- The Keeling Curve is the longest continuous CO2 record.
- Atmospheric CO2 has risen from 313 ppm in 1958 to over 430 ppm.
- The annual CO2 increase rate has tripled since the 1960s.
The Keeling Curve is a graph of atmospheric carbon dioxide concentrations measured continuously at Hawaii's Mauna Loa Observatory since March 1958, representing the longest unbroken instrumental record of CO2 in Earth's atmosphere.
Key figure
430 ppm
Peak CO2 reading, May 2025
Why It Matters
The Keeling Curve transformed atmospheric CO2 from a theoretical concern into a measured fact. Before Charles David Keeling began his readings in 1958, the idea that fossil fuel combustion was raising CO2 levels rested on scattered, disputed measurements. Within a few years, his continuous record at Mauna Loa settled the question.
The curve's value lies in its length. A single year of data shows seasonal breathing. Sixty-seven years of data show a species altering its atmosphere.
The 1958 starting concentration of 313 parts per million has climbed past 430 ppm, according to readings from the Scripps CO2 Program at UC San Diego.
That trajectory now underpins every major climate model, emissions target, and international agreement on carbon reduction. The Intergovernmental Panel on Climate Change uses Mauna Loa data as a baseline for projecting future warming scenarios.
How It Works
Keeling chose Mauna Loa for its isolation. Situated 3,400 meters above sea level on a volcanic island in the mid-Pacific, the observatory sits far from continental vegetation, cities, and industrial sources. Air sampled there reflects well-mixed global concentrations rather than local emissions.
The measurements use infrared gas analyzers calibrated against reference gases. CO2 absorbs infrared light at specific wavelengths. The analyzers measure how much infrared a sample absorbs and convert that to a concentration in parts per million.
Key figure
1958
Year continuous measurement began
The curve's signature feature is its sawtooth pattern. CO2 drops each Northern Hemisphere summer as land plants absorb carbon during photosynthesis, then rises each winter as decomposition releases it. This annual oscillation rides atop the long-term upward trend driven by fossil fuel combustion, cement production, and deforestation.
Ralph Keeling, Charles's son, took over the Scripps CO2 Program after his father's death in 2005. NOAA runs a parallel measurement program at the same observatory, providing independent confirmation.
Key Context
Charles David Keeling earned his Ph.D. in chemistry from Northwestern University in 1954 and developed his precise CO2 measurement technique during a postdoctoral fellowship at Caltech under geochemist Harrison Brown. His work built on Svante Arrhenius's 1896 hypothesis that fossil fuel combustion could warm the planet by increasing atmospheric CO2.
The Mauna Loa record nearly ended several times due to funding cuts. In the 1960s and again in the 1980s, federal budget pressures threatened to shut down the program. Keeling fought to keep it running, arguing that the value of a continuous record depended on its continuity.
The American Chemical Society designated the Keeling Curve measurements a National Historic Chemical Landmark in 2015.
FAQ
Why is the Keeling Curve measured in Hawaii and not closer to industrial sources?
Mauna Loa's mid-Pacific location and 3,400-meter altitude place it far from local pollution sources. Air reaching the observatory has mixed across hemispheres, making it representative of global background concentrations rather than any single region's emissions.
What causes the zigzag pattern in the Keeling Curve?
The seasonal oscillation reflects the Northern Hemisphere's vegetation cycle. Land plants absorb CO2 during spring and summer growth, lowering atmospheric concentrations. In fall and winter, decomposition releases CO2 back. The Northern Hemisphere dominates because it has far more land mass and vegetation than the Southern Hemisphere.
How does the Keeling Curve differ from ice core CO2 records?
Ice core records extend atmospheric CO2 measurements back 800,000 years by analyzing air bubbles trapped in Antarctic ice. The Keeling Curve provides direct, high-precision instrumental readings from 1958 onward. Together they show that current CO2 levels exceed anything in the ice core record.
Has the rate of CO2 increase changed over time?
Yes. In the 1960s, atmospheric CO2 rose by roughly 0.8 ppm per year. By the 2020s, the annual increase had accelerated to about 2.5 ppm per year, according to NOAA's Global Monitoring Laboratory. The acceleration tracks the growth in global fossil fuel consumption.
Sources
- Primary: Scripps CO2 Program (UC San Diego)
- Additional:
Related Reading
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Sources used for verification
- Scripps CO2 Program - scrippsco2.ucsd.edu
- NOAA Global Monitoring Laboratory - gml.noaa.gov
- ACS Keeling Curve Landmark - acs.org
- Britannica Keeling Curve - britannica.com
- Scripps 430 ppm milestone - scripps.ucsd.edu
