Gilbert N. Plass: Climate Science in Perspective

James Rodger Fleming

Gilbert Plass was a scientist on the cutting edge of climate research in 1956. His article in American Scientist, on the recently revived carbon dioxide theory of climate change, and the role that the combustion of fossil fuel was playing in it, was aimed at a broadly educated scientific readership; it was one of six related articles he published that year and one of about a dozen he published that decade. In a 1997 interview, Plass told me, “all sorts of things came together” that placed him at the scientific forefront: new detailed laboratory measurements of the absorption bands of water vapor, carbon dioxide and ozone; theoretical developments involving the influence of temperature and pressure on infrared absorption; new information about the carbon cycle and industrial emissions; and access to a new high-speed electronic computer to facilitate complex calculations of radiative transfer that replaced the older, graphical approximations.

Gilbert Norman Plass was born in Toronto, Ontario, Canada, on March 22, 1920. He was a physicist who developed an early computer model of infrared radiative transfer and published a number of articles on carbon dioxide and climate between 1953 and 1959. He received a B.S. from Harvard University in 1941, where he recalled that his courses on geology, chemistry and physics provided an interdisciplinary foundation for his later work. He was particularly impressed by the experimental techniques of John Strong, one of his physics professors. Plass received his Ph.D. from Princeton University in 1947 and worked as an associate physicist at the Metallurgical Laboratory (Manhattan District) of the University of Chicago from 1942 to 1945. He became an instructor of physics at Johns Hopkins University in 1946 and was subsequently promoted to assistant and then associate professor. At Hopkins he conducted research on infrared radiation with funds provided by the Office of Naval Research. During his sabbatical year, at Michigan State University in 1954–55, he gained access to a large computer and realized it offered the perfect way to construct a better model of radiative transfer. In 1955 Plass moved out of academics, serving for a year as a staff scientist with Lockheed Aircraft Corporation. He then joined the advanced research staff of the Aeronutronic division of the Ford Motor Company. Ford provided him with excellent laboratory facilities where he could continue his experimental work on infrared physics. In 1960, he became manager of the research lab at Ford’s theoretical physics department and a consulting editor of the journal Infrared Physics. In 1963, he accepted a position as the first professor of atmospheric and space science at the Southwest Center for Advanced Studies (now the University of Texas, Arlington) where he remained for five years. In 1968, he arrived at Texas A&M University, where he served as professor of physics and head of the department. He is the author of six books, including Infrared Physics and Engineering (1963) and more than 100 articles on radiative transfer and climate change, nuclear fission and neutron physics, electromagnetic and gravitational action at a distance, electron emission, and electrostatic electron lenses. Plass and his spouse were active supporters of the arts, helping to establish and direct arts societies and producing a radio program in Texas. He passed away in Bryan, Texas, on March 1, 2004.

In 1956 Gilbert Plass was heir to a century of work that identified variations in the trace amounts of carbon dioxide in the atmosphere as a possible cause of ice ages and interglacial periods. John Tyndall wrote in 1861 that slight changes in the amount of any of the radiatively active constituents of the atmosphere—water vapor, carbon dioxide, ozone or hydrocarbons—may have produced “all the mutations of climate which the researches of geologists reveal … they constitute true causes, the extent alone of the operation remaining doubtful.” Thirty-five years later Svante Arrhenius published a landmark paper examining the effect of different concentrations of atmospheric CO2 on the temperature of Earth. His energy budget model, which he calculated by hand, contained estimates of the absorption and emission of terrestrial radiation by water vapor and carbon dioxide, but since infrared research was in its infancy then, Arrhenius had access to only very limited spectroscopic data.

Because of these limitations, the carbon dioxide theory of climate change was in deep eclipse in 1938 when British scientist and engineer Guy Stewart Callendar revived it and placed it on a firm scientific basis. Callendar documented a significant upward trend in temperatures for the first four decades of the 20th century and noted the systematic retreat of glaciers. He compiled estimates of rising concentrations of atmospheric CO2 since pre-industrial times and linked the rise of CO2 to the combustion of fossil fuel. Finally, he synthesized information newly available concerning the infrared absorption bands of trace atmospheric constituents and linked increased sky radiation from increased CO2 concentrations to the rising temperature trend. Today this is called The Callendar Effect.

Building on such foundations, Plass was able to take the next steps in research and provide his masterful overview of the carbon dioxide theory and its implications for the future. He established connections between the physics of infrared absorption by gases, the geochemistry of the carbon cycle, feedback loops in the climate system and computer modeling. Using recent measurements of the influence of the 15-micrometer CO2 absorption band, Plass calculated a 3.6 degrees Celsius surface temperature increase for doubling of atmospheric carbon dioxide and a 3.8 degree decrease if the concentration were halved. Contrary to the assumptions of many scientists at the time, the effect of water vapor absorption did not mask the carbon dioxide effect by any means. He used these results to argue for the applicability of the carbon dioxide theory of climate change for geological epochs and in recent decades.

