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.
That was during the Pleistocene Era. ‘‘It was much warmer than it is today,’’ Tans said. ‘‘There were forests in Greenland. Sea level was higher, between 10 and 20 meters (33 to 66 feet).’’
Other scientists say it may have been 10 million years ago that Earth last encountered this much carbon dioxide in the atmosphere. Full story for BostonGlobe.com subscribers.