By Seth Borenstein, AP Science WriterWASHINGTON -- The old saying that "what goes up must come down" doesn't apply to carbon dioxide pollution in the air, which just hit an unnerving milestone. The chief greenhouse gas was measured Thursday at 400 parts per million in Hawaii, a monitoring site that sets the world's benchmark. It's a symbolic mark that scientists and environmentalists have been anticipating for years. While this week's number has garnered all sorts of attention, it is just a daily reading in the month when the chief greenhouse gas peaks in the Northern Hemisphere. It will be lower the rest of the year. This year will probably average around 396 ppm. But not for long -- the trend is going up and at faster and faster rates. 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. "It means we are essentially passing one in a whole series of points of no return," said Michael Mann, a climate scientist at Pennsylvania State University. Princeton University climate scientist Michael Oppenheimer said the momentum in carbon dioxide emissions has the world heading toward and passing 450 ppm. That is the level which would essentially mean the world warms another 2 degrees, what scientists think of as dangerous, he said. That 2-degree mark is what much of the world's nations have set as a goal to prevent.