Stressing the intrinsic role carbon dioxide plays in our atmosphere, Plass discussed the danger of fossil fuel burning and deforestation. The six billion tons of CO2 being added to the atmosphere each year was sufficient to cause noticeable changes in the Earth’s radiation balance and thus the climate. He noted that the observed 1.1 degree rate of climate warming per century was in agreement with the predictions of the carbon dioxide theory.

Waxing prophetic, Plass wrote that the oceans would be able to sequester only a small amount of the anthropogenic carbon, leaving the majority in the atmosphere. Accumulating atmospheric CO2 content from fossil fuel-based industrial activities would eventually result in a temperature rise of at least 7 degrees. Plass held out little hope for nuclear power—expressing an opinion that would not be widespread for several more decades. Presaging the work of Charles David Keeling, which began two years later, Plass called for new accurate measurements of the increasing CO2 concentration in the atmosphere, which he rightly estimated should be on the order of 0.3 percent per year. Plass pointed out that humanity was conducting a large-scale experiment on the atmosphere, the results of which would not be available for several generations: “If at the end of this century, the average temperature has continued to rise and in addition measurement shows that the atmospheric carbon dioxide amount has also increased, it will be firmly established that carbon dioxide is a determining factor in causing climatic change.”

There have been two interruptions (pauses, if you will) in the rise of global average temperature since 1956, and of course, Earth’s climate is influenced by more than just CO2. Other trace gases and black carbon warm the climate, and aerosols cool it. On a larger scale, the astronomical theory of orbital influences was revived circa 1976, and climate variation attributed to such factors as ENSO, the Pacific Decadal Oscillation, and solar activity (or the lack thereof) are now being widely discussed. Still, more than 50 years later, scientists agree that the uncontrolled experiment pointed out by Plass in 1956 has been verified, and a warmer future caused by the radiative effects of CO2 is in store. The cutting edge question now is, What to do about it?

-------------------------------------------------------------------------------------------------

Does Science Progress? Gilbert Plass Redux

Gavin Schmidt

Considering today’s concerns about human-driven climate change and the need to cut carbon emissions, it’s interesting to look back at a time (not that long ago) when the idea that carbon dioxide (CO2) affected climate was very much a fringe concern. Gilbert N. Plass’s 1956 article (reprinted in this issue of American Scientist) was only the start of a quite rocky road to modern respectability for an idea born in the 19th century; even he might be surprised to see how it has become completely mainstream (despite what one might read on the Internet!).

This paper, the insights it contained, and the calculations and forecasts it made actually constitute a great example of how, despite reaching bottom-line conclusions very similar to pronouncements made in the recent Intergovernmental Panel on Climate Change (IPCC) report, the science that underlies those conclusions has improved remarkably. It is also a great example of the role luck plays in determining one’s role in scientific prosperity (but more on that below).

Before discussing the detail of what was in that paper, it is worth pointing out what Plass could not have known. He did not know how fast CO2 was accumulating in the atmosphere—Charles Keeling would start his seminal measurements at Mauna Loa only in 1957. Neither did he know how CO2 had varied in the past—the first ice core results only emerged in the 1980s. But he was still able, with his understanding of infrared spectroscopy, to write a paper that qualitatively predicted both these results—although with methods that we can now recognise as being incomplete—and correctly concluded that the impact of CO2 on climate would be clear by the end of the 20th century. There are other things that we know now that he could not possibly have known—the importance of other greenhouse gases (methane in particular, which wasn’t recognised as an important contributor to anthropogenic forcing until 1974, but also chlorofluorocarbons and N2O, which have also increased dramatically because of human influence) and the role of human-emitted particulates and low-level ozone precursors.

To be sure, there is much that marks the paper out as a product of its era: There is an excessive focus on single-factor explanations of all climate changes and a penchant for what would now be considered naive back-of-the-envelope estimates of the impacts of small changes on very complex systems. And as befits publication in a popular science magazine, there is a lot of big-picture discussion, although perhaps in excess of what would be considered prudent today.

The paper revolves around three main themes: the modern day carbon cycle and the fate of human-produced CO2, the calculation of the radiative impact of that CO2 and the resulting temperature rise, and the possibilities for CO2 playing a role in climate changes in the past. I’ll review the first two of these themes and leave the far more speculative discussions about the cause of the ice ages for another time.

Plass knew that atmospheric levels of CO2 were around 300 parts per million by volume (ppmv) and correctly noted (as had Guy Stewart Callendar almost 20 years earlier) that human use of fossil fuel would lead to an increase in atmospheric levels of CO2. He was actually a little optimistic, though, in assuming that only 6 × 109 tons of CO2 per year (equivalent to 1.5 gigatonnes of carbon per year, or GtC/yr) were being emitted. Current estimates suggest that emissions in 1956 were already almost 50 percent higher than that (8.8 × 109 tons CO2 /yr or 2.2 GtC/yr).

