World's Air Hits 'Troubling Milestone'

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."

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.

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.