The Keeling Curve is a continuous graph that illustrates the changing concentration of carbon dioxide (CO2) in Earth’s atmosphere. This scientific record began in 1958, with measurements consistently taken at the Mauna Loa Observatory located in Hawaii. The curve is named after its originator, American climate scientist Charles David Keeling, who initiated this long-term monitoring program. It provides a direct, ongoing measure of atmospheric CO2 levels over time.
The Origin of the Measurement
The journey of the Keeling Curve began with Charles David Keeling, a scientist from the Scripps Institution of Oceanography, who aimed to measure atmospheric CO2 with exceptional precision. He sought to establish an unbiased baseline of atmospheric CO2, far from localized sources of pollution. In March 1958, his first instrument was installed at the Mauna Loa Observatory. This remote site, high above the Pacific Ocean, was chosen to ensure air samples were well-mixed and minimally influenced by local vegetation or industrial emissions.
Decoding the Seasonal Cycle
A distinct “saw-tooth” or “zig-zag” pattern is a noticeable feature of the Keeling Curve, reflecting annual fluctuations in atmospheric CO2 levels. This pattern arises from the seasonal cycle of plant life, predominantly in the Northern Hemisphere, which contains most of Earth’s landmass and vegetation. During spring and summer, plants engage in widespread photosynthesis, absorbing substantial amounts of CO2 from the atmosphere as they grow. This biological activity causes a measurable drop in atmospheric CO2.
Conversely, during the fall and winter months, plant growth diminishes, and leaves, along with other organic matter, decay. This decomposition process, carried out by microbes, releases CO2 back into the atmosphere through respiration, leading to an increase in atmospheric concentrations. The continuous rise and fall illustrates the planet’s natural carbon cycle on a yearly basis.
The Upward Trend and Its Implications
Beyond the seasonal oscillations, the Keeling Curve displays an unmistakable long-term upward trajectory in atmospheric CO2 concentrations. When measurements began in 1958, CO2 levels were around 313 parts per million (ppm). By 2024, average concentrations had risen to approximately 425 ppm, marking a substantial increase. This overall rise is directly attributed to human activities, primarily the widespread burning of fossil fuels such as coal, oil, and natural gas, which began accelerating with the Industrial Revolution.
The curve illustrates that roughly half of the CO2 released from fossil fuel combustion remains in the atmosphere, a proportion known as the “airborne fraction”. The rest is absorbed by oceans and terrestrial ecosystems.