing like a 98% consensus among the scientists who ought to know.
The matter is still under intensive study, and the climatologists are
the least certain group in the world on this topic, though the topic
has become a political litmus test.

Joe Sixpack wins again. He'll probably vote for Gore, too.

Leigh