Origins of the CO2 Theory - Four Revolutions in the Earth Sciences - page 201

inclosure [space].” The re-emitted, down-going heat warms the Earth and main-
tains it at a higher temperature than if there were no atmosphere to absorb radi-
ation.
His third insight was that the atmosphere not only absorbs upgoing heat rays; it
also absorbs incoming solar rays, but less effectively. The greater these “unequal
absorbing actions,” the greater the difference in temperature between space, the at-
mosphere, and the Earth.
Over several weeks of sessions of the Académie des Sciences in 1838, Pouillet
presented the results of his mathematical analysis. He showed that if, for example,
the atmosphere absorbs three-tenths of the incoming solar rays but eight-tenths of
the outgoing heat rays, the Earth's surface would be nearly 33°C warmer than the
atmosphere.
In Pouillet's day, scientists did not know how to calculate the amount of heat
escaping from a cooling body. Understandably, he used the wrong formula for heat
loss, but he got the principle right. Pouillet recognized that if, in spite of the differ-
ential absorption of incoming and outgoing radiation by the atmosphere, the Earth
is to avoid runaway heating, the temperature at the surface must increase until it
reestablishes heat balance. For Pouillet, the atmosphere was the glass in de Saus-
sure's hotbox. But as always in science, answering one question raises others, and
a good thing it is too. In this instance, the next question was: “Which atmospheric
gases do the absorbing?”
The Iron Grip of Frost
Forty-one years after Pouillet's presentation, Jozef Stefan showed that a cooling
object loses heat as the fourth power of its temperature. Stefan based his deduction
on measurements made by a punctilious scientist named John Tyndall.
An accomplished Alpinist, like Saussure Tyndall had climbed Mont Blanc. He
was the first to ascend the Weisshorn and “made very frequent excursions on the
Aletsch glacier and went up the Aletschhorn with Mrs. Tyndall.” 16 It was only nat-
ural that Tyndall would look for a way to combine his skill in experimental science
with his interest in glaciology.
No discovery more fascinated scientists of Tyndall's day than the ice ages. The
workofJeandeCharpentier,LouisAgassiz,andothers,reportedinAgassiz's1840
Study on Glaciers , had shown that in the Northern Hemisphere a sheet of ice thou-
sandsoffeetthickhadadvancedsouthward,gougingdeepAlpinevalleysandleav-
ing in its wake telltale moraines and U-shaped valleys, as well as the mild climate
in which human civilization has developed. Scientists could explain a single cycle