Keeling, Charles David

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KEELING, CHARLES DAVID

(b. Scranton, Pennsylvania, 20 April 1928; d. Hamilton, Montana, 20 June 2005),

atmospheric chemistry, geochemistry, carbon cycle, global climate change.

Keeling proved that carbon dioxide (CO₂) was increasing in the atmosphere due to human burning of fossil fuels. Scientists had long been interested in the possibility that such a rise might be occurring and causing global warming, but before Keeling’s systematic measurements, beginning in the 1950s, CO₂ observations were not accurate enough to reliably detect a global trend. Keeling’s time series record of atmospheric CO₂ at Mauna Loa, commonly known as the “Keeling Curve,” is generally regarded as being among the most solid evidence of a human impact on the planet as a whole.

Origins . Charles David Keeling was born in Pennsylvania and grew up in the Chicago area. His father, Ralph Franklin Keeling, was a financial analyst; his mother, Grace Sherburne Keeling, was an English teacher. They encouraged his aptitude and interest in music: he won numerous piano competitions as a boy, and often performed for social events, sometimes accompanying his younger sister, Lyla, a singer. He studied chemistry as an undergraduate at the University of Illinois, where he earned a BA in 1948, but his interest in science as career developed when Malcolm Dole of the Northwestern University Chemistry Department, and a friend of his parents, then offered him a graduate fellowship at Northwestern. He completed his PhD there in 1954. While at Northwestern, he developed lasting scientific interests outside of his doctoral field: in geology, meteorology and astronomy.

Keeling’s doctoral thesis indicated that double bonds between carbon atoms in polyethylene moved to the end of polymer chains when subjected to a high-energy beam of neutrons. Although this research led to job offers from large chemical manufacturers, especially those involved in developing new plastics, and to promising career prospects in nuclear chemistry, Keeling instead sought opportunities that would allow for research in remote mountainous areas of western North America where he had vacationed as a boy and enjoyed hiking during summers in college. In the fall of 1953, he moved to the California Institute of Technology in Pasadena to become the first postdoctoral fellow of Harrison Brown, head of the geochemistry department there, who was then busy working on his influential book, The Challenge of Man’s Future. Brown encouraged Keeling’s first studies of CO₂.

Detecting and Explaining the CO₂ R Rise . Keeling’s CO₂ measurements were initially motivated by the question of whether carbonate in rivers and ground waters was in equilibrium with CO₂ in the nearby air, but he soon turned his principal attention to the air measurements themselves. Keeling’s air sampling was more precise than that of his predecessors, and this led to the awareness that global trends in atmospheric CO₂ concentrations could be determined by careful and continuous monitoring at a few representative sites.

The notion that humans can influence climate has deep historical roots. The modern hypothesis that CO₂ from industrial emissions might produce a sustained rise in global temperatures emerged out of turn-of-the-twentieth century theorizing by Svante Arrhenius, his associate Nils Eckholm, and his contemporary Thomas Chrowder Chamberlin. It was then developed further in the studies of Guy Stewart Callendar, starting in the 1930s, and by Gilbert Plass in the 1950s. By the mid-1950s, oceanographer Roger Revelle, nuclear physicist Hans Suess, and geochemist Harmon Craig at the Scripps Institution of Oceanography in San Diego were shedding light on the questions of how much and how quickly anthropogenic CO₂ might be absorbed by the oceans, although significant uncertainties remained.

In 1954 planners for the upcoming International Geophysical Year (IGY) in 1957–1958 decided to use that funding opportunity to establish a worldwide network of stations for monitoring atmospheric carbon dioxide. Because CO₂ measurements from the techniques used previously (based on “wet chemical” methods) suggested that CO₂ levels varied widely depending on local conditions, a wide observation network was thought necessary in order to properly detect any overall average trend. When Keeling applied the process developed for his air-water comparisons to systematic measurements of CO₂ in air, over time, and in multiple locations, however, he came up with results that led him to question the accuracy of these prior approaches.

