Origins of the CO2 Theory - Four Revolutions in the Earth Sciences - page 200

Fourier explained the reason for the results of Saussure's capital experiment:
heat rays and light rays are different:
The heat of the sun has properties different from those of heat without light. The rays of [the
sun] are transmitted in considerable quantity through the glass plates into all the intervals, even
to the bottom of the vessel. They heat the air and the partitions which contain it. Their heat
thus communicated ceases to be luminous, and preserves only the properties of non-luminous
radiating heat. In this state it cannot pass through the plates of glass covering the vessel. 11
Fourier's interest was in the relative roles of the three sources of Earth's heat:
solar, internal, and heat from space. Scientists of his day had no way to measure
the temperature of space and wrongly believed it to be only a little colder than the
Earth's polar regions, in which case heat from space could have contributed signi-
ficantly to the Earth's heat balance. Fourier thought it at least as important as “the
non-luminous” heat rays rising from the surface.
Fourier clearly grasped the essential point of greenhouse warming: “Luminous
heat flowing in, penetrates, with little difficulty, the interior of the mass, and non-
luminous heat has more difficulty in finding a way out in a contrary direction”
(19). Fourier stood on the shoulders of Mariotte and Saussure, as others would
stand on his.
Heat Lost Must Equal Heat Gained
In 1838, the French physicist Claude Pouillet took the next step in explaining how
greenhouse warming works. 12 He was a professor at the Faculty of Sciences in
Paris and by 1838 the chair of its physics department. Like Fourier, he was unable
to avoid becoming caught up in French politics, being dismissed from his post in
1852 for refusing to swear allegiance to the imperial government. 13
Pouillet had three critical insights about the role of the atmosphere. First, for a
body isolated in space like the Earth, the amount of heat lost must balance the heat
received from the Sun. Were the Earth to receive more heat than it loses, temperat-
ure would rise uncontrollably and, among other things, water at the surface would
boil. Were the Earth to lose more heat than it receives, our planet might become
an uninhabitable snowball. As Pouillet put it: “The sole condition required for the
equilibrium [of the globe] will . . . be, that the quantities of heat lost and absorbed
may be equal.” 14 He recognized that the same principle applies to the atmosphere:
“The sum of the quantities of heat which the envelope [the atmosphere] loses is
equal to the sum of the quantities of heat which it receives.” 15
Pouillet's second insight was that the atmosphere re-emits the absorbed heat ra-
diation in all directions ,some“towardtheglobe[theEarth]”andsome“towardthe