Carbon dioxide levels pass symbolic mark
Carbon dioxide levels in the atmosphere have broken through a symbolic mark.
Daily measurements of CO2 at a US government agency lab on Hawaii have topped 400 parts per million for the first time.
The station, which sits on the Mauna Loa volcano, feeds its numbers into a continuous record of the concentration of the gas stretching back to 1958.
The last time CO2 was regularly above 400ppm was three to five million years ago - before modern humans existed.
Science editor David Shukman explains.