Scientists who operate the National Oceanic & Atmospheric Administration’s Global Greenhouse Gas Reference Network report that Earth’s global average atmospheric carbon dioxide level surpassed 400 ppm in March. NOAA bases the global CO2 average on air samples taken at 40 remote sites around the globe and analyzed at its Earth System Research Laboratory in Boulder, Colo. The NOAA team first recorded levels above 400 ppm at some sites in 2012, but now the average for all reporting stations is above the mark. The last time Earth’s atmospheric CO2 level reached 400 ppm is thought to have been during a peak warm period roughly 4.5 million years ago, which scientists have deduced from a combination of analytical instrument readings since the 1950s and proxy data from ice cores and other sources. The CO2 level was hovering around 280 ppm prior to the Industrial Revolution in the mid-1800s, but the level has been creeping up because of the increased burning of fossil fuels. The data show that half of the 120-ppm increase since 1850 has occurred after 1980 and that the growth rate is increasing and now stands at 2.25 ppm per year. The scientists note that the global average fluctuates seasonally as vegetation and soil organisms wax and wane, but the overall trend continues upward.