LiveScience reports: The proportion of carbon dioxide in the atmosphere broke 400 parts per million Thursday, according to one of the best climate records available.

The Keeling Curve, a daily record of atmospheric carbon dioxide, has been running continuously since March 1958, when a carbon dioxide monitor was installed at Mauna Loa in Hawaii. On the first day, the observatory measured a carbon dioxide concentration of 313 parts per million (ppm). The number means there were 313 molecules of carbon dioxide in the air per every million air molecules.

Now, the Keeling Curve has reached 400 ppm for the first time in human history, with a new measure of 400.03 ppm. The data are preliminary, pending quality control checks, according to the National Oceaninc and Atmospheric Administration (NOAA).