Rather than replicate the existing procedures, Keeling designed and constructed a “manometer” that could measure CO₂ from air—captured in air flasks and extracted by a liquid nitrogen “cold trap”—by means of the pressure it exerted on a column of mercury. With more repeatable gas handling, his approach was much less prone to error than prior wet chemical methods had been. Keeling applied his slow-to-operate but very precise methods to the analysis of samples he took in remote forest locations, where there was little risk of disturbance from local human sources of CO₂. There he consistently found that the concentrations varied systematically with the time of day. The maximum values occurred in the early morning and the minimum values in the early afternoon. He also measured the isotopic ratio of carbon-13 to carbon-12 in his CO₂ samples, which proved that these variations

were the result of natural daily cycles of photosynthesis, respiration, and atmospheric mixing.

During the summer of 1955, as Keeling tracked these “diurnal” fluctuations in parks and wilderness areas up and down the Pacific Coast from Big Sur to the Cascades, the afternoon minimum concentrations of his samples remained surprisingly constant at about 315 parts per million (ppm) of CO₂. This finding was unexpectedly contrary to prior studies, and suggested to Keeling that he was detecting some kind of regional or even global “background level.” Additional samples collected from barren mountain tops, in deserts, and at sea further supported the existence of this background level.

Keeling came to realize that the existence of a measurable, uniform average CO₂ concentration, characteristic of a large portion of Earth’s atmosphere, meant that the global trend of CO₂ could be determined more readily than IGY planners had previously believed possible. By late 1955, Keeling’s CO₂ research had come to the attention of Harry Wexler, director of meteorological research at the U.S. Weather Bureau, and Roger Revelle, the head of the Scripps Institution of Oceanography. Keeling had meanwhile formulated an approach for augmenting manometric methods with much faster infrared analyzers capable of continuous CO₂ measurement. He was invited to interview at both Scripps and the Weather Bureau in Washington, D.C., and was offered jobs by both organizations.

The Weather Bureau was completing construction of an observation station atop Mauna Loa in Hawaii, which Wexler thought would be ideal for the sort of continuous long-term measurement program that Keeling proposed. Revelle, still believing that CO₂ might be too variable to obtain meaningful trends from only a few locations (despite Keeling’s 1955–1956 data suggesting a stable “background” level), favored the original IGY scheme of taking CO₂ “snapshots” from a large number of locations during the eighteen months of the IGY, in order to establish a “baseline” value. Any persisting growth in the atmospheric average of CO₂ could be gauged by repeating

the survey in perhaps ten or twenty years. Keeling preferred the continuous approach endorsed by Wexler but was more attracted by the working environment at Scripps. An agreement was therefore reached to take advantage of the considerable funding under IGY, and pursue both continuous long-term CO₂ measurements and observations to set a “baseline.” Wexler’s Weather Bureau underwrote the establishment of continuous CO₂ measurement at Mauna Loa and Antarctica, while Scripps hired Keeling and helped to fund the geographically widespread sampling from ships and airplanes designed to obtain a “baseline” average. The twenty-eight-year-old scientist, appointed to run the IGY CO₂ program, came to Scripps in the summer of 1956 to take up this time-consuming double job. The baseline sampling was completed by 1962, and the Hawaii and Antarctica measurements continued thereafter.

Keeling’s systematic use of carefully calibrated infrared analyzers and extensive flask sampling, meticulously double-checked, and in locations remote from human activity, yielded data over ten times more accurate than previous measurements. In 1960, after just two years of observations, he reported that the continuous Mauna Loa and Antarctic data (confirmed by the samplings conducted from airplanes and ships) showed that CO₂ fluctuated seasonally in the Northern Hemisphere, due to plant growth (confirmed by concurrent measurement of isotopic carbon), and suggested that global concentrations of CO₂ were rising in line with increasing fossil fuel consumption.

