WASHINGTON (AP) -- Scientists say the world's air has reached a troubling new milestone for carbon dioxide, the main global warming pollutant.
Monitoring stations across the Arctic 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. Government researchers call this milestone depressing and troubling.
Scientists say carbon dioxide levels haven't been this high for at least 800,000 years.
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