Why does 400ppm matter? & what it has to do with the Pliocene

In May 2013, the Mauna Loa Observatory in Hawai’i recorded the first time global carbon dioxide levels surpassed 400ppm (parts per million). As of this Tuesday, we are sitting at 403.98ppm and have been over 400ppm for much of the last year (Scripps). This observatory has been measuring atmospheric carbon dioxide since 1958. So, you might ask, why is the number 400 so important?

Average atmospheric carbon dioxide levels for the last 12 months at the Mauna Loa Observatory in Hawai’i. (From: Scripps Institution of Oceanography

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