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Carbon dioxide in atmosphere reaches landmark level

At 400 parts per million, greenhouse gas concentration is now higher than it has been for millions of years

On May 9, the atmosphere above Hawaii’s Mauna Loa volcano reached a milestone: For the first time since record keeping began there in 1958, the daily mean carbon dioxide concentration reached 400 parts per million.

The Mauna Loa Observatory the world’s oldest continuous monitoring station for carbon dioxide.

In spring of 2012, Alaska, Canada and several other Arctic locations surpassed the 400 ppm benchmark. Parts of the Southern Hemisphere should top 400 ppm within the next few years. And by 2016, scientists at the National Oceanic and Atmospheric Administration expect global average concentrations of the greenhouse gas to hit 400 ppm.

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