Scientists have been anticipating this milestone for a while, but they won't be breaking out the champagne now that it's here: Carbon-dioxide levels in the atmosphere passed the mark of 400 parts per million yesterday for the first time in human history, reports the BBC. The National Oceanic and Atmospheric Administration made the declaration based on readings of the heat-trapping gas at a station on the Mauna Loa volcano in Hawaii. (Another monitoring program run by the Scripps Institution of Oceanography hasn't quite gotten to 400 yet, but it supports the NOAA figure and chalks up the difference to a technicality.)