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.
That was during the Pleistocene era. ‘‘It was much warmer than it is today,’’ Tans said. ‘‘There were forests in Greenland.’’
Other scientists say it may have been 10 million years ago that the earth last encountered this much carbon dioxide in the atmosphere. The first modern humans appeared about 200,000 years ago.
The measurement was recorded Thursday and it is only a daily figure; the monthly and yearly average will be smaller. The number 400 has been anticipated by climate scientists and environmental activists for years as a notable indicator, in part because it’s a round number — not because any changes in man-made global warming happen by reaching it.
‘‘Physically, we are no worse off at 400 ppm than we were at 399 ppm,’’ Princeton University climate scientist Michael Oppenheimer said. ‘‘But as a symbol of the painfully slow pace of measures to avoid a dangerous level of warming, it’s somewhat unnerving.’’
The level of carbon dioxide in the air is rising faster than in the past decades, despite international efforts by developed nations to curb it.
On average the amount is growing by about 2 parts per million per year. That’s 100 times faster than at the end of the Ice Age.