The heat-trapping gas, pumped into the air by cars and smokestacks, was measured at 393.1 parts per million last year, up 2.2 ppm from the previous year, said the Geneva-based World Meteorological Organisation in its annual greenhouse gas inventory.
That is far beyond the 350 ppm that some scientists and environmental groups promote as the absolute upper limit for a safe level.
As the chief gas blamed for global warming, carbon dioxide's 2012 increase outpaced the past decade's average annual increase of 2.02 ppm.
Scientists say the Earth probably last had this much carbon dioxide in the atmosphere at least a few million years ago, when sea levels were higher. Carbon dioxide levels were around 280 ppm before the Industrial Revolution.
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Trapping heat just like in a greenhouse, carbon dioxide accounts for three-quarters of the planet's heat-trapping gases that scientists say are causing sea levels to rise, glaciers to melt and some weather patterns to change. Others, like methane, trap heat much better but have a shorter life span.
The rising amount of greenhouse gases in the atmosphere shows how people have "upset the natural balance of our atmosphere and are a major contribution to climate change," said Michel Jarraud, the secretary general of the World Meteorological Organisation.