Origins of the CO2 Theory - Four Revolutions in the Earth Sciences - page 202

of ice advance and retreat as the result of a fall in global temperature followed by a
rise, but the process had repeated at least four times. Accounting for the multicycle
ice advance and retreat proved so difficult that it took scientists until the 1970s. 17
In 1859, Tyndall summarized the concept of greenhouse warming as elucidated
by Fourier and Pouillet:
The solar heat possesses, in a far higher degree than that of limelight, the power of crossing
an atmosphere; but, when the heat is absorbed by the planet, it is so changed in quality that
the rays emanating from the planet cannot get with the same freedom back into space. Thus
the atmosphere admits of the entrance of the solar heat, but checks its exit; and the result is a
tendency to accumulate heat at the surface of the planet. 18
By this time scientists had isolated the various gases that compose the atmo-
sphere, mainly nitrogen, oxygen, water vapor, and carbon dioxide. But which of
them absorb heat radiation, and how much do they absorb? Tyndall, a great exper-
imentalist, conceived and built an instrument to answer those questions.
His ratio spectrophotometer allowed him to pass heat rays through the different
gases one at a time and measure how much radiation each absorbed. Water vapor
proved to be the strongest absorber, with CO 2 a close second. Tyndall wrote that
the two gases “may have produced all the mutations of climate which the re-
searches of geologists reveal [italics added].However,thismaybe,thefacts above
cited remain; they constitute true causes, the extent [italics original] alone of the
operation remaining doubtful.” 19
Without the benefit of the warming blanket provided by water vapor in the at-
mosphere, he wrote, the Sun over England “would rise upon an island held fast in
the iron grip of frost.” 20 Tyndall did not speculate about the possibility that CO 2
might rise enough to cause a dangerous temperature increase. Who at this still rel-
atively early stage in the Industrial Revolution, before the invention of the internal
combustion engine, could have had that much imagination?
To describe the effect of water vapor (and by implication, CO 2 ) on the Earth's
temperature, Tyndall employed a metaphor: “The aqueous [water] vapor constitu-
tes a local dam, by which the temperature at the earth's surface is deepened: the
dam, however, finally overflows, and we give to space all that we receive from the
sun.” 21
Tyndall realized not only that the absorption of radiation by gases in the atmo-
sphere raises the surface temperature of the Earth but also that since the planet
must over the long run lose as much heat as it gains, as the amount of the absorb-
ing gas increases, the elevation at which the heat is lost to space must rise. Tyndall
showed that atmospheric gases “may have produced all the mutations of climate.”
But “may have produced” is not the same as “did produce.”