Carbon dioxide concentrations in the Earth's atmosphere are on the cusp of reaching 400 parts per million for the first time in 3 million years.

The daily CO2 level, measured at the Mauna Loa Observatory in Hawaii, was 399.72 parts per million last Thursday, and a few hourly readings had risen to more than 400 parts per million.

''I wish it weren't true but it looks like the world is going to blow through the 400 ppm level without losing a beat,'' said Ralph Keeling, a geologist with the Scripps Institution of Oceanography in the US, which operates the Hawaiian observatory.

''At this pace we'll hit 450 ppm within a few decades.''

The 450 ppm level is considered to be the point at which the world has a 50 per cent chance of avoiding dangerous climate change. Any higher and the odds of avoiding searing temperature rises of 4 or 5 degrees by the end of the century become prohibitively risky.