Climate change: Arctic passes 400 parts per million milestone

"It's an important threshold," said Carnegie Institution ecologist Chris Field, a scientist who helps lead the Nobel Prize-winning Intergovernmental Panel on Climate Change. "It is an indication that we're in a different world."

Ronald Prinn, an atmospheric sciences professor at the Massachusetts Institute of Technology, said 400 is more a psychological milestone than a scientific one. We think in hundreds, and "we're poking our heads above 400," he said.

Tans said the readings show how much the Earth's atmosphere and its climate are being affected by humans. Global carbon dioxide emissions from fossil fuels hit a record high of 34.8 billion tons in 2011, up 3.2 percent, the International Energy Agency announced last week.

The agency said it's becoming unlikely that the world can achieve the European goal of limiting global warming to just 2 degrees based on increasing pollution and greenhouse gas levels.

"The news today, that some stations have measured concentrations above 400 ppm in the atmosphere, is further evidence that the world's political leaders — with a few honorable exceptions — are failing catastrophically to address the climate crisis," former Vice President Al Gore, the highest-profile campaigner against global warming, said in an email. "History will not understand or forgive them."

But political dynamics in the United States mean there's no possibility of significant restrictions on man-made greenhouse gases no matter what the levels are in the air, said Jerry Taylor, a senior fellow of the libertarian Cato Institute.

"These milestones are always worth noting," said economist Myron Ebell at the conservative Competitive Enterprise Institute. "As carbon dioxide levels have continued to increase, global temperatures flattened out, contrary to the models" used by climate scientists and the United Nations.

He contends temperatures have not risen since 1998, which was unusually hot.

Temperature records contradict that claim. Both 2005 and 2010 were warmer than 1998, and the entire decade of 2000 to 2009 was the warmest on record, according to NOAA.

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Copyright 2012 The Associated Press.

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