(NOAA)

The first time an atmospheric carbon dioxide concentration of above 400 parts per million was measured was in the Arctic in 2012. That same threshold was reached at the National Oceanic and Atmospheric Administration’s Mauna Loa Observatory in Hawaii in 2013. And now, NOAA reports the monthly global average concentration of the greenhouse gas has surpassed 400 parts per million for the first time.

This isn’t good. In fact, the last time it happened was up to 5 million years ago.

“It’s both disturbing and daunting,” NOAA chief greenhouse-gas scientist Pieter Tans told the Associated Press. “Daunting from the standpoint on how hard it is to slow this down.”

There’s nothing special about 400 ppm of carbon dioxide in our atmosphere. Temperatures around the world continue to rise; our air remains breathable, for now; life goes on. But 400 ppm is a “milestone,” scientists say — one they have feared for some time. The Washington Post thought it worth noting when just Mauna Loa registered 400 ppm two years ago.

[Atmospheric carbon dioxide levels hit worrisome milestone]

“Reaching 400 ppm doesn’t mean much in itself, but the steady increase in atmospheric greenhouse gases should serve as a stark reminder of the task facing politicians” ahead of climate talks in Paris later this year, Ed Hawkins, a climate scientist at the University of Reading, told the Guardian.

One of the problems with carbon dioxide levels: Even if all human carbon emissions stopped immediately, it would take quite a while for CO2 to be cleansed from the atmosphere.

“Elimination of about 80 percent of fossil fuel emissions would essentially stop the rise in carbon dioxide in the atmosphere,” James Butler, director of NOAA’s Global Monitoring Division, said in a statement, “but concentrations of carbon dioxide would not start decreasing until even further reductions are made and then it would only do so slowly.”

What’s scariest: 400 ppm means CO2 levels are increasing faster than ever before in recorded history.

“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,” Tans said. “Half of that rise has occurred since 1980.”

NOAA measures carbon dioxide at 40 remote locations such as islands and on cargo ships to avoid skewing data by measuring levels, say, in the middle of Beijing or near a volcano.

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

Dlugokencky expects the global CO2 average to remain above 400 ppm through May, when carbon dioxide levels in the atmosphere peak before fauna blooms across the globe.