WASHINGTON — The world’s air has reached what scientists call a troubling new milestone for carbon dioxide, the main global warming pollutant.

Monitoring stations across the Arctic this spring are measuring more than 400 parts per million of the heat-trapping gas in the atmosphere. The number isn’t quite a surprise, because it’s been rising at an accelerating pace. Years ago, it passed the 350 ppm mark that many scientists say is the highest safe level for carbon dioxide. It now stands globally at 395.

So far, only the Arctic has reached that 400 level, but the rest of the world will follow soon. “The fact that it’s 400 is significant,” said Jim Butler, global monitoring director at the National Oceanic and Atmospheric Administration’s Earth System Research Lab in Boulder, Colo. “It’s just a reminder to everybody that we haven’t fixed this and we’re still in trouble.”

Carbon dioxide is the chief greenhouse gas and most of it lasts about 100 years in the air, but some of it stays in the atmosphere for thousands of years. Some carbon dioxide is natural, mainly from decomposing dead plants and animals. Before the Industrial Age, levels were around 275 parts per million.

For more than 60 years, readings have been in the 300s, except in urban areas, where levels are skewed. The burning of fossil fuels, such as coal for electricity and oil for gasoline, has caused the overwhelming bulk of the man-made increase in carbon in the air, scientists say.

It’s been at least 800,000 years — probably more — since Earth saw carbon dioxide levels in the 400s, Butler and other climate scientists said.

Until now.

Readings are coming in at 400 and higher all over the Arctic. They’ve been recorded in Alaska, Greenland, Norway, Iceland and even Mongolia. But levels change with the seasons and will drop a bit in the summer, when plants suck up carbon dioxide, NOAA scientists said.

So the yearly average for those northern stations likely will be lower and so will the global number.

Globally, the average carbon dioxide level is about 395 parts per million but will pass the 400 mark within a few years, scientists said.

“The news today, that some stations have measured concentrations above 400 ppm in the atmosphere, is further evidence that the world’s political leaders — with a few honorable exceptions — are failing catastrophically to address the climate crisis,” former Vice President Al Gore, the highest-profile campaigner against global warming, said in an email. “History will not understand or forgive them.”

But political dynamics in the United States mean there’s no possibility of significant restrictions on man-made greenhouse gases no matter what the levels are in the air, said Jerry Taylor, a senior fellow of the libertarian Cato Institute.