Atmospheric CO₂ Data

Scale Update Notice

As of February 21, 2021, the CO₂ mole fraction data from the Scripps CO₂ program are reported on the Scripps X12 CO₂ calibration scale, as opposed to the previous X08A scale (Keeling et al., (2016).

The X12 scale relies on updated determinations of the primary reference gases for the Scripps CO₂ program on the Scripps constant-volume mercury-column manometer (CMM). The update incorporates new assessments of key parameters (e.g. manometer volumes), and changes data only after 1974. The revisions have been uniformly propagated through secondary and tertiary reference gases and to measurements on samples collected in flasks and using the in-situ analyzer at Mauna Loa (Keeling and Walker, 2021).

For the data reported on this website, the values on the X12 scale differ from values on the X08A scale by at most a few tenths of a ppm. The differences are both time- and concentration-dependent (Keeling et al., 2012). For recent measurements, the update lowers the CO2 readings for the primary (in situ) Mauna Loa record by ∼0.08 ppm and increases all other data (which are based on flasks) by ∼0.24 ppm.

Keeling, R F., P. Guenther, S. Walker and D. Moss (2016). Scripps Reference Gas Calibration System for Carbon Dioxide-in-Nitrogen and Carbon Dioxide-in-Air Standards: Revision of 2012. La Jolla, California, Scripps Institution of Oceanography: 1-173. PDF

Walker, S.J. and R.F. Keeling (2021). Special considerations for updating Mauna Loa in situ data to X12 scale. La Jolla, California, Scripps Institution of Oceanography: 1-6. PDF