Keeling Curve

Short description: Graph of atmospheric CO2 from 1958 to the present

"Keeling" redirects here. For other uses, see Keeling (disambiguation).

Steady rise of atmospheric carbon dioxide (CO
2) concentrations measured at Mauna Loa Observatory from 1958 to 2025

The Keeling Curve is a graph of the annual variation and overall accumulation of carbon dioxide in the Earth's atmosphere based on continuous measurements taken at the Mauna Loa Observatory on the island of Hawaii from 1958 to the present day. The curve is named for the scientist Charles David Keeling, who started the monitoring program and supervised it until his death in 2005.

Keeling's measurements showed the first significant evidence of rapidly increasing carbon dioxide (CO2) levels in the atmosphere.[1] According to Naomi Oreskes, Professor of History of Science at Harvard University, the Keeling curve is one of the most important scientific works of the 20th century.[2] Many scientists credit the Keeling curve with first bringing the world's attention to the current increase of CO
2 in the atmosphere.[3]

## Contents

* 1 Background
* 2 Mauna Loa measurements
* 3 Results and interpretation
* 4 Legacy
* 4.1 Global monitoring
* 4.1.1 ICOS Curve
* 4.2 Ralph Keeling
* 4.3 Recognition
* 4.4 Passing 400 ppm in 2013
* 5 Keeling Plot
* 6 See also
* 7 References
* 8 External links

## Background

Prior to the 1950s, measurements of atmospheric CO
2 concentrations had been taken on an ad hoc basis at various locations. In 1938, engineer and amateur meteorologist Guy Stewart Callendar compared datasets of atmospheric CO
2 from Kew in 1898–1901, which averaged 274 parts per million by volume (ppm),[4] and from the eastern United States in 1936–1938, which averaged 310 ppm, and concluded that CO
2 concentrations were rising due to anthropogenic emissions.[5] However, Callendar's findings were not widely accepted by the scientific community due to the patchy nature of the measurements.[6][7]

Charles David Keeling, of the Scripps Institution of Oceanography at UC San Diego, was the first person to make frequent regular measurements of atmospheric CO2 concentrations in Antarctica, and on Mauna Loa, Hawaii from March 1958 onwards.[8] Keeling had previously tested and employed measurement techniques at locations including Big Sur near Monterey, rain forests of the Olympic Peninsula in Washington state, and high mountain forests in Arizona.[1] He observed strong diurnal behavior of CO2, with excess CO2 at night due to respiration by plants and soils, and afternoon values representative of the "free atmosphere" over the Northern Hemisphere.[1]

## Mauna Loa measurements

The Mauna Loa Observatory in Hawaii

In 1957–1958, the International Geophysical Year, Keeling obtained funding from the Weather Bureau to install infrared gas analyzers at remote locations, including the South Pole and on the volcano of Mauna Loa on the island of Hawaii. Mauna Loa was chosen as a long-term monitoring site due to its remote location far from continents and its lack of vegetation. Keeling and his collaborators measured the incoming ocean breeze above the thermal inversion layer to minimize local contamination from volcanic vents.[8] The data was normalized to remove any influence from local contamination. Due to funding cuts in the mid-1960s, Keeling was forced to abandon continuous monitoring efforts at the South Pole, but he scraped together enough money to maintain operations at the Mauna Loa Observatory, which have continued to the present day.[9]

External video Scripps Institution of Oceanography at UC San Diego, The Keeling Curve Animation, Scripps Institution of Oceanography at UC San Diego

Ralph Keeling, "The (Ralph) Keeling Curve", Scripps Institution of Oceanography at UC San Diego
John Barnes, Mauna Loa Observatory I Exploratorium, Exploratorium
Charles David Keeling, "The Keeling Curve Turns 50"
Charles David Keeling, 2005 "Tyler Prize Laureate Lecture", Scripps Institution of Oceanography at UC San Diego

Keeling's Tellus article of 1960 presented the first monthly CO
2 records from Mauna Loa and Antarctica (1957 to 1960), finding a "distinct seasonal cycle...and possibly, a worldwide rise in CO
2 from year to year."[10][9]: 41–42 By the 1970s, it was well established that the increase of atmospheric carbon dioxide was ongoing and due to anthropogenic emissions.[11][12]

