Keeling curve

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The Keeling curve with the measured values ​​of the atmospheric content of carbon dioxide in the earth's atmosphere at the Hawaiian Mauna Loa since 1958

The Keeling curve is the graphic representation of the mean global concentration profile of the trace gas carbon dioxide (CO 2 ) in the earth's atmosphere since 1958. It is named after Charles David Keeling of the Scripps Institution of Oceanography . This was able to show for the first time that the concentration of greenhouse gas increases due to changes in land use and the burning of fossil fuels . The curve shows a characteristic, fluctuating course of the year, which reflects the vegetation cycle of the northern hemisphere . In the (northern) spring and summer, the uptake of CO 2 by plants predominates , which is reflected in a decrease in concentration that is delayed depending on the location of the measurement. In autumn and winter, the vegetation emits net CO 2 , which leads to an increase in concentration.

The Keeling curve is of particular importance in the history of research into climate change , as it was considered to be important evidence of human-caused global warming , which was then only formulated as a hypothesis . Significant scientific findings could be obtained from the measurement data and the course of the curve. In 2015 she was honored by the American Chemical Society with the nomination as a National Historic Chemical Landmark .

In 2005, the physicist and environmental scientist Charles Kennel recognized Keeling's scientific achievements with the words:

“Charles David Keeling's metrological evidence of the global increase in atmospheric carbon dioxide concentration was the starting point for today's major worries about global warming. It is the most important data set of the twentieth century in terms of environmental pollution. "

prehistory

Both Guy Stewart Callendar (1938) and Gilbert Plass (1956) use new, more precise calculations to support the theory of global warming first formulated by Svante Arrhenius in 1895 due to an increase in the atmospheric CO 2 concentration resulting from the combustion of fossil fuels . However, there was no evidence of an increase in concentration, nor was the proportion of carbon dioxide remaining in the atmosphere known at the time. The way and speed with which ocean water mixes was also unknown.

Attempts have been made to determine the atmospheric carbon dioxide content since the 19th century, but the results varied so widely that the majority of researchers assumed that an accurate measurement was not possible. In 1954, a research group led by Stig Fonselius tried to determine the atmospheric CO 2 concentration with the help of 16 measuring stations installed in Scandinavia . The project was soon abandoned, however, as the measurement results differed so greatly that no systematic was discernible. Many scientists therefore assumed that the CO 2 concentrations varied greatly from place to place, that a “ background concentration ” did not exist and that all gas quantities emitted by humans through combustion would be absorbed by the world's oceans .

The then head of the Scripps Institute, Roger Revelle , was an expert in ocean chemistry. He also did not know how quickly and in what quantity additional atmospheric CO 2 introduced by humans would be dissolved in the sea. In the 1950s, Revelle became aware of the research results of Hans E. Suess . He tried to improve the accuracy of radiocarbon dating , and in 1955 discovered the Suess effect named after him . The Suess effect could only be explained by the fact that carbon dioxide, which came from the combustion of fossil fuels, accumulates in the atmosphere. Revelle saw in the evaluation of the nuclear weapons effect a possibility to determine the proportion of the carbon dioxide absorbed by the oceans in order to determine in this way a possible increase in the concentration of the greenhouse gas . In addition, he wanted to gain new knowledge about the mixing of the oceans; the two worked together from then on. After evaluating the 14 C measurements, Revelle calculated that only 20% of the carbon dioxide released into the atmosphere dissolves in the sea. An increase in the concentration in the atmosphere should therefore be detectable by measurement.

As a post-doctoral student , Charles Keeling developed and perfected measuring arrangements from 1953 with which he investigated the CO 2 equilibrium concentrations between the atmosphere, limestone and surface water . He carried out the first measurements with the help of airplanes, weather balloons and ships. In Pasadena he noticed the regionally strongly fluctuating gas concentrations; In a forest area of Big Sur the atmospheric CO 2 concentration showed strong fluctuations during the day . The air always contained more CO 2 at night than during the day, but always around 310 ppm in the afternoon  .

