"The direction we've seen is for blowing through the best benchmark for what's dangerous change," Oppenheimer said.
And to see what the future is, scientists look to the past.
The last time the worldwide carbon level probably hit 400 ppm 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. The first modern humans only appeared in Africa about 200,000 years ago.
Environmental activists, such as former Vice President Al Gore, seized on the milestone.
"This number is a reminder that for the last 150 years -- and especially over the last several decades -- we have been recklessly polluting the protective sheath of atmosphere that surrounds the Earth and protects the conditions that have fostered the flourishing of our civilization," Gore said in a statement. "We are altering the composition of our atmosphere at an unprecedented rate."
Carbon dioxide traps heat just like in a greenhouse. 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 ppm. Some scientists and environmental groups promote 350 ppm 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 ppm per year. That's 100 times faster than at the end of the Ice Age.