Scientists at the National Oceanic and Atmospheric Administration’s Mauna Loa observatory in Hawaii told that June would be be the third month in a row where, for the entire month, average levels of carbon dioxide were above 400 parts per million (ppm). The first time we reached that 400 ppm threshold was in May 2013. But it was brief — just a small moment in time where the levels hit the landmark. there have now been three months in a row of average levels about 400 ppm, climate scientists are worried that the landmark is becoming the norm, which would bring us closer to a level of global warming that will be too difficult to adapt to.