The Keeling Curve is a graph of atmospheric carbon dioxide (CO2) concentrations measured continuously since 1958 at the Mauna Loa Observatory in Hawaii. It is the longest unbroken record of CO2 in the atmosphere and one of the most important datasets in climate science. When measurements began, CO2 stood at 313 parts per million (ppm). As of January 2025, that number had climbed to roughly 427 ppm.
How the Keeling Curve Started
In 1956, a geochemist named Charles David Keeling at the Scripps Institution of Oceanography began a program to measure CO2 in the atmosphere with a level of precision no one had attempted before. He installed an infrared gas analyzer at NOAA’s Mauna Loa Observatory, which sits at about 11,000 feet on a volcanic peak in Hawaii, well above the pollution and vegetation that could skew readings at lower elevations. The instrument recorded its first data point in March 1958: 313 ppm of CO2.
Keeling directed the program until his death in 2005. Today, Scripps Institution of Oceanography at UC San Diego maintains the daily record, with funding from Schmidt Sciences and supplemental support from Earth Networks. NOAA runs a parallel, independent measurement program at the same observatory.
Why Mauna Loa?
Mauna Loa was chosen because it offers some of the cleanest air on Earth for this kind of measurement. Surrounded by thousands of miles of open ocean, the observatory sits far from cities, forests, and industrial sources that would introduce local noise into the data. Its high altitude means the instruments sample well-mixed air representative of the broader atmosphere rather than whatever a nearby highway or power plant happens to emit.
One common question is whether the volcano itself contaminates the readings. It does occasionally vent CO2, but scientists filter out those episodes. The U.S. Geological Survey notes that volcanic CO2 is removed from the long-term atmospheric dataset. That filtered-out data is actually repurposed to study Mauna Loa’s own emissions separately. The instruments detect CO2 using non-dispersive infrared (NDIR) analysis, a technique that measures how much infrared light a sample of air absorbs at wavelengths specific to CO2 molecules.
The Sawtooth Pattern
The most visually striking feature of the Keeling Curve is its zigzag shape. Overlaid on the long upward trend is a seasonal oscillation that repeats every year, giving the graph its characteristic sawtooth appearance. CO2 drops during the Northern Hemisphere’s spring and summer, then rises again in fall and winter.
This happens because of plant activity. During the growing season, forests, grasslands, and crops across the Northern Hemisphere pull enormous quantities of CO2 out of the air through photosynthesis. When those plants shed their leaves or go dormant in autumn, decomposition and respiration release that carbon back. The Northern Hemisphere drives this cycle more than the Southern Hemisphere simply because it contains far more land area and therefore more vegetation. The seasonal swing ranges from about 1 ppm in the tropics to as much as 15 ppm at high northern latitudes like Alaska, where the contrast between summer growth and winter dormancy is most extreme.
What the Numbers Show
Before the Industrial Revolution, atmospheric CO2 hovered around 280 ppm or less, a level reconstructed from air bubbles trapped in ancient ice cores. By 1958, when Keeling started measuring, it had already risen to 313 ppm. Today it is roughly 50 percent higher than pre-industrial levels.
The rate of increase has accelerated substantially. In the 1960s, CO2 rose by less than 1 ppm per year on average. By the 1970s, the annual increase was typically around 1 to 1.5 ppm. In 2023, the growth rate was 2.70 ppm. In 2024, it jumped to 3.74 ppm, the highest single-year increase on record. That acceleration reflects both rising fossil fuel emissions and, in some years, natural factors like El Niño events that temporarily reduce the ability of oceans and land to absorb carbon.
Why the Keeling Curve Matters
Before Keeling’s measurements, scientists debated whether human CO2 emissions were actually accumulating in the atmosphere or being absorbed entirely by the oceans. The curve settled that question within just a few years. The steady upward march was unmistakable, and it tracked closely with the growth in fossil fuel burning.
That clarity had direct consequences for international policy. As attention to greenhouse gases intensified, the United Nations created the Intergovernmental Panel on Climate Change (IPCC) in 1988, tasking it with assessing the scientific evidence on climate change. Its first report in 1990 outlined the need for international coordination and set the stage for the 1992 United Nations Framework Convention on Climate Change, which aimed to stabilize greenhouse gas concentrations at a level that would prevent dangerous human-caused interference with the climate. Five years later, the Kyoto Protocol became the first international agreement committing member countries to actual reductions in CO2 and other greenhouse gases.
None of those efforts would have had the same scientific foundation without the continuous, precise record the Keeling Curve provides. It transformed an abstract hypothesis about carbon and warming into a visible, measurable trend that could be tracked year by year, season by season, and compared against every other climate variable scientists cared about.
Reading the Curve Today
You can view the Keeling Curve in real time on the Scripps website at keelingcurve.ucsd.edu. The site updates daily and shows both the full record stretching back to 1958 and a zoomed-in view of recent months. NOAA publishes its own parallel dataset at gml.noaa.gov. The two programs use slightly different calibration methods but track each other closely, which provides an independent check on data quality.
When you look at the graph, the two features to notice are the overall slope and the seasonal wiggles. The slope tells you how fast CO2 is accumulating. The wiggles tell you the planet’s ecosystems are still breathing, pulling carbon in and pushing it out with the seasons. Both matter: the slope determines how much warming to expect in the decades ahead, and changes in the amplitude of the wiggles can reveal shifts in how ecosystems respond to a warming world.