The Mauna Loa record is the longest series of direct measurements of carbon dioxide in the atmosphere

Background

The carbon dioxide data on Mauna Loa constitute the longest record of direct measurements of CO2 in the atmosphere. They were started by C. David Keeling of the Scripps Institution of Oceanography in March of 1958 at a facility of the National Oceanic and Atmospheric Administration. NOAA started its own CO2 measurements in May of 1974, and they have run in parallel with those made by Scripps since then. The Mauna Loa data are being obtained at an altitude of 3400 m in the northern subtropics, and may not be the same as the globally averaged CO2 concentration at the surface. Every monthly mean is the average of daily means, which are in turn based on hourly averages, but only for those hours during which background conditions prevail. Data are reported as a dry air mole fraction, the number of molecules of carbon dioxide divided by the number of all molecules in air after water vapor has been removed, expressed as parts per million (ppm): 0.000400 is expressed as 400 ppm. Due to the eruption of the Mauna Loa Volcano, measurements from Mauna Loa Observatory were suspended as of Nov. 29, 2022, observations from December 2022 to July 4, 2023 are from a site at the Maunakea Observatories, approximately 21 miles north of the Mauna Loa Observatory, and Mauna Loa observations resumed in July 2023. The CO2 data are now on the WMO X2019 scale.