Within a decade the world will never see days — even in the cleanest of places on days in the fall when greenhouse gases are at their lowest — when the carbon measurement falls below 400 ppm, said James Butler, director of global monitoring at the National Oceanic and Atmospheric Administration’s Earth Science Research Lab in Boulder, Colo.

“The 400 is a reminder that our emissions are not only continuing, but they’re accelerating; that’s a scary thing,” Butler said Saturday. “We’re stuck. We’re going to keep going up.” 

Carbon dioxide stays in the air for a century, some of it into the thousands of years. And the world carbon dioxide pollution levels are accelerating yearly. Every second, the world’s smokestacks and cars pump 2.4 million pounds of the heat-trapping gas into the air.

Carbon pollution levels that used to be normal for the 20th century are fast becoming history in the 21st century.