on 18/4/17 the Mauna Loa Observatory recorded its first-ever carbon dioxide reading in excess of 410 parts per million (it was 410.28 ppm in case you want the full deal).
In what’s become a spring tradition like Passover and Easter, carbon dioxide has set a record high each year since measurements began. It stood at 280 ppm when record keeping began at Mauna Loa in 1958. In 2013, it passed 400 ppm. Just four years later, the 400 ppm mark is no longer a novelty. It’s the norm.
— source climatecentral.org