This is the 4th 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
- Episode 3: Our “large scale geophysical experiment” (1940-1960)
In 1953, Charles David (“Dave”) Keeling, a just-graduated Ph.D. chemist with an interest in geology, was looking for a job. He got one as a postdoctoral researcher at Caltech, where a professor employed him to experimentally confirm a rather esoteric hypothesis about the balance between carbon stored in limestone rocks, carbonate in surface water, and atmospheric carbon dioxide. To do this, Dave realized he would first need to have a very accurate estimate of the CO2 content of the air. In investigating the available data on that subject, he found what we encountered at the end of Episode 3 – a great deal of variability in the reported measurements. In fact, it had become widely believed that the CO2 concentration in air might vary significantly from place to place and from time to time, depending on the movements of various air masses and local effects due to the respiration of plants, etc. Dave decided he would need his own way of very accurately measuring the CO2 concentration in air.
Dave developed a new method of measuring the CO2 content of air by collecting air samples in specialized 5-liter flasks, condensing the CO2 out of the air using liquid nitrogen (which had just recently become commercially available), separating the CO2 from water vapor by distillation, and measuring the condensed CO2 volume using a specialized manometer he developed by modifying a design published in 1914. Dave’s new method was accurate to within 1.0 ppm of CO2 concentration. If you’re interested, you can read more about it in his 1958 paper, “The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas,” in which he reported the results of repeated atmospheric CO2 measurements he made at 11 remote stations, including Big Sur State Park, Yosemite National Park, and Olympic National Park, at different elevations and at all times of the day and night.
In his autobiographical account, Keeling admitted he took many more air samples than probably required for this work largely because he was having fun camping in beautiful state and national parks. The great number of samples paid off, though, as they enabled him to make some important observations about daily fluctuations in the atmospheric CO2 level. He found that, in forested locations, maximum CO2 concentrations occurred in the late evening or early morning hours and minimum CO2 concentrations occurred in the afternoon. In non-forested locations, the CO2 concentrations were very similar to the minimum (afternoon) levels measured in forested locations, as well as earlier published levels in maritime polar air collected north of Iceland. In all these locations, the minimum measured CO2 concentrations were pretty consistent, in the range of 307-317 ppm. By isotopic analysis of the carbon-13/carbon-12 ratio of CO2 collected in the forested areas, Keeling determined that the elevated CO2 levels measured at non-afternoon hours in forested areas were due to respiration of plant roots and decay of vegetative material in the soil. He posited that afternoon meteorological conditions resulted in mixing of the near-surface air layer influenced by vegetative processes with higher air that was constant in CO2 concentration.
Basically, the results of Dave’s camping adventures with 5-liter vacuum flasks suggested three important conclusions: (1) care should be taken to sample air using specific methods and under conditions not influenced by industrial pollution or vegetative processes (sample at rural locations in the afternoon); (2) if such care was taken, maybe the CO2 concentration in the atmosphere was virtually the same everywhere, from the old-growth forests of Big Sur to the pristine sea air north of Iceland; and (3) if that was the case, the global atmospheric CO2 concentration in 1956 was about 310 ppm.
Federal agencies, including the US Weather Bureau, were working to identify scientific studies to undertake using the substantial government geophysical research funding anticipated during the International Geophysical Year. Dave reported to a US Weather Bureau researcher his new CO2 measurement method and his results pointing to a potential constancy of global CO2 levels. This resulted in Dave’s installation at the Scripps Institution of Oceanography, directed by Roger Revelle and his associate, Hans Suess. You may remember Revelle and Suess from Episode 3. They were in the midst of publishing a paper concluding that much of the excess CO2 from fossil fuel combustion should be rapidly conveyed into the deep oceans. However, they remained intrigued by Callendar’s analyses, apparently to the contrary, and thought it worthwhile to undertake a dedicated program of atmospheric CO2 measurements at multiple locations.
With funding from Scripps and the US Weather Bureau, Keeling was to make continuous CO2 measurements with a newly developed infrared instrument at remote locations on a 13,000-foot volcano at Mauna Loa, Hawaii and at Little America, Antarctica. The infrared instruments were to be calibrated by the gas sampling technique Dave had developed at Caltech, and 5-liter flasks were to be collected from other strategic places on the Earth, including on airplane flights and trans-ocean ships. The measurements commenced at Mauna Loa, Hawaii in 1958, and the first measured CO2 concentration was 313 ppm.
