Greenhouse gas levels in atmosphere hit new high in 2018: UN The head of the World Meteorological Organization Petteri Taalas says levels of carbon dioxide, one of the most common greenhouse gases in the atmosphere, hit a record high last year. The WMO's main annual Greenhouse Gas Bulletin listed the atmospheric concentration of CO2 in 2018 at 407.8 parts per million, up from 405.5 parts per million (ppm) in 2017. SOUNDBITE