Carbon dioxide measurements at the Mauna Loa Observatory in Hawaii are made with a type of infrared spectrophotometer, now known as a nondispersive infrared sensor, that is calibrated using World Meteorological Organization standards.[13] This type of instrument, originally called a capnograph, was first invented by John Tyndall in 1864, and recorded by pen traces on a strip chart recorder.[14] Currently, several laser-based sensors are being added to run concurrently with the infrared spectrophotometer at the Scripps Institute of Oceanography, while NOAA measurements at Mauna Loa still use the nondispersive infrared sensor.

## Results and interpretation

The measurements collected at Mauna Loa Observatory show a steady increase in mean atmospheric CO2 concentration from 313 parts per million by volume (ppm) in March 1958 to 406 ppm in November 2018,[15] with a current increase of 2.48 ± 0.26 (mean ± 2 std dev) ppm CO2 per year.[16] This increase in atmospheric CO2 is due to the combustion of fossil fuels, and has been accelerating in recent years. Since CO2 is a greenhouse gas, this has significant implications for global warming. Measurements of CO2 concentration in ancient air bubbles trapped in polar ice cores show that mean atmospheric CO2 concentration was between 275 and 285 ppm during the Holocene epoch (9,000 BCE onwards), but started rising sharply at the beginning of the nineteenth century.[17]

The Keeling Curve also shows a cyclic variation of about 6 ppm each year corresponding to the seasonal change in uptake of CO
2 by the world's land vegetation. Most of this vegetation is in the Northern Hemisphere, where most of the land is located. From a maximum in May, the level decreases during the northern spring and summer as new plant growth takes CO
2 out of the atmosphere through photosynthesis. After reaching a minimum in September, the level rises again in the northern fall and winter as plants and leaves die off and decay, releasing CO
2 back into the atmosphere.[10][12]

## Legacy

### Global monitoring

Due in part to the significance of Keeling's findings,[9] NOAA began monitoring CO2 levels worldwide in the 1970s.[18] Today, atmospheric CO2 levels are monitored at about 100 sites around the globe through the Global Greenhouse Gas Reference Network.[19] Measurements at many other isolated sites have confirmed the long-term trend shown by the Keeling Curve,[20] although no sites have as long a record as Mauna Loa.[21]

ICOS Curve showing the atmospheric CO
2 levels in Europe from 1990 to 2025

#### ICOS Curve

The Integrated Carbon Observation System gathers data from 38 monitoring sites across Europe in a curve similar to the Keeling Curve. It is dubbed the "ICOS Curve".[22] It shows an average CO
2 concentration that is slightly higher than the global average, since Europe is densely populated and still emits high levels of CO
2.[23]

### Ralph Keeling

Since Charles David Keeling's death in 2005, responsibility and oversight of the project were transferred to Keeling's son, Ralph Keeling. On the fiftieth anniversary of the project's start, the younger Keeling wrote an article in Science magazine describing his father's life and work and how the project has grown and evolved over time.[24] Along with more precise measurement materials and funds for the project of monitoring of the Earth's CO
2 levels, Keeling wrote about his pride for his father's work and how he has continued it in his memory.

### Recognition

In 2015, the Keeling Curve was designated a National Historic Chemical Landmark by the American Chemical Society.[25] Commemorative plaques were installed at Mauna Loa Observatory and at the Scripps Institution of Oceanography at the University of California, San Diego.

### Passing 400 ppm in 2013

On May 9, 2013, the daily mean concentration of CO
2 in the atmosphere measured at Mauna Loa surpassed 400 parts per million (ppm).[26] Estimates of CO
2 during previous geologic eras suggest that CO
2 has not reached this level since the mid-Pliocene, 2 to 4 million years ago.[27] This level of carbon dioxide, causing climate change, suggests a continued worsening in natural and ecological disasters, which increasingly threatens human and animal habitats on Earth, if greenhouse gas emissions are not significantly reduced.

