The findings suggest people are falling behind in efforts to limit global warming and its potential impacts, Corinne Le Quere, a physicist at Britain's University of East Anglia, who worked on the study, said in a telephone interview.

"It means that we are not on the track we thought we were in terms of controlling global warming," Le Quere said.

Researchers from the university, the Global Carbon Project and the British Antarctic Survey analysed atmospheric carbon dioxide observations and emissions data since 1959 and compared them with observed and projected trends.

They found that projections made at the end of the 1990s had underestimated the amount of fossil fuel emissions by 4 percent to 17 percent, mainly due to fast economic growth in the developing world.

"Richer countries are improving their energy efficiency when it comes to emissions but as developing countries grow they are using more energy from sources like coal, which pushes out more carbon dioxide in the atmosphere than oil or other resources," Le Quere said.

While increased levels of atmospheric carbon dioxide were no surprise, the causes and pace of the increase were unexpected, the researchers said in the Proceedings of the National Academy of Sciences.

A big reason is that natural sinks such as oceans and forests -- which typically absorb about half of the emissions sent into the atmosphere each year -- are much less efficient due to warming temperatures and related consequences, Le Quere said.

Human emissions of gases such as methane and carbon dioxide that trap heat in the atmosphere are clearly contributing to rising global average temperatures, many experts agree.