This is the 3rd episode in a series recounting the history of measurements and data related to Global Climate Change. If you’re just joining, you can catch up on the previous episodes:
- Episode 1: Beginnings (or two British scientists’ adventures with leaves and CO2 measurements)
- Episode 2: First measurement of anthropogenic global warming
Climate enthusiast Guy Callendar continued to find time, around his day job as a steam engineer, to conduct and publish multiple research studies between 1940 and 1955, proposing increasing evidence of a linkage between fossil fuel use, rising atmospheric CO2 concentration, and warming global surface temperature (G. Callendar, 1940, 1941, 1942, 1944, 1948, 1949, 1952, 1955). In these, Callendar continued to refine estimates of infrared absorption by CO2, catalog CO2 and temperature measurements in various regions during the period since 1850, and refine and update his calculations of the total amount of CO2 that had been produced globally by fossil fuel use. His analyses continued to suggest that most of the CO2 produced by fossil fuel combustion had directly increased the CO2 concentration of the atmosphere.
During this period, Callendar’s influential 1938 paper also served to renew the interest of other scientists in the possibility of anthropogenic global warming. Roger Revelle and Hans Suess, at the Scripps Institution of Oceanography (UC San Diego), summed up the growing interest in the subject particularly well (Revelle & Suess, 1957):
“. . . 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. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years. This experiment, if adequately documented, may yield a far-reaching insight into the processes determining weather and climate.”
Gilbert Plass, a Canadian-born physicist working in the U.S., published a series of papers in 1956 (G. N. Plass, 1956a, 1956b, 1956c, 1956d) in which he brought increased rigor to the calculation of infrared absorption by carbon dioxide in the atmosphere, aided by the new availability of high speed computers to perform complex calculations. These calculations proved wrong a widely held belief at the time, that water vapor absorbed infrared radiation from the Earth’s surface more strongly than carbon dioxide and thus controlled the “greenhouse effect.” With improved calculations, Plass showed that water vapor and carbon dioxide absorbed radiation mainly in different parts of the infrared spectrum. Also, water vapor was present primarily in the region of the atmosphere right next to the Earth’s surface, whereas carbon dioxide was present uniformly at all heights. The new calculations added physical rigor to the theory that the atmospheric carbon dioxide level strongly influences the Earth’s surface temperature. Plass calculated that a doubling of the atmospheric carbon dioxide level would lead to a temperature increase of 3.6 degrees Celsius, and that continued use of fossil fuels would cause about a 1 degree Celsius temperature increase by the year 2000, at which time we would experience easily-observed effects of climate change. As we will see, these 1956 predictions have proven remarkably accurate.
But it was not all agreement during this period. In the tradition of the scientific method, other scientists were questioning the above conclusions. Giles Slocum, a scientist at the U.S. Weather Bureau, pointed out that Callendar’s claim of increasing atmospheric CO2 relied heavily on his selection of particular historical measurements he deemed more accurate than others (G. Slocum, 1995). Slocum’s criticism was illustrated quite well by Stig Fonselius and his coworkers, operators of a network of Scandinavian CO2 measurement sites that had been set up in 1954. Fonselius, et al. (1956) cataloged a large number of CO2 measurements that had been made since the early 1800’s and prepared this graph:
As you can easily see, anyone taking the totality of the data as the CO2 record would be hard pressed to argue there had been an obvious increase over time. Callendar had argued in his papers that many of the measurements, particularly early ones, had been conducted with poor equipment and/or at locations, like the middle of large cities, likely to display elevated CO2 levels due to local sources of CO2 pollution (factories, etc.) While nobody disputed that many CO2 measurements had probably been inaccurate, Slocum argued the totality of data was not yet sufficient to prove atmospheric CO2 had been rising and that a more standardized data set was needed.
