GENEVA: The concentration of carbon dioxide (CO2) in the atmosphere has passed an ominous milestone, ushering the planet into “a new era” of climate change, the UN said on Monday. For the first time on record, the amount of CO2 in the atmosphere averaged 400 parts per million (ppm) in 2015, the World Meteorological Organisation (WMO) said in its latest Greenhouse Gas Bulletin. CO2, the main greenhouse gas driving climate change, has previously passed the 400 ppm threshold on certain months in specific locations but never on a globally averaged basis, WMO said. The UN agency also reported that CO2 concentration rates had surged again to new records in 2016 and predicted the annual average would not dip below (400 ppm) for many generations. WMO Chief Petteri Taalas said that “the year 2015 ushered in a new era of optimism and climate action with the Paris climate change agreement. “But it will also make history as marking a new era of climate change reality with record high greenhouse gas concentrations,” he added in a statement. The CO2 concentration rate for 2014 was 397.2 ppm, just short of the 400 ppm mark that some climate activists once deemed unthinkable.