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Global levels of carbon dioxide, the most prevalent heat-trapping gas, have passed a daunting milestone, US government scientists say.

The National Oceanic and Atmospheric Administration says in March, the global monthly average for carbon dioxide hit 400.83 parts per million.

That is the first month in modern records that the entire globe broke 400 ppm, reaching levels that haven't been seen in about 2 million years. 

The National Oceanic and Atmospheric Administration says in March, the global monthly average for carbon dioxide hit 400.83 parts per million, the first month in modern records that the entire globe broke 400 ppm.

'It's both disturbing and daunting,' said NOAA chief greenhouse gas scientist Pieter Tans. 

'Daunting from the standpoint on how hard it is to slow this down.'

He said it is disturbing because it is happening at a pace so fast that it seems like an explosion compared to Earth's slow-moving natural changes.

Carbon dioxide isn't just higher, it is increasing at a record pace, 100 times faster than natural rises in the past, Tans said.

Pushed by the burning of coal, oil and gas, global carbon dioxide is 18 percent higher than it was in 1980, when NOAA first calculated a worldwide average. 

In 35 years, carbon dioxide levels rose 61 parts per million. 

In pre-human times, it took about 6,000 years for carbon dioxide to rise about 80 parts per million, Tans said.

Monthly levels fluctuate with the season, peaking in May and then decreasing as plants absorb carbon dioxide. 

But they are increasing on a year-to-year basis.

Levels are also higher in the Northern Hemisphere because that's where carbon dioxide is being spewed by power plants and vehicles, Tans said.

The first time levels passed the 400 ppm milestone was for just a few weeks in the Arctic in 2012.

Last year the monthly Northern Hemisphere average measured in Hawaii exceeded 400 and now the global average has as well, said James Butler, head of NOAA's global monitoring division. 

'It was only a matter of time that we would average 400 parts per million globally,' said Tans. 

'We first reported 400 ppm when all of our Arctic sites reached that value in the spring of 2012. 

'In 2013 the record at NOAA's Mauna Loa Observatory first crossed the 400 ppm threshold. Reaching 400 parts per million as a global average is a significant milestone.'

NOAA bases the global carbon dioxide concentration on air samples taken from 40 global sites.

NOAA and partner scientists collect air samples in flasks while standing on cargo ship decks, on the shores of remote islands and at other locations around the world. 

It takes some time after each month's end to compute this global average because samples are shipped from locations for analysis at NOAA's Earth System Research Laboratory in Boulder, Colorado. 

'We choose to sample at these sites because the atmosphere itself serves to average out gas concentrations that are being affected by human and natural forces. 

'At these remote sites we get a better global average,' said Ed Dlugokencky, the NOAA scientist who manages the global network.

Eric Moglia of NOAA's Cooperative Institute for Research in Environmental Sciences, prepares air sample canisters. NOAA says in March, the global monthly average for carbon dioxide hit 400.83 parts per million. That is the first month in modern records that the entire globe broke 400 ppm, reaching levels that haven't been seen in about 2 million years.

Dlugokencky said he expects the global average will remain above 400 ppm through May, the time of year when global carbon dioxide concentrations peak due to natural cycles on top of the persistent rising greenhouse gases. 

Decaying plant matter and soil organisms give off carbon dioxide gas all year long, but the dormant period in plant growth allows the respiration of carbon dioxide to dominate during those months.

Carbon dioxide levels drop back down as plants begin to bloom, using carbon dioxide for photosynthesis in late spring and summer.

James Butler, director of NOAA's Global Monitoring Division, added that it would be difficult to reverse the increases of greenhouse gases which are driving increased atmospheric temperatures.

'Elimination of about 80 percent of fossil fuel emissions would essentially stop the rise in carbon dioxide in the atmosphere, but concentrations of carbon dioxide would not start decreasing until even further reductions are made and then it would only do so slowly.'