(BBC News) 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.

Scientists say the climate back then was also considerably warmer than it is today.

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