By SETH BORENSTEIN, Associated Press
WASHINGTON (AP) — Levels of heat-trapping carbon dioxide in the air have hit a milestone of 400 parts per million.
Pieter Tans, who monitors levels for the U.S. government, said the last time carbon dioxide levels were this high was about 2 million years ago.
When measurements of this chief greenhouse gas were first taken in 1958, carbon dioxide levels were at 315. Levels are now growing about 2 parts per million per year. That's 100 times faster than at the end of the Ice Age.
The measurements taken in Hawaii, at the world's oldest monitoring station, are considered the global benchmark.
Last year, regional monitors briefly hit 400 ppm in the Arctic. Generally carbon levels peak in May then fall slightly.
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