Graph depicting the Scripps carbon dioxide (CO2) measurements recorded at the Mauna Loa Observatory.

Image by the Scripps Institution of Oceanography

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  • The Keeling Curve is a graph that represents the concentration of carbon dioxide (CO2) in Earth’s atmosphere since 1958. The Keeling Curve is named after its creator, Dr. Charles David Keeling. 

    Keeling began studying atmospheric carbon dioxide in 1956 by taking air samples and measuring the amount of CO2 they contained. Over time he noticed a pattern. The air samples taken at night contained a higher concentration of CO2 compared to samples taken during the day. He drew on his understanding of photosynthesis and plant respiration to explain this observation: plants take in CO2 during the day to photosynthesize—or make food for themselves—but at night, they release CO2. By studying his measurements over the course of a few years, Keeling also noticed a larger seasonal pattern. He discovered CO2 levels are highest in the spring, when decomposing plant matter releases CO2 into the air, and are lowest in autumn when plants stop taking in CO2 for photosynthesis. 

    Keeling was able to establish a permanent residence at the Mauna Loa Observatory in Hawaii to continue his research. At Mauna Loa, he discovered global atmospheric CO2 levels were rising nearly every year. By analyzing the CO2 in his samples, Keeling was able to attribute this rise to the use of fossil fuels. Since its creation, the Keeling Curve has served as a visual representation of Keeling’s data, which scientists have continued to collect since his death in 2005.


    The Keeling Curve

    Graph depicting the Scripps carbon dioxide (CO2) measurements recorded at the Mauna Loa Observatory.

  • Term Part of Speech Definition Encyclopedic Entry
    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    autumn Noun

    season between summer and winter. Also called fall.

    carbon dioxide Noun

    greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

    Charles Keeling Noun

    (1928-2005) American scientist who confirmed the accumulation of CO2 in the atmosphere.

    concentration Noun

    measure of the amount of a substance or grouping in a specific place.

    fossil fuel Noun

    coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.

    Keeling curve Adjective

    graph illustrating the amount of carbon dioxide (CO2) in Earth’s atmosphere as measured at the Mauna Loa Observatory in Hawaii.

    pattern Noun

    arrangement of people, places, or things across a specific space.

    photosynthesis Noun

    process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

    seasonal Adjective

    likely to change with the seasons.