## Keeling Plot

A Keeling Plot is a graphical representation of variations in δ13C (carbon isotope ratios) (y-axis), plotted against the inverse of CO
2 concentration (1/[CO
2]) (x-axis), used to identify sources and sinks of carbon within ecosystems, for paleoclimate studies,[28] or for carbon cycle researches.[29]

## See also

* Greenhouse gas monitoring
* Action for Climate Empowerment (ACE)
* Paris Agreement

## References

1. ↑ 1.0 1.1 1.2 "The Early Keeling Curve | Scripps CO2 Program" (in en). https://scrippsco2.ucsd.edu/history_legacy/early_keeling_curve.html.
2. ↑ Naomi Oreskes (23 January 2017). Climate Disruption (video). Awesome Documentaries TV. Archived from the original on 2021-12-12. Retrieved 27 August 2017.
3. ↑ Nisbet, Euan (2007). "Cinderella science". Nature 450 (7171): 789–790. doi:10.1038/450789a. PMID 18063983. https://www.esrl.noaa.gov/gmd/co2conference/Reporters/EarthmonitoringCinderellascience_Nature.pdf.
4. ↑ Brown, Horace Tabberer; Escombe, F. (1905). "On the variations in the amount of carbon dioxide in the air of Kew during the years 1898-1901" (in en). Proc. R. Soc. Lond. B 76 (507): 118–121. doi:10.1098/rspb.1905.0004. ISSN 0950-1193. Bibcode: 1905RSPSB..76..118B.
5. ↑ Callendar, Guy Stewart (1938). "The artificial production of carbon dioxide and its influence on temperature". Quarterly Journal of the Royal Meteorological Society 64 (275): 223–240. doi:10.1002/qj.49706427503. Bibcode: 1938QJRMS..64..223C. https://www.eas.ualberta.ca/jdwilson/EAS372_15/exams/Callendar_QJRMS1938.pdf. Retrieved 2018-11-24.
6. ↑ Fleming, James Rodger (1998). Historical Perspectives on Climate Change. Oxford: Oxford University Press. ISBN 978-0195078701.
7. ↑ "The Carbon Dioxide Greenhouse Effect" (in en). https://history.aip.org/history/climate/co2.htm#SC.
8. ↑ 8.0 8.1 Harris, Daniel C. (2010). "Charles David Keeling and the Story of Atmospheric CO2 Measurements". Analytical Chemistry 82 (19): 7865–7870. doi:10.1021/ac1001492. ISSN 0003-2700. PMID 20536268.
9. ↑ 9.0 9.1 9.2 Keeling, Charles D. (1998). "Rewards and Penalties of Monitoring the Earth". Annual Review of Energy and the Environment 23: 25–82. doi:10.1146/annurev.energy.23.1.25.
10. ↑ 10.0 10.1 Keeling, Charles D. (1960). "The concentration and isotopic abundances of carbon dioxide in the atmosphere". Tellus 12 (2): 200–203. doi:10.3402/tellusa.v12i2.9366. Bibcode: 1960Tell...12..200K. http://scrippsco2.ucsd.edu/assets/publications/keeling_tellus_1960.pdf.
11. ↑ Pales, Jack C.; Keeling, Charles David (1965). "The Concentration of Atmospheric Carbon Dioxide in Hawaii". Journal of Geophysical Research 70 (24): 6053–6076. doi:10.1029/JZ070i024p06053. Bibcode: 1965JGR....70.6053P.
12. ↑ 12.0 12.1 Keeling, Charles D.; Bacastow, Robert B.; Bainbridge, Arnold E.; Ekdahl Jr., Carl A.; Guenther, Peter R.; Waterman, Lee S.; Chin, John F. S. (1976). "Atmospheric carbon dioxide variations at Mauna Loa Observatory, Hawaii" (in en). Tellus 28 (6): 538–551. doi:10.3402/tellusa.v28i6.11322. ISSN 0040-2826. Bibcode: 1976Tell...28..538K.
13. ↑ Tans, Pieter; Thoning, Kirk (March 2018). "How we measure background CO2 levels on Mauna Loa". https://www.esrl.noaa.gov/gmd/ccgg/about/co2_measurements.pdf.
14. ↑ "Sampling the Air". The New York Times. December 22, 2010. https://www.nytimes.com/interactive/2010/12/22/science/earth/20101222-carbon/index.html.
