Monthly Average Mauna Loa CO 2

August 2020: 412.55 ppm August 2019: 409.95 ppm Last updated: September 9, 2020

The graphs show monthly mean carbon dioxide measured at Mauna Loa Observatory, Hawaii. The carbon dioxide data (red curve), measured as the mole fraction in dry air, on Mauna Loa constitute the longest record of direct measurements of CO 2 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 [Keeling, 1976]. NOAA started its own CO 2 measurements in May of 1974, and they have run in parallel with those made by Scripps since then [Thoning, 1989].

The last four complete years of the Mauna Loa CO 2 record plus the current year are shown in the first graph. The full record of combined Scripps data and NOAA data are shown in the second graph. The dashed red lines with diamond symbols represent the monthly mean values, centered on the middle of each month. The black lines with the square symbols represent the same, after correction for the average seasonal cycle. The latter is determined as a moving average of SEVEN adjacent seasonal cycles centered on the month to be corrected, except for the first and last THREE and one-half years of the record, where the seasonal cycle has been averaged over the first and last SEVEN years, respectively.