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. Carbon dioxide is regarded as the...