On May 10, the National Oceanic and Atmospheric Administration (NOAA) announced that the daily mean carbon dioxide concentration surpassed the 400 parts per million (ppm) threshold. Scientists, who have anticipated the milestone, have been monitoring the level of carbon since 1958; the measurement has not surpassed 400 ppm since measurements began. The level has been rising at a rate of 2.21 ppm per year in the last decade.