The concentration of carbon dioxide (CO2) in the atmosphere is increasing faster than expected, according to a new study published in Tuesday's edition of the Proceedings of the National Academy of Sciences. The study said that rising levels of CO2, the main greenhouse gas blamed for climate change, threaten to accelerate global warming. The study attributes the change in CO2 levels to increased use of fossil fuels and a decline in the amount of gas being absorbed by the oceans and land.

CO2 emissions have increased by an average of 3.3 percent per year since 2000, compared with 1.1 percent per year in the 1990s. The rate has increased 35 percent more than scientists had anticipated based on economic growth, according to Corinne Le Quere, one of the paper's authors.

"The causes of the acceleration are surprising," Le Quere said. "One is that we are not as efficient at using CO2 as we had anticipated, and the second cause is that it appears that the CO2 sinks are weakening." The main natural CO2 sinks are the oceans and plants.

The study attributed approximately half of the unexpected growth to an increase in the carbon intensity of the global economy, a measure of the amount of carbon emissions per unit of economic activity. "For 30 years we were becoming more efficient at using CO2 for producing energy and wealth, and this has completely stalled since 2000," Le Quere said, adding that increased use of coal by developing countries such as China and India was partially to blame.

The remaining unexpected growth was attributed to a decline in the ability of the oceans and land to absorb CO2. Most climate models do not account for a weakening of CO2 sinks, Le Quere said. Alan Robock, associate director of the Center for Environmental Prediction at Rutgers University, called the reduction of the oceanic CO2 sink "really shocking".

The study "raises some very important issues that the public should be aware of: Namely that concentrations of CO2 are increasing at much higher rates than previously expected and this is in spite of the Kyoto Protocol that is designed to hold them down in western countries," said Kevin Trenberth, the head of the Climate Analysis Section at the National Center for Atmospheric Research and a lead author of the 2007 assessment report of the U.N. Intergovernmental Panel on Climate Change (IPCC).

The IPCC predicted in February of this year that temperatures would rise by up to 6.4 degrees Celsius over the next century. However, Le Quere suggested that this scenario might be overly optimistic. "There’s quite a significant difference from what was forecast,” she said. "It’s rather scary and the IPCC scenarios are, therefore, rather too optimistic — as if they weren’t bleak enough already. The whole thing is likely to mean mitigation is rather more difficult than was thought."