The difference between the seasonal peak in May and the seasonal trough in October was only about 5 ppm. This was small relative to the diurnal swing of as much as 100 ppm in the forested areas where Keeling’s first samples were taken in 1955 and 1956, and thus went unnoticed at first. At Mauna Loa, however, the seasonal fluctuations were clearly apparent by 1959, the second year of data gathering, because the diurnal fluctuations there were only about 1 ppm. The annual CO₂ rise at Mauna Loa averaged slightly under 1 ppm in the late 1950s and early 1960s. Apart from a number of small but interesting temporary variations, the seasonal cycle and the rate of annual increase changed only slowly over the subsequent decades of Keeling’s continuous measurements. By 2000 the seasonal amplitude was about 6 ppm,

the year-to-year rise about 2 ppm, and the average overall level had grown to 370 ppm.

Once Keeling’s initial results were publicized in 1960, interest in “snapshots” at ten- or twenty-year intervals quickly dwindled. The reliability and value of continuous, careful measurements from remote locations was undisputed and has remained so. By the early mid-1960s, Keeling had shown incontrovertibly that CO₂ was increasing in the atmosphere, and his subsequent research became the focal point for growing scientific interest in the implications of rising atmospheric CO₂ for the future climate of the planet.

After five years of measurements, Keeling also had enough observations to credibly estimate that approximately half of all industrially emitted CO₂ was remaining in the atmosphere on a consistent basis. This result was supported by figures showing that industrial production and atmospheric accumulation of the gas were increasing in tandem. In the early 1970s, using well over a decade of data revealing the long-term rate of CO₂ increase more precisely, he also refined estimates of emission levels by making adjustments for the lower carbon content of soft coals, and he developed global carbon cycle models to help track and predict the relative portions going into the air, oceans, and biosphere. Further insights into the carbon cycle also came from the pioneering surveys that Keeling performed in the 1960s concerning CO₂ partial pressure in seawater.

Variations in CO₂ and Changes in Climate . Having identified the global average trend of CO₂ and related it to fossil fuel consumption, Keeling turned his attention in the 1970s and 1980s to investigations of the causes and consequences of variations and anomalies in that average across space and time. Early in his continuous CO₂ tracking, for instance, it became evident that the characteristic seasonal swings due to biotic growth and decay were more pronounced in the Northern Hemisphere because of the preponderance of plant life there. The CO₂ observations also revealed systematic variations in the annual mean concentration with latitude. These variations were featured already in a model of atmospheric mixing developed with Bert Bolin in the early 1960s, and Keeling worked further on such modeling with Martin Heimann and other collaborators in the 1980s.

As the Mauna Loa record was extended, decade by decade, further variations around the general seesawing upward trend also became evident. There were ups and

downs on both a three-to-four-year and a ten-year time scale. Unlike the rise of the Keeling Curve itself, these fluctuations were determined to have natural origins. In the mid-1970s Keeling’s associate Robert Bacastow established that the three-to-four-year oscillations were tied to the El Niño weather phenomenon, and this synchronicity was incorporated into the carbon cycle modeling collaborations that followed in the 1980s. Isotopic measurements, made later on Keeling’s flask samples in a collaboration with Wilhelm Mook, showed that these El Niño oscillations were mainly caused by surges in plant growth and decay and wildfires. In the 1990s Keeling also worked on explaining the ten-year swings in CO₂ by correlating them to cycles in ocean temperatures.

By this point, the longevity and precision of Keeling’s CO₂ time series, and his accumulating experience in interpreting its nuances, were enabling him to examine possible interactions between CO₂ levels and changes in climate. In collaboration with biologists and geographers in the mid-1990s, he showed that a long-term increase in the magnitude and duration of seasonal declines in CO₂ was associated with increasing temperatures, vegetation, and longer growing seasons in higher northern latitudes. Seeking to also explain the ten-year ocean temperature cycles correlated with ten-year fluctuations in seasonal CO₂ amplitude, Keeling and his associate Tim Whorf hypothesized that long-term tidal cycles had been causing episodic cooling of ocean surface waters. Their preliminary findings, published in 1997, showed a close match between tides and the periodicity of air temperatures since 1855.