He also understood enough of the terrestrial and oceanic carbon cycle to know that uptake of the anthropogenic carbon would be slow. He had two quite telling insights: First, although the residence time for carbon dioxide in the atmosphere (the total amount of CO2 divided by the flux in and out of the ocean) is on the order of a few years, the perturbation time is much longer—even up to a few tens of thousands of years—because of the slow uptake in the deep ocean and the buffering effects of the ocean chemistry. Second, he realised that the added carbon in the ocean would cause increasing acidification with consequent impacts on marine life (although he did underestimate how big this effect would be).

Combining the rate of increase of fossil carbon and lack of uptake in the ocean, he estimated that the CO2 levels might increase 30 percent in a century. Since 1850, CO2 has actually increased by more than 100 ppmv (36 percent above pre-industrial values), and so this appears to be a reasonable prediction. However, Plass was lucky. In underestimating both the current anthropogenic emissions and the uptake by the ocean, his two errors roughly cancelled.

Plass’s real contribution, however, is in his assessment of what that extra CO2 would do to the climate. His calculations included the fact that you have to consider the whole atmospheric column, and that despite the large amount of water vapor near the surface, there are always large parts of the atmosphere where CO2 is a very important absorber and emitter. This meant that the impact of changing CO2 would not be as negligible as had been thought over previous decades. These calculations require good knowledge of how all the various wavelengths are absorbed by each component in the atmosphere (including clouds), and how that changes as a function of temperature and (most importantly) pressure. The data for these absorption spectra have improved enormously in the past 50 years as has the capacity to do all these calculations, so one might anticipate that this is where Plass would have been most overtaken by scientific progress. However, Plass actually did a pretty good job. Converting to more modern units and doing a little publication archaeology, we can see that he estimated the radiative forcing by a doubling of CO2 in clear sky conditions at 8.3 watts per square meter (W/m2) and that in cloudy conditions it would be 5.8 W/m2. The accepted value for the global average today is around 4 W/m2 with about a 10 percent uncertainty (including both cloudy and clear-sky conditions). Thus while his numbers were a little large, they were within a factor of two of the right answer, and much closer than the near-zero impact that had been previously considered to be the best estimate.

To convert the radiative forcing into a temperature change, Plass relied on a conversion factor of about 0.43°C/(W/m2) (again, updated to more modern units). This was not independently calculated by him and referred only to the “no-feedback” case where all other atmospheric components (for example, water vapor and clouds) stayed the same. Modern estimates for the no-feedback sensitivity are a little lower (around 0.3°C/(W/m2)). The basis of his 3.6°C change for a doubling of CO2 is then seen as a combination of his over estimated forcing and a slightly high no-feedback sensitivity. Modern estimates of this number are around 1.2°C. Plass was aware of the potential for amplifying feedbacks, particularly via water vapor and cloud changes, but the quantification of these effects would have to wait another 10 years for the work of Fritz Möller and subsequently Suki Manabe and colleagues.

Thus even though the headline number in the Plass article is well within the range of the modern IPCC reports (which give a total sensitivity of between 2 to 4.5°C for a doubling of CO2), it isn’t quite fair to give him full credit since his number doesn’t include many important factors that he was not able to quantify. Nonetheless, he realized full well the importance of numerical computation for these estimates but was working at the edge of what was then possible.

Similarly, his estimate for the temperature change for the 20th century of 1.1°C was uncannily close to the actual change of roughly 0.7°C. However, as he himself admits, this “could merely be coincidence,” and unfortunately I have to confirm that. Two other factors that he was not really aware of complicate this estimate dramatically. The first is the thermal lag of the system due to the heat capacity of the oceans. This delays substantially (by decades to centuries) the full impact of a change in greenhouse gases, as it takes a long time for the ocean surface temperatures to equilibrate with the new radiation balance. Secondly, he probably wasn’t aware that other aspects of atmospheric composition—as mentioned above—were being greatly affected by human activity as well.

Nonetheless, a number of conclusions that he drew were almost prophetic. He was correct in assuming (against the conventional wisdom of his time) that moves towards nuclear energy would not make a substantial difference to carbon emissions. He was also correct in thinking that the price of removing the carbon from the air would be prohibitively expensive (as it has turned out, although some progress is being made).

So does science progress? Yes, of course. Gilbert Plass had the right framework for this problem and foresaw most of the issues, but the detailed rendering of the calculations—for the carbon cycle, for the radiative transfer, for the existence of feedbacks, for the temperature response—have all become much more sophisticated and complete. What once were rough estimates have been much more tightly constrained. Speculations about growth rates and past behavior have been confirmed by multiple observations.

Nonetheless, the coincidences of some of his numbers and the ones we know today are just that, coincidences, and so some part of the high regard in which we hold Plass today may simply be due to luck. Indeed, Lewis Kaplan, the author of a subsequent and more accurate calculation, has been all but forgotten since he incorrectly concluded that CO2 could not play a role in climate change. In 50 years time if someone reviews my work, I would hope to have been as lucky as Gilbert Plass.