View source for Keeling Curve

{{Short description|Graph of atmospheric CO2 from 1958 to the present}}
{{Redirect|Keeling}}
[[File:Mauna Loa CO2 monthly mean concentration.svg|thumb|Atmospheric [[carbon dioxide]] ({{CO2}}) concentrations from 1958 to 2023|500px]]

The '''Keeling Curve''' is a graph of the annual variation and overall accumulation of [[carbon dioxide in Earth's atmosphere|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]] (CO<sub>2</sub>) levels in the atmosphere.<ref name="UCSD">{{Cite web|url=https://scrippsco2.ucsd.edu/history_legacy/early_keeling_curve.html|title=The Early Keeling Curve {{!}} Scripps CO<sub>2</sub> Program|website=scrippsco2.ucsd.edu|language=en|access-date=2018-11-24}}</ref>  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.<ref name="clidis">{{cite AV media|url=https://www.youtube.com/watch?v=ENvJ2WqxNgQ |archive-url=https://ghostarchive.org/varchive/youtube/20211212/ENvJ2WqxNgQ| archive-date=2021-12-12 |url-status=live|title=Climate Disruption|date=23 January 2017|people=Naomi Oreskes|language=en|publisher=Awesome Documentaries TV|access-date=27 August 2017|medium=video}}{{cbignore}}</ref> Many scientists credit the Keeling curve with first bringing the world's attention to the current increase of {{CO2}} in the atmosphere.<ref>{{Cite journal|last=Nisbet|first=Euan|date=2007|title=Cinderella science|url=https://www.esrl.noaa.gov/gmd/co2conference/Reporters/EarthmonitoringCinderellascience_Nature.pdf|journal=Nature|volume=450|issue=7171|pages=789–790|doi=10.1038/450789a|pmid=18063983|doi-access=free}}</ref>

== Background ==
Prior to the 1950s, measurements of atmospheric {{CO2}} concentrations had been taken on an [[ad hoc]] basis at a variety of locations. In 1938, engineer and amateur meteorologist [[Guy Stewart Callendar]] compared datasets of atmospheric {{CO2}} from [[Kew]] in 1898–1901, which averaged 274 parts per million by volume ([[Parts-per notation|ppm]]),<ref>{{Cite journal|last1=Brown|first1=Horace Tabberer|last2=Escombe|first2=F.|date=1905|title=On the variations in the amount of carbon dioxide in the air of Kew during the years 1898-1901|journal=Proc. R. Soc. Lond. B|language=en|volume=76|issue=507|pages=118–121|doi=10.1098/rspb.1905.0004|bibcode=1905RSPSB..76..118B|issn=0950-1193|doi-access=|s2cid=97664092 }}</ref> and from the eastern [[United States]] in 1936–1938, which averaged 310 [[Parts-per notation|ppm]], and concluded that {{CO2}} concentrations were rising due to [[Anthropogeny|anthropogenic]] emissions.<ref>{{Cite journal|last=Callendar|first=Guy Stewart|date=1938|title=The artificial production of carbon dioxide and its influence on temperature|url=https://www.eas.ualberta.ca/jdwilson/EAS372_15/exams/Callendar_QJRMS1938.pdf|journal=Quarterly Journal of the Royal Meteorological Society|volume=64|issue=275|pages=223–240|doi=10.1002/qj.49706427503|bibcode=1938QJRMS..64..223C|access-date=2018-11-24|archive-date=2020-11-12|archive-url=https://web.archive.org/web/20201112015420/https://www.eas.ualberta.ca/jdwilson/EAS372_15/exams/Callendar_QJRMS1938.pdf|url-status=dead}}</ref> However, [[Guy Stewart Callendar|Callendar]]'s findings were not widely accepted by the scientific community due to the patchy nature of the measurements.<ref name="Fleming">{{cite book|title=Historical Perspectives on Climate Change|last1=Fleming|first1=James Rodger|date=1998|publisher=[[Oxford University Press]]|isbn=978-0195078701|location=Oxford}}</ref><ref>{{Cite web|url=https://history.aip.org/history/climate/co2.htm#SC|title=The Carbon Dioxide Greenhouse Effect|website=history.aip.org|language=en|access-date=2018-11-24}}</ref>

