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Earth’s atmosphere is entering a new era. A mountaintop research station that has been tracking carbon dioxide for more than 50 years says the level of that gas in our air has reached a milestone: 400 parts per million.

That number is one of the clearest measures of how human beings are changing the planet. It shows how much carbon we have put into the air from burning fossil fuels — and that carbon dioxide drives global warming.

This measurement comes from Mauna Loa, Hawaii, a remote volcano where the air is largely free of local influences.

Read more here:  http://www.npr.org/2013/05/10/182029983/dangerous-territory-carbon-dioxide-levels-reach-iconic-high