A Unique Experiment of Planetary Dimensions

Heat Seeker

One reason that Callendar’s 1938 paper did not inspire follow-up was that scientists could think of nothing to do to test the CO₂ theory. In the short run it did not appear to be true, as through the 1940s global temperatures fell. And as Callendar had pointed out, the oceans might well soak up enough excess CO₂ so that even though the greenhouse effect is real, it is too small to matter. Then came a world war. New instruments and methods, whose origins lay in their military applications, brought a revolution in the practice of science.

During the war and in the Cold War years that followed, infrared radiation became the subject of intense study and generous government funding, as detecting and interpreting the heat radiation given off by a jet engine or a human body had obvious military benefits. One result was the heat-seeking missile, an early version of which the United States nicknamed the Sidewinder for its curving and ominous flight pattern.

One of the most accomplished infrared researchers of the postwar years was Gilbert N. Plass, who wrote six books and more than one hundred technical articles on infrared radiation and a wide variety of other topics in physics. Early in the 1950s, like Hulburt and Callendar before him, Plass realized that enough new information had become available to allow a reappraisal of the CO₂ theory of climate change. In addition, a new machine, the digital computer, could reduce the tedium of the calculations.

Plass had read Tyndall, Callendar, and the meteorologists who had dismissed the CO₂ theory. In a 1956 paper, he cited as an example C. E. P. Brooks, who had written in 1951 that scientists had abandoned the theory “when it was found that all the long-wave radiation absorbed by CO₂, is also absorbed by water vapour.”¹ With the primitive instruments available to them, the early spectroscopists saw infrared absorption as taking place in broad, virtually opaque frequency bands, as though blocked by a board fence with no gaps between the boards. By Plass’s day, scientists had learned that CO₂ and other molecules have complex, multifaceted spectra whose effect more resembles a picket fence, some of whose laths are exceedingly thin. Plass understood that “the [carbon dioxide saturation] argument neglects the 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 CO₂ to absorb. 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 carbon 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 CO₂ theory of climatic change.” Emulating Arrhenius and Callendar—but using 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 CO₂ concentration is doubled and the surface temperature must fall 3.8°C if the CO₂ 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 CO₂ concentration to double? Plass identified mankind’s activities, which were adding “[6 billion] tons per year of CO₂ 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 CO₂ concentration was rising at about 90 ppm per century, Plass reported. (Today CO₂ 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 burn all the then-known coal reserves, which amounted to 10 trillion 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, instruments, 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 warming 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 CO₂ theory to point to obsolete fifty-year-old spectrographic findings 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 CO₂ theory into the realm of modern science.

As if to ensure that no one missed Plass’s conclusions, in 1953 Time ran an article saying that

this spreading envelope of gas around the earth, says Johns Hopkins Physicist Gilbert N. Plass, serves as a great greenhouse. Transparent to the radiant heat from the sun, it blocks the longer wave lengths of heat that bounce back from the earth. At its present rate of increase, says Plass, the CO₂ in the atmosphere will raise the earth’s average temperature 1.5° Fahrenheit every 100 years.⁶

In his review three years later, Panofsky would refer to Ångström’s work but not Hulburt’s.

Contaminating the Atmosphere

By the mid-1950s, the big question mark was how much CO₂ the oceans had absorbed and how long a molecule of atmospheric CO₂, once dissolved in the ocean, would stay dissolved. A byproduct of Cold War research provided the answer. Even more critical to national security than infrared radiation was the ability of the United States to detect Soviet nuclear tests and to learn what type of bomb the Russians had exploded. One important clue was the amount of an isotope of carbon, C-14, made in atomic explosions and present in the fallout. Fortunately, C-14 had other benefits.

C-14 is radioactive and decays with a half-life of 5,730 years, allowing scientists to use the isotope to date archeological objects. After about fifty thousand years, C-14 decays to undetectable levels. It did not take scientists long to realize that they could also use C-14 to trace the source and measure the age of carbon not just in manmade objects but also in various earthly environments.