Around the same time, oceanographer Roger Revelle and physical chemist Hans Suess were starting to bring nuclear physics to bear on the question (Revelle & Suess, 1957). Their work involved carbon-14, an isotope of carbon present in atmospheric carbon dioxide but not present in fossil fuels (if you’re interested, see my primer on carbon-14). Revelle and Suess and other scientists reasoned that, if atmospheric CO2 levels were increasing due mainly to the burning of fossil fuels, the proportion of atmospheric CO2 containing carbon-14 should be decreasing. In fact, Suess did find that tree rings from recent years were depleted in carbon-14 compared with old tree rings:
But the reductions appeared lower than could be expected based on Callendar’s estimate that the atmospheric CO2 level had increased by some 6% or more. Further, using data on the carbon-14 contents of the atmosphere and of carbonaceous materials extracted from the ocean surface (namely, seashells, fish flesh, and seaweed), Revelle & Suess calculated that a molecule of CO2 in the atmosphere would be absorbed into the ocean surface within an average of about 10 years, and that the overall ocean was mixed within several hundred years. Based on the enormity of the oceans, Revelle & Suess concluded that Callendar’s claims seemed improbable. Moreover, assuming fossil fuels continued to be used at about the rate they were being used in the mid-1950’s, they calculated that the ocean would prevent anything but a modest increase in atmospheric CO2 well into the future.
Guy Callendar’s “last word” during this period was in a 1958 paper applying an additional 20 years of measurements and analysis to his 1938 catalog of atmospheric CO2 measurements, as shown in the graph at the top of this post. Dr. Browne & Mr. Escombe’s year 1900 measurements of about 290 ppm CO2 are the point labelled “d” in the plot. The atmospheric CO2 concentration in the North Atlantic region appeared to have increased to around 320 ppm by the year 1956. At the same time, Callendar (1961) and Landsberg & Mitchell, Jr. (1961) independently continued to document that the Earth, at all latitudes, had been warming over the same period:
Callendar acknowledged the contradiction between his analyses and the carbon-14 measurements, but was unapologetic:
“. . . the observations show a rising trend which is similar in amount to the addition from fuel combustion. This result is not in accordance with recent radio carbon data, but the reasons for the discrepancy are obscure, and it is concluded that much further observational data is required to clarify this problem.”
On the need for further measurements, Callendar, Revelle, Suess, and other scientists agreed. If you read the linked papers on this page, you’ll find many mentions of the upcoming International Geophysical Year (1957-1958), a period of international governmental funding of Earth sciences interestingly intertwined with a Cold War competition for scientific prestige, the launching of the first satellites by the Soviet Union and the United States, and the beginning of the Space Race. As you will see in the next episode of this series, new measurements were coming largely as a result of this funding.
This period is a confusing chapter of climate science, but it presents a terrific example of the self-correcting nature of the scientific method. Pioneering scientists like Callendar test obscure hypotheses, often relying on scant initial data. Their conclusions, if compelling, inspire other scientists both to make more measurements and to check their work. “Watchdog” scientists (like Slocum) point out deficiencies in their analyses. Scientists from other disciplines (Plass, Revelle & Suess) apply alternative techniques to see whether the results are consistent. Predictive scientists (Plass) extend the conclusions of early work to formulate predictions that can be tested. If a hypothesis is correct – if it’s the truth – then any accurate measurement will confirm it. Any prediction based on it will come true. Where there is an apparent contradiction, or where a prediction fails to come true, more measurements are needed to resolve the contradiction.
Keep this mind as we go forward. We will, of course, be applying these principles to the findings supporting the hypothesis of anthropogenic global warming. But also bear in mind that any alternative hypothesis must stand up to the same tests. It’s not enough to say, as my own Senator Ron Johnson (R-WI) did,
“It’s far more likely that it’s sunspot activity or just something in the geologic eons of time.” [Journal Sentinel 8/16/2010]
Well, okay, if it’s sunspots or something (what thing?), let’s see the data. Do measurements of sunspot activity correlate with our observations of Earth’s climate? Scientists have been thinking about and studying this since the 1800’s and making concerted measurements since the early 1900’s. We should be in a position, after all that work, to support our claims with evidence.
In upcoming episodes of this series, we will get into the data that resulted from the calls for study by Callendar, Revelle, and Suess. As for the “geologic eons of time,” we will actually take a look at that, too. Has the scientific controversy evident in this episode persisted? Or, are people who claim it’s controversial stuck in the ’50’s? Find out in upcoming episodes!
To be continued…