Greenhouse gas level highest in two million years, NOAA reports

Worldwide levels of the greenhouse gas that plays the biggest role in global warming have reached their highest level in almost 2 million years—an amount never before encountered by humans, U.S. scientists said Friday.

Carbon dioxide was measured at 400 parts per million Thursday at the oldest monitoring station in Hawaii, which sets the global benchmark.

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.

"What we see today is 100 percent due to human activity," said Pieter Tans, a senior scientist with the National Oceanic and Atmospheric Administration. The burning of fossil fuels, such as coal for electricity and oil for gasoline, has caused the overwhelming bulk of the man-made increase in carbon in the air, scientists say.