WASHINGTON -- 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.
"The direction we've seen is for blowing through the best benchmark for what's dangerous change," Oppenheimer said.
And to see what the future is, scientists look to the past.
The last time the worldwide carbon level probably hit 400 ppm was about 2 million years ago, said Pieter Tans of the National Oceanic and Atmospheric Administration.
That was during the Pleistocene Era. "It was much warmer than it is today," Tans said. "There were forests in Greenland. Sea level was higher, between 10 and 20 meters (33 to 66 feet)."
Other scientists say it may have been 10 million years ago that Earth last encountered this much carbon dioxide in the atmosphere. The first modern humans only appeared in Africa about 200,000 years ago.
Environmental activists, such as former Vice President Al Gore, seized on the milestone.
"This number is a reminder that for the last 150 years -- and especially over the last several decades -- we have been recklessly polluting the protective sheath of atmosphere that surrounds the Earth and protects the conditions that have fostered the flourishing of our civilization," Gore said in a statement. "We are altering the composition of our atmosphere at an unprecedented rate."
Carbon dioxide traps heat just like in a greenhouse. It accounts for three-quarters of the planet's heat-trapping gases. There are others, such as methane, which has a shorter life span but traps heat more effectively. Both trigger temperatures to rise over time, scientists say, which is causing sea levels to rise and some weather patterns to change.
When measurements of carbon dioxide were first taken in 1958, it measured 315 ppm. Some scientists and environmental groups promote 350 ppm as a safe level for CO2, but scientists acknowledge they don't really know what levels would stop the effects of global warming.
The level of carbon dioxide in the air is rising faster than in the past decades, despite international efforts by developed nations to curb it. On average the amount is growing by about 2 ppm per year. That's 100 times faster than at the end of the Ice Age.
Back then, it took 7,000 years for carbon dioxide to reach 80 ppm, Tans said. Because of the burning of fossil fuels, such as oil and coal, carbon dioxide levels have gone up by that amount in just 55 years.
Before the Industrial Revolution, carbon dioxide levels were around 280 ppm, and they were closer to 200 during the Ice Age, which is when sea levels shrank and polar places went from green to icy. There are natural ups and downs of this greenhouse gas, which comes from volcanoes and decomposing plants and animals. But that's not what has driven current levels so high, Tans said. He said the amount should be even higher, but the world's oceans are absorbing quite a bit, keeping it out of the air.
"What we see today is 100 percent due to human activity," said Tans, a NOAA senior scientist. 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.
The world sent 38.2 billion tons of carbon dioxide into the air in 2011, according international calculations published in a scientific journal in December. China spews 10 billion tons of carbon dioxide into the air per year, leading all countries, and its emissions are growing about 10 percent annually. The U.S. at No. 2 is slowly cutting emissions and is down to 5.9 billion tons per year.
The speed of the change is the big worry, said Pennsylvania State's Mann. If carbon dioxide levels go up 100 ppm over thousands or millions of years, plants and animals can adapt. But that can't be done at the speed it is now happening.
"We are a society that has inadvertently chosen the double-black diamond run without having learned to ski first," NASA climate scientist Gavin Schmidt said. "It will be a bumpy ride."