The Keeling Curve is a graph of atmospheric carbon dioxide (CO2) concentrations measured continuously since 1958 at Hawaii’s Mauna Loa Observatory. When measurements began, CO2 stood at 315 parts per million (ppm). The 2024 annual average was 424.61 ppm. That steady, decades-long climb, punctuated by a rhythmic seasonal zigzag, makes it one of the most important datasets in climate science and the first direct proof that human activity was raising CO2 levels in the atmosphere.
How the Measurements Began
In the 1950s, scientists suspected that burning fossil fuels was adding CO2 to the atmosphere, but they couldn’t prove it. Existing measurements varied wildly from place to place and day to day, making trends impossible to detect. Charles David Keeling, a geochemist at the Scripps Institution of Oceanography, set out to solve this problem with painstaking attention to precision and consistency.
Keeling chose Mauna Loa, a volcano on Hawaii’s Big Island, because its remote, high-altitude location (about 11,000 feet) sits far from cities, forests, and other local sources that could skew readings. The air sampled there reflects a well-mixed slice of the global atmosphere rather than conditions in any one region. He began collecting data in March 1958, and within two years he had already uncovered two groundbreaking findings: the atmosphere had a regular seasonal CO2 pulse, and underneath that pulse, CO2 was rising year after year.
Why It Looks Like a Sawtooth
The Keeling Curve’s most recognizable feature is its jagged, sawtooth shape. Each year, CO2 dips during the Northern Hemisphere’s spring and summer as plants photosynthesize and pull carbon from the air. It rises again in fall and winter as leaves drop, decay accelerates, and respiration from soils and decomposing plant matter releases carbon back. The Northern Hemisphere drives this pattern because it contains far more land (and therefore more vegetation) than the Southern Hemisphere.
The size of this annual swing is larger at high northern latitudes and smaller near the equator. In recent decades, the swing has been getting bigger. Research published in Science found that photosynthetic carbon uptake in northern ecosystems has increased more strongly in response to warming than carbon release processes have, amplifying the seasonal cycle. Changing vegetation cover, including the greening of Arctic and boreal regions, also plays a role. The result is a deeper annual inhale and exhale layered on top of the long-term upward trend.
What the Curve Proved
Before Keeling’s work, many scientists believed that the carbon cycle would naturally absorb excess CO2. The oceans, forests, and soils exchange enormous quantities of carbon with the atmosphere every year, and a widespread assumption held that these exchanges would buffer any human additions. Keeling’s continuous, precise record disproved that idea. Year after year, CO2 kept climbing. Natural sinks were absorbing some of the extra carbon, but not nearly enough to keep pace with emissions from coal, oil, and natural gas.
The American Chemical Society describes the dataset as providing “one of the most important scientific linkages between fossil fuel combustion and global climate change due to the greenhouse effect.” Before the Keeling Curve, the connection between fossil fuels and rising temperatures was theoretical. Afterward, scientists had real atmospheric measurements, not estimates, to anchor their understanding. The data also deepened knowledge of how carbon moves between the oceans, atmosphere, living organisms, and rocks, a set of processes collectively called the carbon cycle.
Key Milestones on the Curve
The pre-industrial CO2 concentration, reconstructed from air bubbles trapped in ancient ice cores, was roughly 278 ppm. That level held relatively stable for thousands of years before the Industrial Revolution. By the time Keeling started measuring in 1958, it had already climbed to 315 ppm. The pace of increase has accelerated since then.
CO2 first crossed 400 ppm at Mauna Loa in May 2013, a symbolic threshold because it had not been that high in at least 800,000 years of ice core records. By March 2014, readings exceeded 400 ppm nearly two months earlier in the seasonal cycle than the year before, illustrating how quickly the baseline was shifting. In May 2024, the monthly peak hit just under 427 ppm, another record. The annual average for 2024 settled at 424.61 ppm. CO2 has risen more than 45 percent above pre-industrial levels.
How CO2 Is Measured
The core technology uses infrared light. CO2 molecules absorb infrared radiation at specific wavelengths, so by passing a beam of infrared light through a sample of dried air and measuring how much light gets absorbed, instruments can calculate the precise concentration of CO2 in that sample. These devices, called non-dispersive infrared analyzers, run continuously at the observatory. Flask samples are also collected weekly and analyzed independently as a cross-check.
Getting accurate numbers requires careful control. Water vapor interferes with CO2 readings, so air samples are thoroughly dried before analysis. Temperature and pressure must be measured precisely. NOAA’s Global Monitoring Laboratory maintains a set of primary reference standards calibrated using a method that physically extracts CO2 from air and measures its pressure in a tiny, precisely known volume. Those standards are used to calibrate the field instruments twice a year, keeping the entire record on a consistent scale across decades.
Who Maintains the Record Today
Two institutions run parallel CO2 measurements at Mauna Loa. Scripps Institution of Oceanography has maintained the original Keeling program since 1958; after Charles David Keeling’s death in 2005, his son Ralph Keeling took over. NOAA’s Global Monitoring Laboratory began its own independent measurements at the same site in 1974. Having two groups measure the same atmosphere with different instruments provides a built-in quality check. Both datasets are publicly available and updated regularly.
In late 2022, a lava flow from Mauna Loa temporarily disrupted access to the observatory. Backup measurements from a nearby site on Mauna Kea filled the gap until the primary station was restored, preserving the continuity of the longest direct CO2 record on Earth. That unbroken timeline, now spanning more than 65 years, is what gives the Keeling Curve its scientific power: a single, consistent measurement showing exactly how fast the composition of the atmosphere is changing.