For first time in 2 million years, levels of chief greenhouse gas hit 400 parts per million

Orbiting Carbon Observatory 2 (Photo credit: Wikipedia)

Levels of heat-trapping carbon dioxide in the air are now the highest they've been in about 2 million years.

Pieter Tans, who monitors levels for the U.S. government, says carbon dioxide levels have hit 400 parts per million.

When measurements of this chief greenhouse gas were first taken in 1958, carbon dioxide levels were at 315. Levels are now growing about 2 parts per million per year.

That's 100 times faster than at the end of the Ice Age.

The measurements taken in Hawaii, at the world's oldest monitoring station, are considered the global benchmark.

Last year, regional monitors briefly hit 400 ppm in the Arctic. Generally carbon levels peak in May then fall slightly.