Carbon dioxide levels in the atmosphere have topped 400 parts per million for the first time at a laboratory atop a volcano in Hawaii.
Carbon dioxide levels in the atmosphere have broken through a symbolic mark.
Daily measurements of CO2 at the Keeling lab on Hawaii have topped 400 parts per million for the first time.
Measurements by the laboratory, which sits atop the Mauna Loa volcano, of the concentration of the gas, began in 1958, when the reading was around 315ppm.
The BBC reports the last time CO2 was regularly above 400ppm was about three - five million years ago, when the climate was considerably warmer than it is today.
Last year, all Arctic sites reached 400ppm for the first time.