Because carbon dioxide typically peaks in May, researchers are expecting the Keeling Curve to break a milestone of 400 ppm this year. (If not, the number will almost certainly be reached in May 2014.) As of May 1 of this year, the last day data was available, the Mauna Loa observatory recorded 399.39 ppm of carbon dioxide in the air.
There will be no huge atmospheric or climatic shift once carbon dioxide hits 400 ppm, but the milestone has symbolic significance, said Michael Mann, a climate scientist at Pennsylvania State University.
"It is a reminder of just how uncontrolled this dangerous experiment we're playing with the planet really is," Mann told LiveScience.
What 400 ppm means
In the 1,000 years that occurred before the Industrial Revolution of the 18th century, atmospheric carbon dioxide held steady at around 270 to 280 parts per million.
Scientists believe that the most recent period to reach 400 ppm was the Pliocene Epoch, between 5 million and 3 million years ago, according to the Scripps Institution of Oceanography, which keeps track of the Keeling Curve.