In contrast to the group around Fonselius, Keeling postulated on the basis of the data he had obtained that the atmospheric CO 2 concentration had to be largely constant far away from disturbing sources and sinks. In 1956 he proposed a global measurement program to Harry Wexler of the US Weather Bureau (now: National Weather Service ) and Roger Revelle of the Scripps Institution of Oceanography . Since Revelle and Suess were also interested in precise CO 2 measurements, the Scripps Institute finally successfully applied for the international geophysical year 1957/58 with the large-scale measurement program, and Keeling was entrusted with the management of the project.

Newer development

In addition to the station that is still active today, the Scripps Institute in Hawaii operates other facilities and a. in Alert (Nunavut) and Barrow (Alaska) , also at the Trinidad Head Observatory in California , on the island of Tutuila ( American Samoa ) and at the South Pole ; the NOAA takes over 60 stations twice a week samples. Since 2009, the regional distribution of greenhouse gas concentrations has also been recorded from space with the GOSAT satellite .

The monitoring of the series of measurements in Hawaii, which was started by Charles Keeling, who died in 2005, was taken over by his son Ralph F. Keeling, who is also a professor of oceanography and who extended the series of measurements to include atmospheric oxygen content.

The measurements

The Mauna Loa Observatory

With the additional financial support, Keeling was able to procure four gas analyzers , and in 1958 he set up the first measuring station on the Mauna Loa volcano in Hawaii . There he began in March 1958 with systematic measurements of the greenhouse gas carbon dioxide. He placed the other three gas analyzers in Antarctica , in California and in his laboratory. With the latter device, he evaluated samples collected at different locations. B. were obtained with the help of aircraft. The first measured value used for the later Keeling curve was 313  ppm CO 2 .

According to measurements published by Kurt Buch (1881-1967) in 1948 , typical concentrations of the greenhouse gas were between 150-230 ppm (Arctic) and 319-349 ppm (tropics). The values ​​published by Keeling for the first time in 1962 were much more precise. He gave a range between 313 and 325 ppm for the Arctic and a range of 317–321 ppm for the equatorial Pacific. Previous measurements were based on wet chemical methods , while Keeling used a more precise measuring non-dispersive infrared sensor and eliminated the disruptive influence of changing humidity. The uncertainty was his procedure Keeling in 1960 with ± 0.3 ppm, which he was able to further reduce ppm by a later possible recalibration to ± 0.1.

Disturbances in the measurement results

In contrast to the recently failed project in Scandinavia, the Keeling measuring stations were placed far away from interfering carbon dioxide sources. The measuring station on Mauna Loa was set up at a great height on the windward side. The measuring station in Antarctica was also far away from CO 2 sources and sinks. In spite of this, the carbon dioxide concentrations determined at all measuring locations were sometimes falsified by impurities. At the South Pole, for example, it was once caused by an internal combustion engine that was running close to the measuring station. At Mauna Loa, these were outgassing from the volcano, which in rare cases could reach the measuring station. The disturbances were always clearly recognizable in the recordings, however, because they were only of short duration and high amplitude , so that these measured values ​​could easily be recognized and discarded.

Results and Impact

Keeling's measurements laid the foundation for further series of measurements of other atmospheric gases. Scientists were inspired by Keeling's successes to determine the concentration curves of the greenhouse gases methane and nitrous oxide and measurements of the ozone concentration were initiated. Nevertheless, the measurement program was repeatedly endangered and the continuous measurements should be stopped several times, as it was argued that random measurements would show the same results as continuous measurements.

Early results

CO 2 increase and spectra of changes in concentration; the Fourier analysis reveals a pronounced annual cycle (first and second curve from above); if you smooth this annual cycle, a continuous increase can be seen (3rd and 4th curves from the top); Curve 5: the measurement data

In the first few months of his measurements, Keeling doubted whether his theory of background concentration could be correct, as the values ​​rose for a few months and then fell. However, since he did not give up and ran the measurements over several years, he was able to show that the fluctuations had an annual period, which he attributed to the seasonally changing growth period of the vegetation . In the northern hemisphere , with its larger land area, there are considerably more plants than in the southern hemisphere . During the northern spring, therefore, more CO 2 is bound from the atmosphere than during the northern autumn, when the strongest growth occurs south of the equator .

