The concentration of carbon dioxide in the atmosphere on Dec. 9 was 398.48 parts per million (ppm).

One year ago = 395.80 ppm

1958 = 318 ppm (the year records began to be kept)

800,000 years ago = 260 ppm

In April, May, and June of this year the level exceeded 400 ppm, highest in almost one million years.

Readings are taken daily at the Mauna Loa Observatory in Hawaii for the Scripps Institution of Oceanography. Go to https://scripps.ucsd.edu/programs/keelingcurve/