FILE - Smoke billows from a coal-fired power plant in Beijing, China, Nov. 13, 2014.
FILE - Smoke billows from a coal-fired power plant in Beijing, China, Nov. 13, 2014.

Global carbon concentrations in the atmosphere reached a record level in March, U.S. government statistics showed.

For the first time since the government started tracking carbon dioxide in the atmosphere around the world, the monthly global average surpassed 400 parts per million.

Carbon dioxide is a potent greenhouse gas that occurs in nature but is also a byproduct of burning fossil fuels such as oil and coal.

The National Oceanic and Atmospheric Administration's Global Greenhouse Gas Reference Network had previously collected readings of 400 ppm at individual sites, such as its Arctic locations in 2012 and its Hawaiian observatory in 2013. But this was the first time 400 ppm had been reached as a global average.

“This marks the fact that humans burning fossil fuels have caused global carbon dioxide concentrations to rise more than 120 parts per million since pre-industrial times,” said Pieter Tans, the network's lead scientist. “Half of that rise has occurred since 1980.”

NOAA collects air samples in flasks from 40 sites around the world. The sites include ship decks and remote islands.

Ed Dlugokencky, the NOAA scientist who manages the global network, said his team gets a better global average at the remote sites. “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," he said.

Dlugokencky said he expected the global average to remain above 400 ppm through May, the time of year when global carbon dioxide concentrations peak because of natural cycles on top of the persistent human production of greenhouse gases.

Carbon dioxide levels drop back down as more plants emerge from their dormant period, using carbon dioxide for photosynthesis in late spring and summer.