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% 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.

___

Online:

NOAA monitoring at Mauna Loa: http://www.esrl.noaa.gov/gmd/ccgg/trends/weekly.html

Seth Borenstein can be followed at http://twitter.com/borenbears
Copyright 2011 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

If you liked this article you might like

What's Behind the Surge in Energy Stocks

Hillary Clinton Says Prosecuting Individuals is Key to Wall Street Reform