Recognition, Influence, and Nonprofessional Life . Charles David Keeling was appointed as a tenured professor at the newly created University of California (UC), San Diego, in 1964. He retired in 2003 but remained active in science until his unexpected death of a heart attack in 2005. His collaborations with European scientists were furthered by sabbatical leaves in Stockholm in 1961–1962 (with Bert Bolin), Heidelberg in 1969–1970 (with Karl Otto Münnich), and Bern in 1979–1980 (with Hans Oeschger). Keeling wrote more than one hundred published scientific papers, many of them coauthored with scientific collaborators from around the world, or with associates working in the Scripps CO₂ Program that he established at UC San Diego and that continues in the early twenty-first century, directed by his son Ralph Keeling. Charles David Keeling co-convened international conferences on oceanic and atmospheric carbon dioxide, served on the Commission on Atmospheric Chemistry and Global Pollution of the International Association of Meteorology and Atmospheric Physics, and was the scientific director of the Central CO₂ Calibration Laboratory of the World Meteorological Organization from 1975 to 1995. The inexorable upward trend of the Keeling Curve

underscored the long-term importance of research into the effects of increasing CO₂ upon climate, influenced early pioneers of global climate modeling such as Syukuro Manabe, and helped inspire the global measurement and study of other atmospheric gases

Over the course of his career, Keeling faced considerable and repeated difficulty in securing financial support for his CO₂ research due to shifting priorities and budgets within funding agencies, and, after the 1960s, by agency administrators who maintained that CO₂ monitoring had become “routine” and was duplicating newer government-run measuring programs. Except for a hiatus from February to April 1964, when Mauna Loa measurements were temporarily shut down, Keeling always managed to obtain funding and keep the measurements going, by assiduously persuading agencies and officials of the significance and unmatched reliability, consistency, and long-lived continuity of the data series being generated by his program.

Keeling received the Second Half Century Award of the American Meteorology Society in 1981, the Maurice

Ewing Medal of the American Geophysical Union in 1991, the Blue Planet Prize in 1993, the National Medal of Science in 2002, and the Tyler Prize for Environmental Achievement in 2005. He was a Fellow of the American Academy of Arts and Sciences, the American Geophysical Union, and the American Association for the Advancement of Science, and a member of the National Academy of Sciences and the American Philosophical Society.

Keeling married Louise Barthold in 1954, and they had five children. In addition to his scientific career, he was an avid hiker and conservationist, and in 1974–1975 chaired the citizens’ General Plan revision committee of Del Mar, California, where he lived from the late 1950s. He enjoyed hiking near the summer home he established in Montana in the 1980s, sometimes taking air samples in the forest there. Keeling was an accomplished nonprofessional musical performer, teacher, and composer, and, throughout his life, often played and performed chamber music with family and friends. From 1964 to 1969 he directed the UC San Diego Madrigal Singers.

The author gratefully acknowledges the careful and extensive scrutiny and assistance of Professor Ralph Keeling, Scripps Institution of Oceanography.

BIBLIOGRAPHY

Archival Sources: C. D. Keeling Collection, Scripps Institution of Oceanography Archives, University of California, San Diego. A full list of Keeling’s publications can be found on the Scripps CO₂Program Web site, which is referenced under “Other Sources” below.

WORKS BY KEELING

“Variations in Concentration and Isotope Abundances of Atmospheric Carbon Dioxide.” In Proceedings of the Conference on Recent Research in Climatology, edited by H. Craig, 43–49. San Diego: Committee on Research in Water Resources and University of California, Scripps Institution of Oceanography, 1957.

“The Concentration and Isotopic Abundances of Carbon Dioxide in Rural Areas.” Geochimica et Cosmochimica Acta 13 (1958): 322–334.

