By David Shukman
BBC science correspondent
US climate scientists have recorded a significant rise in the concentration of carbon dioxide in the atmosphere, pushing it to a new record level.
Air samples have been taken from Colorado's Rocky Mountains
BBC News has learned the latest data shows CO2 levels now stand at 381 parts per million (ppm) - 100ppm above the pre-industrial average.
The research indicates that 2005 saw one of the largest increases on record - a rise of 2.6ppm.
The figures are seen as a benchmark for climate scientists around the globe.
The US National Oceanic and Atmospheric Administration (Noaa) has been analysing samples of air taken from all over the world, including America's Rocky Mountains.
The chief carbon dioxide analyst for Noaa says the latest data confirms a worrying trend that recent years have, on average, recorded double the rate of increase from just 30 years ago.
"We don't see any sign of a decrease; in fact, we're seeing the opposite, the rate of increase is accelerating," Dr Pieter Tans told the BBC.
The precise level of carbon dioxide in the atmosphere is of global concern because climate scientists fear certain thresholds may be "tipping points" that trigger sudden changes.
The UK government's chief scientific adviser, Professor Sir David King, said the new data highlighted the importance of taking urgent action to limit carbon emissions.
"Today we're over 380 ppm," he said. "That's higher than we've been for over a million years, possibly 30 million years. Mankind is changing the climate."