According to the World Meteorological Organization (WMO), that means CO2 concentrations have risen by more than 40 percent above the 278 ppm level that existed before the Industrial Revolution launched the widespread burning of fossil fuels.
The daily average broke the 400 ppm level for the first time on record last May.
Because deciduous trees in the highly industrialized northern hemisphere store CO2 in their thick summer foliage, atmospheric carbon dioxide levels fluctuate in an annual cycle, typically peaking in April.
“This should serve as yet another wake-up call about the constantly rising levels of greenhouse gases, which are driving climate change,” WMO Secretary-General Michel Jarraud said in a statement.
“If we are to preserve our planet for future generations, we need urgent action to curb new emissions of these heat-trapping gases. Time is running out,” he added.
His comments were virtually echoed by Ed Chen, a spokesman for the Natural Resources Defense Council, who said in an email: “The next big step is to limit, for the first time, carbon pollution being spewed by our power plants.”
Graphic Data: NOAA - Scripps Institution of Oceanography