15. ↑ "Recent Monthly Average Mauna Loa CO2". http://www.esrl.noaa.gov/gmd/ccgg/trends/.
16. ↑ Rasmussen, Carl Edward. "Atmospheric Carbon Dioxide Growth Rate". http://mlg.eng.cam.ac.uk/carl/words/carbon.html.
17. ↑ Neftel, A.; Moor, E.; Oeschger, H.; Stauffer, B. (1985). "Evidence from polar ice cores for the increase in atmospheric CO2 in the past two centuries". Nature 315 (6014): 45–47. doi:10.1038/315045a0. Bibcode: 1985Natur.315...45N.
18. ↑ Keeling, Charles D. (1978). "The Influence of Mauna Loa Observatory on the Development of Atmospheric CO2 Research". In Mauna Loa Observatory: A 20th Anniversary Report. (National Oceanic and Atmospheric Administration Special Report, September 1978), edited by John Miller, pp. 36–54. Boulder, CO: NOAA Environmental Research Laboratories.
19. ↑ NOAA, Earth System Laboratory, US Department of Commerce Research. "ESRL Global Monitoring Division - Global Greenhouse Gas Reference Network" (in EN-US). https://www.esrl.noaa.gov/gmd/ccgg/.
20. ↑ Global Stations CO
2 Concentration Trends. Scripps CO2 Program.
21. ↑ Keeling, Charles D.; Whorf, T. P. (2004). "Atmospheric CO2 from Continuous Air Samples at Mauna Loa Observatory, Hawaii, U.S.A.". http://cdiac.ornl.gov/trends/co2/sio-keel-flask/sio-keel-flaskmlo_c.html.
22. ↑ "New ICOS Curve shows CO2 concentrations and trends over Europe | ICOS" (in en). https://www.icos-cp.eu/news-and-events/news/new-icos-curve-shows-co2-concentrations-and-trends-over-europe.
23. ↑ "ICOS Curve" (in en). https://www.icos-cp.eu/science-and-impact/icos-curve.
24. ↑ Keeling, Ralph F. (2008). "Recording Earth's Vital Signs" (in en). Science 319 (5871): 1771–1772. doi:10.1126/science.1156761. ISSN 0036-8075. PMID 18369129.
25. ↑ "Keeling Curve – American Chemical Society" (in en). https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/keeling-curve.html.
26. ↑ Showstack, Randy (2013). "Carbon dioxide tops 400 ppm at Mauna Loa, Hawaii". Eos, Transactions American Geophysical Union 94 (21): 192. doi:10.1002/2013eo210004. ISSN 0096-3941. Bibcode: 2013EOSTr..94Q.192S.
27. ↑ Montaigne, Fen. "Son of Climate Science Pioneer Ponders A Sobering Milestone". Yale School of Forestry & Environmental Studies. http://e360.yale.edu/feature/keeling_curve_son_of_climate_science_pioneer_on_co2_milestone/2650/.
28. ↑ Köhler, Peter; Schmitt, Jochen; Fischer, Hubertus (2006). "On the application and interpretation of Keeling plots in paleo climate research – Deciphering δ13C of atmospheric CO2 measured in ice cores". Biogeosciences Discussions 3: 513–573. ISSN 1810-6285. https://epic.awi.de/id/eprint/13636/. Retrieved 2026-01-20.
29. ↑ Pataki, D. E.; Ehleringer, J. R.; Flanagan, L. B.; Yakir, D.; Bowling, D. R.; Still, C. J.; Buchmann, N.; Kaplan, J. O. et al. (2003). "The application and interpretation of Keeling plots in terrestrial carbon cycle research". Global Biogeochemical Cycles (American Geophysical Union (AGU)) 17 (1). doi:10.1029/2001gb001850. ISSN 0886-6236.

## External links

* Official Keeling Curve website. Scripps Institution of Oceanography, UC San Diego
* CO
2 earth: Annually-updated version of the Keeling curve
* Climate Change Is Clear Atop Mauna Loa, NPR, Day to Day, May 1, 2007
* Scripps Institution of Oceanography CO2-Program: Home of the Keeling Curve

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