This graph showing the data is called the Keeling Curve.
It is named after the scientist who first produced accurate measurements of carbon dioxide in the air - Charles David Keeling.
The small up-and-down pattern is caused by changes during each year - in the summer in the northern hemisphere, there is more photosynthesis which causes CO2 levels to fall.
Charles David Keeling in the lab.
Keeling's collection of data began in 1958.
Concentration is measured in parts per million (ppm)
March 2015 was the first month when CO2 exceeded 400 ppm for the whole month.
Ice core data shows humans have never breathed air containing so much CO2.
Since Keeling began recording, CO2 has risen from 318 ppm to 400 ppm... a rise of around 25%.
There is evidence that CO2 has never risen so fast since the extinction of the dinosaurs.