How Accurate are Our CO2 Ice Core Data?
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| The Keeling Curve |
I sometimes hear claims that our measurements of CO2 concentrations, especially as recovered in ice core data, are unreliable indicators of variability in CO2 concentrations, often stating that CO2 may have varied by far more than scientists are letting us know about. The implication is that scientists are covering up evidence for the variability of atmospheric CO2 concentrations to sell a narrative that humans are responsible for the increase in CO2. There are at least two forms of lines of argument here, one arguing that ice core data is unreliable and the other arguing that other measurements of CO2 contradict the ice core record.
1. Zbigniew Jaworowski
It appears that even in the 1990s there were attempts by contrarian scientists to undermine the reliability of the empirical data extracted from ice cores to reconstruct background CO2 concentrations over the last several hundred thousand years. One such paper, Jaworowski 1994[1], argued that the CO2 data was unreliable for several reasons. He claimed that CO2 at depths of about 70 m solidifies to solid clathrates and redated the ages of the ice relative to the occluded gases to push the increase in CO2 concentrations backwards about 100 years. The paper received the attention of Hans Oeschger, who worked on a team studying ice cores at the University of Bern. Oeschger notes that already by the mid-1990s there was independent confirmation that the ice core reconstructions of CO2 concentrations were accurate.
The project to reconstruct the history of the grennhouse(sic) gases was conducted; it was, and is, very successful much above expectation. The CO₂ concentrations measured on the SIPLE core, Antarctica, serve as a measure of that success. They illustrate (JAWOROWSKI, Fig. 5 a, p. 168) the history of atmospheric CO₂ increase since the middle of the 18th century. Another important result was the observation of low CO₂ concentrations of the gases extracted from ice-age ice. The low glacial CO₂ concentrations have been confirmed in ice cores with different physical and chemical properties both from Greenland and Antarctica and in-dependently from 13C measurements on carbonate of foraminifera shells in ocean cores and, yet again, more recently in moss samples.[2]
Oeschger also notes that even in the 1990s, Jaworowski's claims were "not taken seriously by the science community" and that his paper was not only "incorrect, but irresponsible." R. B. Alley provides a helpful summary of the evidence we have for the reliability of CO2 concentrations from ice cores,[3] and concludes,
More work remains to be done, and as increasingly precise measurements on increasingly small samples are developed, some of the issues discussed above may become more important. However, the main results of ice-core science have not been overturned despite being tested in many ways by many groups over many years and decades, and the main results are based strongly on physical understanding of relatively simple systems. Thus, these results are especially robust.
There are uncertainties associated with ice core data having do wo with diffusion of gas molecules within the firn layer and dating the occluded gases relative to the ice layer. After accounting for sources of uncertainty, ice core measurements are regarded as being accurate to a few ppm. The uncertainty for CO2 concentrations at the Law Dome in Antarctica are accurate to 1.2 ppm since 1006 a.d. And while smoothing does occur via diffusion, this is largely limited to the firn layer. Diffusion continues the ice layer, but it doesn't become comparable to that in the firn layer until 930-950 m (~60-70K years).[9] The diffusion effects are pretty well-understood, and comparing CO2 measurements between various ice cores and and to other sources like foraminifera shells, ocean cores, etc. show that diffusion smoothing has an overall small effect that does not support the notion of large swings in atmospheric CO2 on century time scales like we've seen since the 20th century.
2. Ernst-Georg Beck
In papers published in 2007[4] and 2008[5] argued that scientists have been systematically ignoring chemical measurements of CO2 concentrations that contradict a predetermined narrative. The IPCC and scientists like Guy Callendar and Charles Keeling were claiming that CO2 concentrations were in an "equilibrium state" of about 280 ppm and were rising due to human carbon emissions from fossil fuels. Keeling claimed that measurements from Mauna Loa showed rapid and accelerating increases in CO2 concentrations since measurements began in 1958. Beck's argument was that these scientists and the IPCC were ignoring other CO2 measurements that contradicted this narrative. He argued that chemical methods for measuring CO2 between 1800 and 1961 showed that CO2 changed by much more than what these scientists and the IPCC were leading us to believe.
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| From Beck's 2008 Paper |
Beck's 2007 paper was published in Energy & Environment, which has a long history of publishing politically-motivated pseudoscience papers. But to their credit, they did publish two comments to Beck's paper that thoroughly refute Becks' argument.[6][7] Simply stated, the paper is fatally flawed in multiple ways, but two of the most significant problems are:
- The "chemical methods" reported by Beck are not very reliable compared to modern methods. C. D. Keeling developed a CO2 measuring method based on liquid-nitrogen extraction that was a more reliable indicator of CO2 concentrations.
- Many of the measurements Beck is reporting were taken near major sources of CO2 (forests, cities, etc.) and so these measurements are not indicative of background levels of CO2, which is what scientists like Callendar, Keeling were interested in, and what is important in IPCC assessment reports.
Conclusion
References:
[1] Jaworowski Z. (1994). Ancient Atmosphere - Validity of Ice Records. ESPR 1 (3): 161–171.
[2] Oeschger, Hans. (1995) Letters to the Editor. "Z. JAWOROWSKI: Ancient Atmosphere - Validity of Ice Records ESPR 1 (3): 161–171 (1994)." ESPR-Environ. Sci. & Pollut. Res. 2 (1): 60-61.
https://web.archive.org/web/20070927024724/http://www.scientificjournals.com/sj/espr/Pdf/aId/7394
[3] Alley RB. Reliability of ice-core science: historical insights. Journal of Glaciology. 2010;56(200):1095-1103. doi:10.3189/002214311796406130
[4] Beck, Ernst-Georg. 180 Years of Atmospheric CO2 Gas Analysis by Chemical Methods. Energy & Environment Vol. 18, No. 2 (2007): 259-282.
[5] Beck, Ernst-Georg. The Historical Data the IPCC Ignored: 180 Years of Atmospheric CO2 Measurement By Chemical Methods. 21st Century Science and Technology. Spring 2008. pp. 41-52.
[6] Meijer, Harro A.J. Comment on 180 Years of Atmospheric CO2 Gas Analysis by Chemical Methods. Energy & Environment Vol. 18(2), 2007.
https://journals.sagepub.com/doi/10.1260/0958-305X.18.5.635
[7] Keeling, Ralph F. Comment on 180 Years of Atmospheric CO2 Gas Analysis by Chemical Methods. Energy & Environment Vol. 18(2), 2007.
https://www.ncei.noaa.gov/pub/data/paleo/icecore/ice-cores.pdf
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