After a few years it was evident that the measured values ​​were also increasing from year to year. With this, Keeling was able to prove with measurement technology that the atmospheric carbon dioxide concentration increases from year to year and thus support Svante Arrhenius' theory of a possible global warming caused by humans. The measured increase in concentration corresponded almost exactly to the value expected from burning fossil fuels.

Keeling series of measurements included the determination of the isotope ratios of 13 C / 12 C . Through isotope studies , he was able to prove the influence of land plants on the course of the carbon dioxide concentration, because land plants prefer to absorb the lighter 12 C. The ratio was lower at night than during the day and showed a similar pattern over the year. He published these results as early as 1960 in the magazine Tellus.

Results from the long-term analysis of the series of measurements

CO 2 flow in the biosphere on December 23, 2006; The concentration changes depending on the time of day are color-coded (source: NOAA Carbon tracker).

Since Keeling managed to maintain the continuity of the measurements, analyzes of the curves over several years and decades became possible.

In the mid-1970s, the accuracy of the measurement series was high enough to determine the proportion of anthropogenic CO 2 emissions that remain in the atmosphere and are not absorbed by the oceans: the value is 57%. This relationship is described by the Revelle factor .

In the mid-1990s, by precisely determining the rise and fall points of the curves, it was clear that spring - due to global warming - began in the northern hemisphere around a week earlier than at the beginning of the series of measurements.

Through long-term observation of the concentration course of atmospheric 13 CO 2 and molecular oxygen , the combustion of fossil fuels could be identified without a doubt as the main source of the strong increase in the concentration of the greenhouse gas carbon dioxide.

The comparison between the equatorial CO 2 concentrations with the data obtained in arctic latitudes showed the seasonal influence of the growth period: While the annual variation of the curves near the equator is only approx. 3 ppm, in arctic latitudes it is 20 ppm. The annual amplitude of this oscillation has also increased by 20% (Hawaii) and 40% (Arctic region) in the mid-1990s compared to the curves obtained in the 1960s. Keeling suspected that this was due to an increased assimilation of the land plants, which react to the increased CO 2 supply and the increased temperatures with increased growth.

The CO 2 absorption capacity of seawater fluctuates depending on the sea surface temperature ; the Henry constant is highly temperature dependent and for carbon dioxide; Cold water can dissolve the trace gas well, but it is only sparingly soluble in warm water. El Niño and La Niña influence the surface temperature of the Pacific over a large area. Years of low CO 2 uptake by the oceans could thus be correlated with El Niño events.

Classification of the results

Course of the atmospheric carbon dioxide concentration over the past 420,000 years, reconstructed from the climate archive of the Vostok ice core

With the exception of an interruption in 1964, the data from Mauna Loa to the present day are in an uninterrupted series. At that time, the measurement had to be suspended due to a device defect that could not be repaired at short notice.

During the past 800,000 years, the concentration of carbon dioxide in the earth's atmosphere has never exceeded 300 ppm. In the 750 years between 1000 and 1750 AD, the CO 2 content in the earth's atmosphere was between 275 and 285 ppm. While the CO 2 content in the earth's atmosphere was around 310 ppm in the early 1950s, it rose to 400 ppm for the first time in spring 2013 at the Mauna Loa measuring station. A new high of 415.26 ppm was reached on May 11, 2019.

Between 1770 and 1970, over 200 years, an increase of 50 ppm was observed, i.e. that is, the annual rate of increase was 0.25 ppm. In just 30 more years there was a further increase of 50 ppm: in the 1960s the annual increase was around 0.8 ppm, in the 1980s the value was already twice as high at 1.6 ppm per year. In the 1990s it was fairly constant at around 1.5 ppm per year, rose to around 2 ppm per year in the 2000s and was around 2.4 ppm per year in the 2010s.

In 2018, the increase in CO 2 concentration of 2.87 ppm was the fourth highest ever measured in a year since accurate records exist. In May 2020, the carbon dioxide concentration was 417.2 ppm, 2.4 ppm above the previous year's value. The increase was slightly lower than in the same period of the previous year, which is probably related to the slightly lower carbon dioxide emissions as a result of the COVID-19 pandemic .

