Atmospheric CO₂ Over Time: The Keeling Curve Explained
Since 1958, scientists atop Hawaii's Mauna Loa have measured the carbon dioxide in Earth's atmosphere every day. The result — the famous Keeling Curve — is one of the most important graphs in science: a relentless upward climb from about 315 parts per million to over 430 today, wrapped in a yearly sawtooth as the planet breathes in and out. This guide explains what the curve shows, why it zigzags, how fast it's rising, and why the pre-industrial level of 280 ppm is the number everything is measured against.
What is the Keeling Curve?
In 1958 the scientist Charles David Keeling began continuously recording CO₂ at the Mauna Loa Observatory, a remote peak far from cities and forests where the air is well-mixed and representative of the whole Northern Hemisphere. That unbroken record — now the longest direct measurement of atmospheric CO₂ in existence — is what we call the Keeling Curve. Its message is unmistakable: every single year since 1958 has averaged more CO₂ than the year before.
Why does CO₂ zigzag every year?
Look closely and the line wobbles up and down within each year by roughly 6–7 ppm. That's the planet breathing. Most of Earth's land — and therefore most of its plant life — sits in the Northern Hemisphere. Each spring and summer, growing plants pull CO₂ out of the air and the curve dips; each fall and winter, as leaves decay, that carbon is released and the curve climbs. The smooth dashed line removes this seasonal cycle to show the underlying trend, which only ever goes one way.
How fast is CO₂ rising — and is it speeding up?
Yes, it's accelerating. In the 1960s CO₂ rose by under 1 ppm per year. Over the past decade it has averaged well over 2 ppm per year — the fastest sustained increase in the entire record, and far faster than any natural change seen in the ice-core data spanning hundreds of thousands of years. The rise tracks closely with global fossil-fuel use, which is the dominant source of the added carbon.
Why 280 ppm matters: the pre-industrial baseline
Before the Industrial Revolution, ice-core records show atmospheric CO₂ held near 280 ppm for thousands of years. Today's level is more than 50% above that baseline — a level the planet has not experienced in at least 800,000 years, and possibly several million. Because CO₂ traps heat, that increase is the principal driver of modern global warming, which is why this single curve sits at the center of climate science.
Frequently asked questions
What is the current atmospheric CO₂ level?
It is now above 430 parts per million (ppm) at Mauna Loa — see the latest monthly figure in the chart above — compared with about 315 ppm when measurements began in 1958.
Why does atmospheric CO₂ rise and fall each year?
Northern Hemisphere plants absorb CO₂ during the spring and summer growing season and release it in fall and winter, producing an annual sawtooth of roughly 6–7 ppm on top of the long-term rise.
What was the CO₂ level before the Industrial Revolution?
Ice-core data show it held near 280 ppm for thousands of years before large-scale fossil-fuel use began. Today's level is more than 50% higher.
How is CO₂ measured at Mauna Loa?
An observatory on the Mauna Loa volcano in Hawaii samples well-mixed air far from local sources of pollution or vegetation, giving a clean reading representative of the broader atmosphere — a record kept continuously since 1958.
Is CO₂ rising faster now than it used to?
Yes. The annual increase has grown from under 1 ppm in the 1960s to well over 2 ppm per year in the past decade — the fastest sustained rise in the record.