CO2 concentration in atmosphere rises to dangerous levels: Report
The greenhouse gas has previously passed the 400 ppm threshold on certain months in specific locations but never on an annual global basis, the United Nations World Meteorological Organization said.
In its latest Greenhouse Gas Bulletin, WMO also reported that CO2 concentration rates had "surged again to new records in 2016" and predicted the annual average would stay above 400 ppm "for many generations."
The concentration of CO2 spiked in part because of a strong El Nino, the whether phenomenon that occurs every four to five years with a broadly warming effect.
But the head of the Geneva-based agency, Petteri Taalas, warned against complacency just because El Nino has subsided for now.
"The El Nino event has disappeared. Climate change has not," he said.
He applauded the landmark deal reached in Rwanda's capital Kigali earlier this month to phase out hydrofluorocarbons, a category of dangerous greenhouse gases widely used in refrigerators and air conditioners.
But he said that without similarly bold action against C02 emissions, the world will fail to meet the goals laid out in the historic Paris Agreement on climate change.