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The Boston Globe

Science

Scientists say greenhouse gases are highest in human history

WASHINGTON — 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. 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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