What unit is commonly used for atmospheric CO2 concentration in the Keeling Curve?

The Keeling Curve commonly expresses atmospheric carbon dioxide concentration in parts per million, abbreviated ppm.

A reading of 420 ppm means that, on average, 420 out of every million air molecules are CO₂ molecules. The Keeling Curve is the long-running graph of atmospheric CO₂ measurements begun by Charles David Keeling at Mauna Loa Observatory in Hawaii in 1958. It shows both a long-term rise and a regular seasonal zigzag.

The seasonal cycle occurs mainly because land vegetation in the Northern Hemisphere absorbs CO₂ during spring and summer through photosynthesis, then releases more of it during autumn and winter as plants respire and decay. The upward trend reflects the accumulation of CO₂ from fossil-fuel burning, land-use change, and industry.

Parts per million is a concentration unit, not a measure of temperature, pressure, or energy. Pascals measure pressure, watts per square metre measure radiative flux, and degrees Celsius measure temperature. Earlier measurements and ice-core records extend the story beyond the modern instrumental curve.