Which gas is measured by the Keeling Curve?

The Keeling Curve measures the concentration of carbon dioxide in Earth’s atmosphere.

It is a graph built from continuous atmospheric CO₂ measurements that began at the Mauna Loa Observatory in Hawaii in 1958. Scientist Charles David Keeling established the monitoring program, and the curve was later named after him. Its regular up-and-down pattern records seasonal plant growth: CO₂ generally falls during the Northern Hemisphere growing season and rises when vegetation decays and respiration dominates.

The curve’s long-term climb shows that atmospheric CO₂ has increased substantially since the Industrial Revolution. The rise is primarily linked to fossil-fuel combustion, with additional contributions from land-use change such as deforestation. Because carbon dioxide is a greenhouse gas, the record is central to understanding modern climate change.

A common mix-up is thinking the Keeling Curve measures temperature or all greenhouse gases. It specifically tracks atmospheric CO₂ concentration, usually reported in parts per million. Similar measurements from ice cores extend the record far beyond the modern observatory era.