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.

According to the UW paleontologist Peter Ward, all mass extinction events of the last 500 million years occurred when CO2 levels exceeded 1000 parts per million (ppm).