[[Charles David Keeling]], of the [[Scripps Institution of Oceanography]] at [[UC San Diego]], was the first person to make frequent regular measurements of atmospheric CO<sub>2</sub> concentrations in [[Antarctica]], and on [[Mauna Loa]], [[Hawaii]] from March 1958 onwards.<ref name=":2">{{Cite journal|last=Harris|first=Daniel C.|date=2010|title=Charles David Keeling and the Story of Atmospheric CO<sub>2</sub> Measurements|journal=Analytical Chemistry|volume=82|issue=19|pages=7865–7870|doi=10.1021/ac1001492|pmid=20536268|issn=0003-2700}}</ref> Keeling had previously tested and employed measurement techniques at locations including [[Big Sur]] near [[Monterey]], rain forests of the [[Olympic Peninsula]] in [[Washington (state)|Washington state]], and high mountain forests in [[Arizona]].<ref name="UCSD" /> He observed strong [[Diurnal cycle|diurnal]] behavior of CO<sub>2</sub>, with excess CO<sub>2</sub> at night due to [[Respiration (physiology)|respiration]] by plants and soils, and afternoon values representative of the "free atmosphere" over the [[Northern Hemisphere|Northern hemisphere]].<ref name="UCSD" />

==Mauna Loa measurements==
[[File:Mauna Loa Observatory from air.jpg|The Mauna Loa Observatory|thumb|400x400px|alt=]]In 1957–1958, the [[International Geophysical Year]], Keeling obtained funding from the [[Weather Bureau]] to install [[infrared gas analyzer]]s at remote locations, including the [[South Pole]] and on the volcano of [[Mauna Loa]] on [[Hawaii (island)|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 [[Inversion (meteorology)|thermal inversion layer]] to minimize local contamination from volcanic vents.<ref name=":2" /> The data was [[normalization (statistics)|normalized]] to remove any influence from local contamination. Due to funding cuts in the mid-1960s, [[Charles David Keeling|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.<ref name="rewards">{{cite journal|last1=Keeling|first1=Charles D.|date=1998|title=Rewards and Penalties of Monitoring the Earth|journal=[[Annual Review of Energy and the Environment]]|volume=23|pages=25–82|doi=10.1146/annurev.energy.23.1.25|doi-access=free|citeseerx=10.1.1.173.2051}}</ref> {{external media | width = 210px | float = right | headerimage=  | video1 = Scripps Institution of Oceanography at UC San Diego, [https://www.youtube.com/watch?v=rEbE5fcnFVs The Keeling Curve Animation], Scripps Institution of Oceanography at UC San Diego | video2 = Ralph Keeling, [https://www.youtube.com/watch?v=6WFCoJgt71A "The (Ralph) Keeling Curve"], Scripps Institution of Oceanography at UC San Diego | video3 = John Barnes, [https://www.youtube.com/watch?v=3jOAlC2dVtA Mauna Loa Observatory I Exploratorium], [[Exploratorium]] | video4 = Charles David Keeling, [https://www.youtube.com/watch?v=K0Z7RRKzrdg "The Keeling Curve Turns 50"]|video5=Charles David Keeling, 2005 [https://www.youtube.com/watch?v=aH1837EUvTI "Tyler Prize Laureate Lecture"], Scripps Institution of Oceanography at UC San Diego}}

Keeling's ''[[Tellus A|Tellus]]'' article of 1960 presented the first monthly {{CO2}} records from [[Mauna Loa Observatory|Mauna Loa]] and Antarctica (1957 to 1960), finding a "distinct seasonal cycle...and possibly, a worldwide rise in {{CO2}} from year to year."<ref name=":0">{{Cite journal|last=Keeling|first=Charles D.|date=1960|title=The concentration and isotopic abundances of carbon dioxide in the atmosphere|url=http://scrippsco2.ucsd.edu/assets/publications/keeling_tellus_1960.pdf|journal=Tellus|volume=12|issue=2|pages=200–203|doi=10.3402/tellusa.v12i2.9366|bibcode=1960Tell...12..200K|doi-access=free}}</ref><ref name="rewards"/>{{rp|pages=41–42}} By the 1970s, it was well established that the increase of atmospheric carbon dioxide was ongoing and due to anthropogenic emissions.<ref>{{cite journal|year=1965|last1=Pales|first1=Jack C.|last2=Keeling|first2=Charles David|title=The Concentration of Atmospheric Carbon Dioxide in Hawaii|volume=70|issue=24|pages=6053–6076|doi=10.1029/JZ070i024p06053|journal=Journal of Geophysical Research|bibcode=1965JGR....70.6053P}}</ref><ref name=":3">{{Cite journal|last1=Keeling|first1=Charles D.|last2=Bacastow|first2=Robert B.|last3=Bainbridge|first3=Arnold E.|last4=Ekdahl Jr.|first4=Carl A.|last5=Guenther|first5=Peter R.|last6=Waterman|first6=Lee S.|last7=Chin|first7=John F. S.|date=1976|title=Atmospheric carbon dioxide variations at Mauna Loa Observatory, Hawaii|journal=Tellus|language=en|volume=28|issue=6|pages=538–551|doi=10.3402/tellusa.v28i6.11322|bibcode=1976Tell...28..538K|issn=0040-2826|doi-access=free}}</ref>