Web links

Commons : Keeling Curves  - collection of images, videos and audio files

Individual evidence

  1. ^ Scripps Institution of Oceanography (Ed.): American Chemical Society Honors Keeling Curve and NOAA Observatory. Press release of April 24, 2015, accessed on February 23, 2020
  2. ↑ NASA Earth Observatory: Image of the day: The keeling curve (will no longer be updated)
  3. ↑ GS Callendar: The artificial production of carbon dioxide and its influence on temperature . In: Quarterly Journal of the Royal Meteorological Society . tape 64 , no. 275 , April 1, 1938, pp. 223-240 , doi : 10.1002 / qj.49706427503 .
  4. ↑ Dr. Max Pettenkofer : About a method to determine the carbon dioxide in the atmospheric air in Journal für Praktische Chemie 85 , 165-184 (1862)
  5. ↑ Go ye, MJ, Recherches sur la proportion de l'acide carbonic dans l'air in Comptes Rendus 90, 1144-1150 (1880)
  6. ↑ Stig Fonselius, Folke Koroleff and Kurt Buch: Microdetermination of CO2 in the Air, with Current Data for Scandinavia. In: Tellus . tape 7 , no. 2 , May 1955, p. 258–265 , doi : 10.1111 / j.2153-3490.1955.tb01160.x ( PDF ). and S. Fonselius, F. Koroleff and Karl-Erik Wärme: Carbon Dioxide Variations in the Atmosphere . In: Tellus . tape 8 , no. 2 , May 1956, p. 176-183 , doi : 10.1111 / j.2153-3490.1956.tb01208.x ( PDF ).
  7. ↑ American Institute of Physics: Money for Keeling
  8. ^ A b c d American Institute of Physics: The Discovery of Global Warming: Roger Revelles Discovery
  9. ↑ a b c d Scripps CO 2 Program: The Early Keeling Curve.
  10. ↑ Measuring stations of the Scripps Institute
  11. ↑ Tracking Global Carbon - Atmospheric Scientists Monitor Global Carbon ( Memento from March 21, 2011 in the Internet Archive )
  12. ↑ Reuters.com: Japan launches satellite to monitor greenhouse gases. January 23, 2009, accessed February 23, 2020.
  13. ↑ Scripps CO 2 Program - Carbon Dioxide Measurements. Scripps Institute of Oceanography, accessed February 23, 2020 .
  14. ↑ a b c d Charles D. Keeling: The Concentration and Isotopic Abundances of Carbon Dioxide in the Atmosphere . In: Tellus . tape 12 , no. 2 , 1960, p. 200–203 , doi : 10.1111 / j.2153-3490.1960.tb01300.x ( PDF ). PDF ( Memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / scrippsco2.ucsd.edu
  15. ↑ a b Tyler Price Lecture ( Memento of November 7, 2013 in the Internet Archive ) 2005 (Online, pdf).
  16. ^ Geophysica, vol. 3, 1948, pp. 63-79
  17. ↑ a b c Scripps CO 2 Program: Keeling Curve Lessons
  18. ↑ a b C. D. Keeling, JFS Chin, TP Whorf: Increased activity of northern vegetation inferred from atmospheric CO2 measurements . In: Nature . tape 382 , no. 6587 , 1996, pp. 146-149 , doi : 10.1038 / 382146a0 .
  19. ↑ IPCC AR4 , Chapter 1.3.1 The Human Fingerprint on Greenhouse Gases Online, PDF
  20. ↑ a b IPCC AR4 , Chapter 2.3.1 Atmospheric Carbon Dioxide Online, PDF
  21. ↑ American Institute of Physics: Illustration of the measurement interruption
  22. ^ The Keeling Curve Daily Reading
  23. ↑ CO2 content in the atmosphere reaches a new high. In: Spiegel online. May 14, 2019, accessed June 10, 2019 .
  24. ↑ a b Atmospheric CO2 levels rise sharply despite Covid-19 lockdowns . In: The Guardian , June 4, 2020. Retrieved June 5, 2020.
  25. ↑ Global carbon dioxide growth in 2018 reached 4th highest on record. In: https://www.noaa.gov/ . National Oceanic and Atmospheric Administration, March 22, 2019, accessed May 15, 2019 .
This version was added to the list of articles worth reading on February 10, 2011 .