The Keeling Curve shows the steady rise of carbon dioxide (CO2) in Earth’s atmosphere since 1958, measured continuously at Hawaii’s Mauna Loa Observatory. It is the longest unbroken record of atmospheric CO2 ever collected, and it was the first direct evidence that burning fossil fuels was changing the composition of the atmosphere. When measurements began, CO2 levels were around 315 parts per million (ppm). Today they exceed 420 ppm.
How the Curve Got Its Start
In March 1958, a geochemist named Charles David Keeling began measuring CO2 from a research station perched at 3,400 meters on the slopes of Mauna Loa. The location was chosen deliberately: high above local pollution sources, far from large landmasses and forests, surrounded by ocean air. This isolation gave Keeling a cleaner sample of background atmospheric CO2 than almost any other place on Earth could provide.
Keeling worked out of the Scripps Institution of Oceanography in San Diego, and the measurements were hosted at a facility run by what is now NOAA. Within just a couple of years, two things became obvious in his data. First, CO2 levels rose and fell in a repeating annual cycle. Second, the overall trend was climbing upward, year after year. That upward climb has never reversed.
The Sawtooth Pattern: Earth Breathing
One of the most striking features of the Keeling Curve is its zigzag shape. CO2 doesn’t just go up in a straight line. It rises each winter, drops each summer, then rises again, creating a sawtooth pattern that repeats every year. This cycle reflects the seasonal rhythm of plant life, primarily in the Northern Hemisphere, where most of the world’s land and vegetation are concentrated.
Starting in May, as forests, grasslands, and crops across North America, Europe, and Asia photosynthesize in full swing, they pull CO2 out of the air and lock it into leaves, wood, and roots. Atmospheric CO2 dips. When winter arrives and photosynthesis slows, the balance tips the other way: bacteria decomposing dead leaves, animals exhaling, and soil organisms all continue releasing CO2, pushing levels back up. The curve reaches its annual peak around May, just before the growing season kicks in again.
Ocean photosynthesis, while enormous in scale, barely registers in this cycle. Marine algae absorb and release CO2 mostly within the water itself, so very little of that exchange reaches the atmosphere. The seasonal zigzag is driven almost entirely by land plants.
The Long-Term Rise
Zoom out from the seasonal pattern and the real story emerges: a relentless upward trend. When Keeling started his measurements, CO2 stood at about 315 ppm. By the early 2000s it had crossed 370 ppm. It passed 400 ppm around 2013 and now sits above 420 ppm. The rate of increase has also accelerated. In the 1960s, CO2 rose by roughly 0.8 ppm per year. In recent years, the annual increase has more than tripled that pace.
This matters because of what ice core records tell us about the past. Scientists have extracted mile-thick cylinders of ancient ice from Antarctica and Greenland, trapping tiny bubbles of air that preserve the atmosphere from hundreds of thousands of years ago. Those records stretch back 800,000 years, covering multiple ice ages and warm periods. Throughout that entire span, CO2 never exceeded 300 ppm. Before the Industrial Revolution began in the mid-1700s, the level was around 280 ppm or less. The current concentration is not just higher than anything in recorded history. It is higher than anything the planet has experienced in at least 800 millennia.
Why the Curve Was So Important for Climate Science
Before Keeling’s measurements, scientists debated whether CO2 released by burning coal, oil, and gas actually accumulated in the atmosphere or whether the oceans and forests simply absorbed it all. The Keeling Curve settled the question. NASA describes it as the first confirmation that atmospheric CO2 was rising due to fossil fuel combustion. Each year’s peak was higher than the last, and the trend tracked closely with global fossil fuel use.
The curve also gave researchers a precise baseline against which to measure everything else: the effectiveness of carbon sinks like oceans and forests, the pace of emissions growth, and the relationship between CO2 concentration and global temperature. Without it, climate science would have lacked its most fundamental data point.
How CO2 Is Actually Measured
The original instrument Keeling installed in 1958 was an infrared gas analyzer. CO2 molecules absorb infrared light at specific wavelengths, so shining infrared light through a sample of air and measuring how much gets absorbed gives you a precise CO2 reading. That original analyzer ran almost continuously for nearly 50 years before being replaced in 2006 with a newer model. Scripps has since begun upgrading again to an even more advanced technology called cavity ring-down spectroscopy, which measures how quickly pulses of laser light decay inside a mirrored chamber filled with air.
The precision involved is remarkable. Researchers filter the raw data by looking for stretches of at least five consecutive hours where readings are stable, with hourly averages agreeing to within 0.22 ppm of each other. Multiple analyzers running simultaneously typically agree within 0.2 to 0.3 ppm. This tight consistency is what makes the dataset so reliable across decades.
Beyond Mauna Loa
While the Keeling Curve from Mauna Loa remains the most famous CO2 record, it is no longer the only one. NOAA’s Global Greenhouse Gas Reference Network now collects air samples from more than 50 sites around the world, staffed largely by trained volunteers. These stations span from the Arctic to the South Pole, filling in a global picture that a single Hawaiian mountaintop cannot capture on its own. NOAA notes that the Mauna Loa data, collected in the northern subtropics at high altitude, may not perfectly match the global average surface concentration. The worldwide network helps scientists calculate that global average and track how CO2 varies by latitude and season.
Still, the Mauna Loa record holds a unique place. Its unbroken 60-plus-year timeline provides something no other station can: a direct, continuous line from the late 1950s to today, showing exactly how fast the atmosphere has changed within a single human lifetime.