Carbon dioxide measurements at the [[Mauna Loa Observatory]] in [[Hawaii]] are made with a type of [[infrared spectroscopy|infrared spectrophotometer]], now known as a [[nondispersive infrared sensor]], that is calibrated using [[World Meteorological Organization]] standards.<ref>{{cite web|url=https://www.esrl.noaa.gov/gmd/ccgg/about/co2_measurements.pdf|title=How we measure background CO<sub>2</sub> levels on Mauna Loa|last1=Tans|first1=Pieter|last2=Thoning|first2=Kirk|date=March 2018}}</ref> 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.<ref>{{cite news| url=https://www.nytimes.com/interactive/2010/12/22/science/earth/20101222-carbon/index.html | work=[[The New York Times]] | title=Sampling the Air | date=December 22, 2010}}</ref> Currently, several laser-based sensors are being added to run concurrently with the infrared spectrophotometer at the [[Scripps Institution of Oceanography|Scripps Institute of Oceanography]], while [[NOAA]] measurements at [[Mauna Loa Observatory|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 CO<sub>2</sub> concentration from 313 parts per million by volume ([[Parts-per notation|ppm]]) in March 1958 to 406 [[Parts-per notation|ppm]] in November 2018,<ref name="MaunaMonthly">{{cite web|title=Recent Monthly Average Mauna Loa CO2|url=http://www.esrl.noaa.gov/gmd/ccgg/trends/|website=Earth System Research Laboratory|access-date=9 May 2016}}</ref> with a current increase of 2.48 ± 0.26 (mean ± 2 std dev) [[Parts-per notation|ppm]] CO<sub>2</sub> per year.<ref name="Atmospheric Carbon Dioxide Growth Rate">{{cite web | url=http://mlg.eng.cam.ac.uk/carl/words/carbon.html | title=Atmospheric Carbon Dioxide Growth Rate | author=Rasmussen, Carl Edward}}</ref> This increase in atmospheric CO<sub>2</sub> is due to the [[combustion]] of [[fossil fuel]]s, and has been accelerating in recent years.  Since CO<sub>2</sub> is a [[greenhouse gas]], this has significant implications for [[global warming]]. Measurements of CO<sub>2</sub> concentration in ancient [[air bubble]]s trapped in polar [[ice core]]s show that mean atmospheric CO<sub>2</sub> concentration was between 275 and 285 [[Parts-per notation|ppm]] during the [[Holocene]] epoch (9,000 [[Common Era|BCE]] onwards), but started rising sharply at the beginning of the nineteenth century.<ref name="Neftel">{{cite journal|last1=Neftel|first1=A.|last2=Moor|first2=E.|last3=Oeschger|first3=H.|last4=Stauffer|first4=B.|date=1985|title=Evidence from polar ice cores for the increase in atmospheric CO<sub>2</sub> in the past two centuries|journal=Nature|volume=315|issue=6014|pages=45–47|bibcode=1985Natur.315...45N|doi=10.1038/315045a0|s2cid=4321970}}</ref>

The Keeling Curve also shows a cyclic variation of about 6 [[Parts-per notation|ppm]] each year corresponding to the seasonal change in uptake of {{CO2}} 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 {{CO2}} 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 {{CO2}} back into the atmosphere.<ref name=":0" /><ref name=":3" />

