Although the “Greenhouse Effect” is of crucial importance to modern climatology and is the putative cornerstone of the Anthropogenic Global Warming hypothesis, it lacks clear thermodynamic definition. This forecasts the likelihood that the name is misapplied. Even general descriptions of the “Greenhouse Effect” may seem confused when compared to one another. In the first year university geology text by Press & Siever (1982, p. 312) we read:
“The atmosphere is relatively transparent to the incoming visible rays of the Sun. Much of that radiation is absorbed at the Earth’s surface and then reemitted as infrared, invisible long-wave rays that radiate back away from the surface (Fig. 12-14). The atmosphere, however, is relatively opaque and impermeable to infrared rays because of the combined effect of clouds and carbon dioxide, which strongly absorbs the radiation instead of allowing it to escape into space. This absorbed radiation heats the atmosphere, which radiates heat back to the Earth’s surface. This is called the ‘greenhouse effect’ by analogy to the warming of greenhouses, whose glass is the barrier to heat loss.”
This explanation is fundamentally confusing because it is seemingly contradictory, as impermeable materials cannot absorb on the minute to minute timescale that applies to the “Greenhouse Effect”, even if such an impermeable material has a very high fluid storage capacity or porosity. According to Press & Siever’s explanation above, the atmosphere is relatively impermeable due to the presence of clouds and carbon dioxide, which are part of the atmosphere. How then, can the part of the atmosphere that makes it impermeable to infrared, simultaneously facilitate infrared absorption? Moreover, the idea of thermal permeability is a product of the 19th century pseudoscientific notion that heat was actually a fluid (called “caloric”). This led to a great deal of misunderstanding amongst the scientifically illiterate when it came to the findings of Fourier (e.g. Kelland, 1837). We may compare this description of the “Greenhouse Effect” with that of Whitaker (2007, pp. 17-18), which lacks the misplaced 19th century usage:
“The incoming solar radiation that the earth absorbs is re-emitted in the form of so-called infra-red radiation – this is where the vital ‘greenhouse effect’ begins. Because of the chemical structure of the greenhouse gases in the atmosphere, they absorb the infra-red radiation from the Earth, and then emit it, into space and back into the atmosphere. The atmospheric re-emission helps heat the surface of the Earth – as well as the lower atmosphere – and keeps us warm.”
This explanation describes the “Greenhouse Effect” as “vital”, perhaps because, as Whitaker points out, it warms the earth’s surface. Wishart (2009, p. 24) explains that this “Greenhouse Effect” is useful for a completely different reason:
“The Moon is another excellent example of what happens with no greenhouse effect. During the lunar day, average surface temperatures reach 107ºC, while the lunar night sees temperatures drop from boiling point to 153 degrees below zero. No greenhouse gases mean there’s no way to smooth out temperatures on the moon. On Earth, greenhouse gases filter some of the sunlight hitting the surface and reflect some of the heat back out into space, meaning the days are cooler, but conversely the gases insulate the planet at night, preventing a lot of the heat from escaping.”
In Wishart’s explanation above, the Greenhouse Effect” is no longer a warming mechanism but a thermal buffer that moderates the extremes of temperature. In fact, Plimer (2001) uses the term “greenhouse” to denote interglacial periods (e.g. Plimer, 2001, p. 80). In describing the conditions when life evolved on earth 3800 million years ago, Plimer (2001, p. 43), like Wishart, is more reminiscent of Frankland (1864) and Tyndall (1867):
“The Earth’s temperature had moderated because the atmosphere was rich in carbon dioxide and water vapour created a greenhouse.”
The above quotes demonstrate a confusing array of “Greenhouse Effect” definitions, including the first one which seems to contradict itself. Plimer (2009, p. 365) really describes this situation very well when he writes:
“Everyone knows what the greenhouse effect is. Well … do they? Ask someone to explain how the greenhouse effect works. There is an extremely high probability that they have no idea. What really is the greenhouse effect? The use of the term ‘greenhouse effect’ is a complete misnomer. Greenhouses or glasshouses are used for increasing plant growth, especially in colder climates. A greenhouse eliminates convective cooling, the major process of heat transfer in the atmosphere, and protects the plants from frost.”
The “Greenhouse Effect” was originally defined around the hypothesis that visible light penetrating the atmosphere is converted to heat on absorption and emitted as infrared, which is subsequently trapped by the opacity of the atmosphere to infrared. In Arrhenius (1896, p. 237) we read:
“Fourier maintained that the atmosphere acts like the glass of a hothouse, because it lets through the light rays of the sun but retains the dark rays from the ground.”
This quote from Arrhenius establishes the fact that the “Greenhouse Effect”, far from being a misnomer, is so-called because it was originally based on the assumption that an atmosphere and the glass of a greenhouse are the same in their workings. Interestingly, Fourier doesn’t even mention hothouses or greenhouses, and actually stated that in order for the atmosphere to be anything like the glass of a hotbox, such as the experimental aparatus of de Saussure (1779), the air would have to solidify while conserving its optical properties (Fourier, 1827, p. 586; Fourier, 1824, translated by Burgess, 1837, pp. 11-12).
In spite of Arrhenius’ misunderstanding of Fourier, the Concise Oxford English Dictionary (11th Edition) reflects his initial opening description of the “Greenhouse Effect”:
“Greenhouse Effect noun the trapping of the sun’s warmth in the planet’s lower atmosphere, due to the greater transparency of the atmosphere to visible radiation from the sun than to infrared radiation emitted from the planet’s surface.”
These descriptions of the “Greenhouse Effect” all evade the key question of heat transfer. Given that the “Greenhouse Effect” profoundly affects heat transfer and distribution, what are the thermodynamic properties that govern the “Greenhouse Effect” and how, exactly, is this “Greenhouse Effect” governed by these material properties? Moreover, all of the elements expressed in the preceding quotations can be found in Arrhenius’ proposition of the “Greenhouse Effect”. While Arrhenius credits Tyndall with the thermal buffer idea expressed in Plimer (2001) and Wishart (2009), he then goes on to express the more complicated idea described in Press & Siever (1982) and Whitaker (2007). The “atmospheric re-emission” that “helps heat the surface of the earth” of Whitaker (2007, pp. 17-18) is the key to Arrhenius’ original proposition, which revolves around the backradiation notion first proposed by Pouillet (1838, p. 42; translated by Taylor, 1846, p. 61). However, Pouillet used this idea to explain rather than add to the thermal gradient measured in transparent envelopes while, as we shall see, Arrhenius treated backradiation as an addition to the conductive (i.e. net) heat flow indicated by the thermal gradient.
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