Updated 11:27 A.M. E.T. WASHINGTON (AP) — 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 which is in Hawaii 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.
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