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 Hawai'i, United States, 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.