The world's carbon dioxide levels are on the cusp of reaching 400 parts per million in the atmosphere for the first time in 3 million years.

The daily CO2 level, measured at the Mauna Loa Observatory in Hawaii, stood at 399.72 parts per million last Thursday, and a few hourly readings had already risen above 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 geochemist at 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 parts per million level is considered the point where the world has a 50 per cent chance of avoiding dangerous climate change - any higher, and the odds of avoiding searing temperature rises of four or five degrees by the end of the century become prohibitively risky.

The rise in greenhouse gases corresponds with the extra CO2 emitted by human activities such as burning fossil fuels and cutting down forests. More greenhouse gases mean more heat builds up at the Earth's surface.