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, which is in Hawaii and sets the global benchmark. The last time the worldwide carbon level was probably that high was about two 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. The first modern humans appeared in Africa about 200,000 years ago.

The measurement was recorded Thursday and 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 human-made global warming happen by reaching it.

"Physically, we are no worse off at 400 p.p.m. than we were at 399 p.p.m.," 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."

Environmental activists, such as former Vice President Al Gore, seized on the milestone.

"We are altering the composition of our atmosphere at an unprecedented rate," Gore said in a statement.

Carbon dioxide traps heat just like in a greenhouse and most of it stays in the air for a century; some lasts for thousands of years, scientists say. It accounts for three-quarters of the planet's heat-trapping gases.

There are others, such as methane, which has a shorter life span but traps heat more effectively. Both trigger temperatures to rise over time, scientists say, which is causing sea levels to rise and some weather patterns to change.

When measurements of carbon dioxide were first taken in 1958, it measured 315 parts per million. Some scientists and environmental groups promote 350 parts per million as a safe level for CO2, but scientists acknowledge they don't really know what levels would stop the effects of global warming.

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.

Back then, it took 7,000 years for carbon dioxide to reach 80 parts per million, Tans said. Because of the burning of fossil fuels, such as oil and coal, carbon dioxide levels have gone up by that amount in just 55 years.

Before the Industrial Revolution, carbon dioxide levels were around 280 p.p.m.

There are natural ups and downs of this greenhouse gas, which comes from volcanoes and decomposing plants and animals. But that's not what has driven current levels so high, Tans said. He said the amount should be even higher, but the world's oceans are absorbing quite a bit, keeping it out of the air.

"What we see today is 100 percent due to human activity," said Tans, a NOAA senior scientist.Generally carbon levels peak in May then fall slightly, so the yearly average is usually a few parts per million lower than May levels.