During the 1950s, Hans Suess (1909–1993), the grandson of Eduard Suess, whom we met in part 2, was working for the U.S. Geological Survey measuring the amount of C-14 in ancient tree rings. By counting the rings inward, scientists could date them independently and use the result to calibrate the C-14 age obtained from the same rings. In 1955 Suess reported that some modern wood revealed “contamination” from “the introduction of C-14-free CO₂ into the atmosphere by artificial coal and oil combustion.”⁷ But how did Suess know that the contaminating “C-14-free CO₂” had come from burning fossil fuels? Coal and oil come from organisms that died millions of years ago. Their original C-14 has long since decayed to undetectable levels. Thus the only source of C-14-free carbon in the atmosphere is the combustion of ancient fossil fuels. This was the first proof that human activities are changing the composition of the atmosphere. The dilution of atmospheric CO₂ by ancient plant carbon was then and remains today dispositive evidence that humans are the cause of the observed temperature rise.

Suess and others soon realized that radiocarbon provided a way to trace the movement of CO₂ between the atmosphere and the oceans—and even the movement of water masses between the shallow and deeper parts of the ocean. Within a few years, at the invitation of its dynamic director, Roger Revelle, Suess had moved to the Scripps Institute of Oceanography in La Jolla, California.

No End in Sight

Following the lead of Callendar and Plass, Revelle (1909–1991) had become interested in the role of CO₂ in climate. In 1957, he and Suess published a classic paper in which they concluded that

the average lifetime of a CO₂ molecule in the atmosphere before it is dissolved into the sea is of the order of 10 years. This means that most of the CO₂ released by artificial fuel combustion since the beginning of the industrial revolution the oceans must have absorbed. The increase of atmospheric CO₂ from this cause is at present small but may become significant during future decades if industrial fuel combustion continues to rise exponentially.8

The carbon chemistry of seawater is extraordinarily complex, but the upshot is that seawater is “buffered” against changes. When seawater absorbs CO₂ from the atmosphere, the pH (acidity) of the water changes, which causes other changes, and so on, the effect rippling through a chain of reactions whose result is to move the pH partway back toward where it started. Seawater absorbs CO₂, yes, but it also expels CO₂ back into the atmosphere. As an incoming molecule of atmospheric CO₂ dissolves in the oceans, we might say that it meets another molecule on the way out, traveling back into the atmosphere. The net effect is to cause much more of the carbon emitted from fossil fuel combustion to wind up in the atmosphere than the ten-year average lifetime would indicate.

At the end of their article, Revelle and Suess wrote that “the probably large increase in CO₂ production by fossil fuel combustion in coming decades” would cause “a total increase of 20 to 40 percent in atmospheric CO₂.”⁹ Their estimate assumed “a constant rate of addition of industrial CO₂” of about 0.8 ppm per year. This turned out to be a serious underestimate.

The import of research on the chemistry of CO₂ in seawater was that humanity could not depend on the oceans to absorb an unlimited amount of added carbon dioxide. Revelle summed up: “Human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future.”¹⁰

In 1956, when two of his papers on CO₂ appeared, Gilbert Plass worked at the Lockheed Aircraft Corporation in southern California. Not far away at the California Institute of Technology, a young postdoctoral fellow named Charles David Keeling (1928–2005) had read Plass’s papers and become curious about the amount of CO₂ in the atmosphere. The astute Revelle soon recruited Keeling to Scripps. It took only two years of measurement for Keeling to find that the amount of CO₂ in the atmosphere was indeed increasing. At that time, the atmosphere contained about 315 ppm of CO₂. Keeling and his successors, one of them his son, have continued their measurements to the present day, but never once has the annual level of CO₂ in the atmosphere fallen. On May 9, 2013, at the height of the spring upswing in CO₂ emitted by plants, atmospheric CO₂ exceeded 400 ppm for the first time in human history. Following the seasonal cycle, it then began to decline. But as surely as the Sun will rise tomorrow, for the indefinite future each successive monthly peak and trough in the Keeling Curve will be higher than the ones before. Within a year or two of the publication of this book, the yearly average will exceed 400 ppm, and by the early 2030s it will reach 450 ppm. How long and how far CO₂ levels will rise after that, no one can say. At the moment, no end is in sight.