Geoscience Reference
In-Depth Information
hundreds of spectral lines from carbon dioxide that are outside [the] interval of
complete absorption.”
2
Plass also joined the line of scientists who had dispelled the argument that water
vapor would absorb all the available heat radiation, leaving none for CO
2
to ab-
sorb. Because the amount of water vapor in the atmosphere falls off sharply with
elevation, he said: “Even if the water vapor absorption were larger than that of car-
bon dioxide at the surface of the Earth, at only a short distance above the ground
the carbon dioxide absorption would be considerably larger than that of the water
vapor.”
3
With the two long-standing objections obviated, Plass was ready to “reappraise
the CO
2
theory of climatic change.” Emulating Arrhenius and Callendar—but us-
ing the MIDAC digital computer at Michigan State University—Plass calculated
the amount of upgoing and down-going infrared radiation in 1 km bands from the
surface up to 75 km. He found that “in order to restore equilibrium, the surface
temperature must rise 3.6°C if the CO
2
concentration is doubled and the surface
temperature must fall 3.8°C if the CO
2
concentration is halved.” Such temperature
changes, Plass wrote, “are sufficiently large to have an appreciable influence on
the climate.”
4
But what process could cause CO
2
concentration to double? Plass identified
mankind's activities, which were adding “[6 billion] tons per year of CO
2
to the
atmosphere.” He estimated that during the 1950s average global temperature was
increasing at a rate of 1.1°C per century. Today we know that Plass's estimate was
high, though in the right direction. Atmospheric CO
2
concentration was rising at
about 90 ppm per century, Plass reported. (Today CO
2
is rising at a rate of about
250 ppm per century.) If fossil fuel consumption continued at the then-current rate,
could humans consume all the coal that they could ever find, he wondered? Plass
calculated that to burnall the then-known coal reserves, which amounted to 10tril-
lion tons, would take “less than 1,000 years” and raise global temperature more
than 7°C.
5
With the advantage of hindsight, we can see that Plass's use of new methods, in-
struments, and data marks a turning point in the history of global warming science.
From then on, scientists who wished to understand the potential for global warm-
ing would have to build better quantitative models and, given their complexity, test
the new models on increasingly powerful computers. No longer would it suffice
for critics of the CO
2
theory to point to obsolete fifty-year-old spectrographic find-
ings or to indulge in what amounted to little more than arm-waving and the citing
of one another as authorities. In this sense too, Plass brought the study of the CO
2
theory into the realm of modern science.
