World carbon dioxide pollution levels are accelerating and reached a record high in 2012, the UN weather agency says.
The heat-trapping gas, pumped into the air by cars and chimneys, was measured at 393.1 parts per million last year, up 2.2ppm from the previous year, said the Geneva-based World Meteorological Organisation.
That is far beyond the 350ppm that some scientists and environmental groups promote as the absolute upper limit for a safe level.
As the main gas blamed for global warming, carbon dioxide's 2012 increase outpaced the past decade's average annual increase of 2.02ppm.
Based on that rate, the organisation says the world's carbon dioxide pollution level is expected to cross the 400ppm threshold by 2016. That level was already reached at some individual measurement stations in 2012 and 2013.
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 280ppm before the Industrial Revolution.
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.
Carbon dioxide remains in the air for a century, some of it far longer, which means that a lot of future warming is already locked in.
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.