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Rebecca Leber reports that “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. [Washington Post]

It’s been at least 800,000 years — probably more — since Earth saw carbon dioxide levels in the 400s….

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.

“It’s an important threshold,” said Carnegie Institution ecologist Chris Field, a scientist who helps lead the Nobel Prize-winning Intergovernmental Panel on Climate Change. “It is an indication that we’re in a different world."

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