What atmospheric measurement is expressed in parts per million on the Keeling Curve?

The Keeling Curve shows atmospheric carbon dioxide concentration in parts per million.

Charles David Keeling began precise carbon dioxide measurements at Mauna Loa Observatory in Hawaii in 1958. The record revealed a clear long-term upward trend superimposed on a yearly cycle. Northern Hemisphere plant growth generally lowers carbon dioxide during spring and summer, while seasonal decay raises it during autumn and winter.

Mauna Loa’s remote volcanic location helps provide measurements representative of broad atmospheric conditions, although the observatory can occasionally be affected by local volcanic emissions. The record became one of the clearest visual demonstrations of rising greenhouse-gas concentration.

The curve is not a temperature graph. It records carbon dioxide abundance, while global temperature responds to many influences and varies from year to year. Similar monitoring stations now operate around the world.