Continuous weekly CO2 measurements have been conducted at Mauna Loa ever since. The results are freely available to the public here. You can download the data yourself (as can, presumably, House Representatives, Senators, and the President). I did, and I plotted the weekly measurements as this blue curve which has become known as the “Keeling Curve“:
Keeling’s very first observation was a seasonal cycle in atmospheric CO2 concentration. The atmospheric CO2 concentration reached a maximum in May, just before the local plants put on new leaves. It then declined, as the plants withdrew CO2 from the atmosphere through photosynthesis, until October, when the plants dropped their leaves. This was, incredibly and quite literally, the breathing of the Earth, which you can clearly see in Keeling’s first measurements (1960, 1963, 1965).
The first few years of measurements also confirmed remarkable agreement between measurements taken at Mauna Loa, in Antarctica, on trans-Pacific air flights, and at other locations:
By 1960, the Scripps workers had concluded that the average atmospheric CO2 concentration was rising year-on-year. As you can see by the blue curve above, both the seasonal “breathing” of the Earth’s plants and increasing average CO2 concentration, measured at Mauna Loa, have continued every single year, without interruption, since Keeling’s first measurement in 1958.
No informed person disputes the correctness of the blue curve above. The Mauna Loa CO2 record makes the most compelling graph because it is our only uninterrupted CO2 record. But it has been corroborated for decades by many other scientists who have made measurements all over the world. The Scripps Institution of Oceanography has made measurements at 12 sampling stations from the Arctic to the South Pole, and spread across the latitudes in between. You can get daily updates of the Mauna Loa CO2 concentration here. The National Oceanic and Atmospheric Administration also operates a globally distributed system of air sampling sites, based on which it calculates a global average atmospheric CO2 concentration that is periodically updated here.
In fact, we now know 57% of the CO2 produced by the burning of fossil fuels has stayed in the atmosphere, according to the Mauna Loa CO2 record (see here for more information). So, what about the analysis of Roger Revelle and Hans Suess (1957) from Episode 3, which suggested the CO2-absorbing power of Earth’s deep oceans would save us the hassle of worrying about our CO2 emissions? The early 1957 conclusions were based on measurement of the steady-state rate of exchange of CO2 between air and seawater. That is, the average time a CO2 molecule floats around in the atmosphere before it is “traded” for one dissolved in the surface of the ocean, independently of any net change of the CO2 concentration in either the air or the seawater. Revelle and Suess estimated that steady state exchange rate at around 10 years, and reasoned this meant that, if new CO2 were introduced into the atmosphere, a matching increase in the CO2 surface concentration of the seawater would occur within about 10 years.
Around the same time Dave Keeling was beginning his CO2 measurements at Mauna Loa, Roger Revelle and other scientists were learning the above assumption ignored an important buffering effect of the dissolved salts in seawater, which causes seawater to “resist” increases in its CO2 concentration (see more in this 1959 paper). Thus, when the concentration of CO2 in the atmosphere increases, the net concentration of CO2 in the ocean surface increases by an amount over 10 times less. After decades of further study, this buffering effect is well understood and is routinely measured in the oceans as a quantity known as the Revelle Factor. It explains why Callendar was right about increasing atmospheric CO2, and why we can’t count on the deep oceans to help with our CO2 problem on any but geological time scales of several thousands of years (for more details see this paper).
So, at least, we can say with certainty we’ve settled the question of whether combustion of fossil fuels has increased atmospheric CO2. A multitude of independent measurements tell us it has. When we started this story in Episode 1 around the year 1900, the atmospheric CO2 concentration at the Royal Botanical Gardens was 290 ppm. Dave Keeling’s first measurement at Mauna Loa in 1958 was 8% higher. When I first watched Star Wars at the drive-in in 1977, the CO2 concentration in the air around me was 16% higher. By the time Barack Obama was elected President in 2008, it was 32% higher. The March 18, 2017 Mauna Loa reading was 406.92 ppm, 40% higher than the CO2 concentration in the year 1900. As you can see by the upward bend of the blue curve above, the atmospheric CO2 concentration is increasing at an accelerating rate.
So, how big is that change in the context of Earth’s history? To find out, it would seem we would have to go backward in time. As it turns out, we can! (Sort of.) Stay tuned!
To be continued…