This represents an increase of 0.44 ppm per century [from 21,000 B.P. to 1800 A.D.], which may be partly attributed to the PIAC of the agricultural revolution, which started 9,000 years ago.

The concentration was 299 ppm in 1900.

This represents an increase of 17 ppm in the 19th century.

The concentration was 372 ppm in 2000.

This represents an increase of 73 ppm in the 20th century.

The current value (July 2011) is 392 ppm.