“The Concentration and Isotopic Abundances of Carbon Dioxide in the Atmosphere.” Tellus 12 (1960): 200–203.

With Bert Bolin. “Large-Scale Atmospheric Mixing as Deduced from the Seasonal and Meridional Variations of Carbon Dioxide.” Journal of Geophysical Research 68 (1963): 3899–3920.

With Jack C. Pales. “The Concentration of Atmospheric Carbon Dioxide in Hawaii.” Journal of Geophysical Research 70, no. 24 (1965): 6053–6076.

With Craig W. Brown, “The Concentration of Atmospheric Carbon Dioxide in Antarctica.” Journal of Geophysical Research 70, no. 24 (1965): 6077–6085.

With Norris W. Rakestraw and Lee S. Waterman. “Carbon Dioxide in Surface Waters of the Pacific Ocean. 1. Measurements of the Distribution.” Journal of Geophysical Research 70 (1965): 6087–6097.

“Carbon Dioxide in Surface Waters of the Pacific Ocean. 2. Calculation of the Exchange with the Atmosphere.” Journal of Geophysical Research 70 (1965): 6099–6102.

“Is Carbon Dioxide from Fossil Fuel Changing Man’s Environment?” Proceedings of the American Philosophical Society 114 (1970): 10–17.

“The Carbon Dioxide Cycle: Reservoir Models to Depict the Exchange of Atmospheric Carbon Dioxide with the Oceans and Land Plants.” In Chemistry of the Lower Atmosphere, edited by S. I. Rasool, 251–329. New York: Plenum Press, 1973.

“Industrial Production of Carbon Dioxide from Fossil Fuels and Limestone.” Tellus 25 (1973): 174–198.

“The Influence of Mauna Loa Observatory on the Development of Atmospheric COResearch.” In Mauna Loa Observatory 20th Anniversary Report, edited by J. Miller, 36–54. Washington, DC: National Oceanographic and Atmospheric Administration, 1978.

With Roger Revelle. “Effects of El Niño/Southern Oscillation on the Atmospheric Content of Carbon Dioxide.” Meteoritics 20 (1985): 437–450.

With Eric From. “Reassessment of Late 19th-Century Atmospheric Carbon Dioxide Variations.” Tellus 38B (1986): 87–105.

With Martin Heimann, et al. “A Three Dimensional Model of Atmospheric CO₂ Transport Based on Observed Winds.” In Aspects of Climate Variability in the Pacific and the Western Americas, edited by David H. Peterson, 165–363. Washington, DC: American Geophysical Union, 1989.

With Timothy P. Whorf, Martin Wahlen, and Johannes van der Plicht. “Interannual Extremes in the Rate of Rise of Atmospheric Carbon Dioxide since 1980.” Nature 375 (1995): 666–670.

With John F. S. Chin and Timothy P. Whorf. “Increased Activity of Northern Vegetation Inferred from Atmospheric CO₂ Measurements.” Nature 382 (1996): 146–149.

With Ranga B. Myneni, Compton J. Tucker, Ghassem Asrar, and Ramakrishna R. Nemani. “Increased Plant Growth in the Northern High Latitudes Due to Enhanced Spring Time Warming.” Nature 386 (1997): 698–702.

With Timothy P. Whorf. “Possible Forcing of Global Temperature by the Oceanic Tides.” Proceedings of the National Academy of Sciences of the United States of America 94 (1997): 8321–8328.

“Rewards and Penalties of Monitoring the Earth.” Annual Review of Energy and the Environment 23 (1998): 25–82.

With Timothy P. Whorf. “The 1,800-Year Oceanic Tidal Cycle: A Possible Cause of Rapid Climate Change.” Proceedings of the National Academy of Sciences of the United States of America 97 (2000): 3814–3819.

With Nicolas Gruber and Nicholas R. Bates. “Interannual Variability in the North Atlantic Ocean Carbon Sink.” Science 298 (2002): 2374–2378.