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 in the last week, and on occasion, at observatories in the high Arctic. But the Mauna Loa station, sited at 3,400m and far away from major pollution sources in the Pacific Ocean, has been monitoring levels for more than 50 years and is considered the gold standard.
"I wish it weren't true but it looks like the world is going to blow through the 400ppm level without losing a beat. At this pace we'll hit 450ppm within a few decades," said Ralph Keeling, a geologist with the Scripps Institution of Oceanography which operates the Hawaiian observatory.