Guardian: The concentration of carbon dioxide in the atmosphere has reached 399.72 parts per million (ppm) and is likely to pass the symbolically important 400ppm level for the first time in the next few days.
Readings at the US government's Earth Systems Research laboratory in Hawaii, are not expected to reach their 2013 peak until mid May, but were recorded at a daily average of 399.72ppm on 25 April. The weekly average stood at 398.5 on Monday.
Hourly readings above 400ppm have been recorded six times...