Global carbon emissions are at the highest level in three million years and about to hit a milestone, scientists say.

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 ever, in the next few days.

The U.S. government's Earth Systems Research laboratory in Mauna Loa, Hawaii recorded the levels of CO2. The Mauna Loa site is situated far away from major sources of pollution and has been monitoring atmospheric levels for more than 50 years. The observatory is considered the most accurate source of such information.

"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 that operates the Hawaiian observatory.

"Each year, the concentration of CO2 at Mauna Loa rises and falls in a sawtooth fashion, with the next year higher than the year before. The peak of the sawtooth typically comes in May. If CO2 levels don't top 400ppm in May 2013, they almost certainly will next year," Keeling added.

Levels of CO2 in the atmosphere have been steadily rising for 200 years. At the start of the industrial revolution, carbon emissions levels were at 280ppm. The CO2 level was recorded at 316ppm in 1958, when the Mauna Loa observatory started measurements.

Some experts say if levels exceed 350ppm, climate change will disrupt daily life on our planet. Much of the increase in carbon levels is tied to the burning of fossil fuels.

Although many industrialized nations have already reduced their carbon emissions, burning of fossil fuels still adds to the increase in atmospheric concentrations.

The last time CO2 levels were so high was likely in the Pliocene epoch, between 3.2m and 5m years ago, when Earth's climate was significantly warmer than today.