==Legacy==

===Global monitoring===
Due in part to the significance of Keeling's findings,<ref name="rewards" /> [[National Oceanic and Atmospheric Administration|NOAA]] began monitoring CO<sub>2</sub> levels worldwide in the 1970s.<ref>Keeling, Charles D. (1978). "The Influence of Mauna Loa Observatory on the Development of Atmospheric CO<sub>2</sub> 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.</ref> Today, atmospheric CO<sub>2</sub> levels are monitored at about 100 sites around the globe through the Global Greenhouse Gas Reference Network.<ref>{{Cite web|url=https://www.esrl.noaa.gov/gmd/ccgg/|title=ESRL Global Monitoring Division - Global Greenhouse Gas Reference Network|last=NOAA, Earth System Laboratory|first=US Department of Commerce Research|website=www.esrl.noaa.gov|language=EN-US|access-date=2018-11-25}}</ref> Measurements at many other isolated sites have confirmed the long-term trend shown by the Keeling Curve,<ref>[https://scrippsco2.ucsd.edu/graphics_gallery/other_stations/global_stations_co2_concentration_trends.html Global Stations {{CO2}} Concentration Trends]. Scripps CO<sub>2</sub> Program.</ref> although no sites have as long a record as [[Mauna Loa Observatory|Mauna Loa]].<ref name="KeelingWhorfContinuous">{{cite web|url=http://cdiac.ornl.gov/trends/co2/sio-keel-flask/sio-keel-flaskmlo_c.html|title=Atmospheric CO<sub>2</sub> from Continuous Air Samples at Mauna Loa Observatory, Hawaii, U.S.A.|last1=Keeling|first1=Charles D.|last2=Whorf|first2=T. P.|date=2004|access-date=2007-10-17|archive-url=https://web.archive.org/web/20160303173428/http://cdiac.ornl.gov/trends/co2/sio-keel-flask/sio-keel-flaskmlo_c.html|archive-date=2016-03-03|url-status=dead}}</ref>

===Ralph Keeling===
Since [[Charles David Keeling]]'s death in 2005, responsibility and oversight of the project was transferred to Keeling's son, [[Ralph Keeling]]. On the fiftieth anniversary of the beginning of the project, the younger Keeling wrote an article in [[Science (journal)|Science magazine]] describing his father's life and work, along with how the project has grown and evolved over time.<ref>{{Cite journal|last=Keeling|first=Ralph F.|date=2008|title=Recording Earth's Vital Signs|journal=Science|language=en|volume=319|issue=5871|pages=1771–1772|doi=10.1126/science.1156761|issn=0036-8075|pmid=18369129|s2cid=206512305}}</ref> Along with more precise measurement materials and funds for the project of monitoring of the Earth's {{CO2}} 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 Landmarks|National Historic Chemical Landmark]] by the [[American Chemical Society]].<ref>{{Cite web|url=https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/keeling-curve.html|title=Keeling Curve - American Chemical Society|website=American Chemical Society|language=en|access-date=2018-11-25}}</ref> 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 {{CO2}} in the atmosphere measured at Mauna Loa surpassed 400 parts per million ([[Parts-per notation|ppm]]).<ref>{{Cite journal|last=Showstack|first=Randy|date=2013|title=Carbon dioxide tops 400 ppm at Mauna Loa, Hawaii|journal=Eos, Transactions American Geophysical Union|volume=94|issue=21|pages=192|doi=10.1002/2013eo210004|bibcode=2013EOSTr..94Q.192S|issn=0096-3941|doi-access=free}}</ref> Estimates of {{CO2}} during previous [[geologic era]]s suggest that {{CO2}} has not reached this level since the [[Pliocene climate|mid-Pliocene]], 2 to 4 million years ago.<ref name="Montaigne">{{cite web|url=http://e360.yale.edu/feature/keeling_curve_son_of_climate_science_pioneer_on_co2_milestone/2650/|title=Son of Climate Science Pioneer Ponders A Sobering Milestone|last1=Montaigne|first1=Fen|website=Yale Environment 360|publisher=Yale School of Forestry & Environmental Studies|access-date=14 May 2013|archive-url=https://web.archive.org/web/20130608174549/http://e360.yale.edu/feature/keeling_curve_son_of_climate_science_pioneer_on_co2_milestone/2650/|archive-date=8 June 2013|url-status=dead}}</ref> 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.

==See also==
* [[Greenhouse gas monitoring]]
* [[Action for Climate Empowerment]] (ACE)
* [[Paris Agreement]]

==References==
{{Reflist}}

==External links==
{{Commons category|Keeling Curves}}
*[https://keelingcurve.ucsd.edu/ Official Keeling Curve website. Scripps Institution of Oceanography, UC San Diego]
*[https://co2.earth {{CO2}} earth]: Annually-updated version of the Keeling curve
*[https://www.npr.org/templates/story/story.php?storyId=9885767 Climate Change Is Clear Atop Mauna Loa, NPR, Day to Day, May 1, 2007]
*[https://scrippsco2.ucsd.edu/ Scripps Institution of Oceanography CO<sub>2</sub>-Program: Home of the Keeling Curve]

{{Global warming}}

[[Category:Atmosphere]]
[[Category:Historical climatology]]
[[Category:Carbon dioxide]]
[[Category:20th-century neologisms]]