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Atmospheric carbon dioxide levels reach milestone

A monitoring station in Hawaii has measured carbon dioxide levels of 400 parts per million, a concentration not seen on Earth since the Pleistocene Era.

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A flock of Geese fly past the smokestacks at the Jeffrey Energy Center coal power plant as the suns sets near Emmett, Kan., in December 2012.

Charlie Riedel/AP

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Worldwide levels of the chief greenhouse gas that causes global warming have hit a milestone, reaching an amount never before encountered by humans, federal scientists said Friday.

Carbon dioxide was measured at 400 parts per million at the oldest monitoring station in Hawaii which sets the global benchmark. The last time the worldwide carbon level was probably that high was about 2 million years ago, said Pieter Tans of the National Oceanic and Atmospheric Administration.

That was during the Pleistocene Era. "It was much warmer than it is today," Tans said. "There were forests in Greenland. Sea level was higher, between 10 and 20 meters (33 to 66 feet)."

Other scientists say it may have been 10 million years ago that Earth last encountered this level of carbon dioxide.

The measurement was recorded Thursday. The number 400 has been anticipated by climate scientists and environmental activists for years as a notable indicator, in part because it's a round number — not because any changes in man-made global warming happen by reaching it.

When measurements of this chief greenhouse gas were first taken in 1958, carbon dioxide was measured 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.

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