The average concentration of CO2 in the atmosphere hit the symbolic level of 400 parts per million (ppm) for the first time in 2015 and has continued to surge in 2016, according to the World Meteorological Organization. From a report on Time:Scientists say humans may need to take some carbon dioxide out of the atmosphere to stop global warming. The carbon dioxide concentration is unlikely to dip below the 400 ppm mark for at least several decades, even with aggressive efforts to reduce global carbon emissions, according to the WMO report, which confirms similar findings reported last month. Carbon dioxide can last in the atmosphere for thousands of years without efforts to remove it. "The year 2015 ushered in a new era of optimism and climate action," said WMO Secretary-General Petteri Taalas, referring to the landmark Paris Agreement to address climate change. "The real elephant in the room is carbon dioxide, which remains in the atmosphere for thousands of years and in the oceans for even longer."