Image caption Coal and oil contain far less carbon-14 than is contained in the CO2 produced by life today

Researchers have demonstrated a way of distinguishing between carbon dioxide in the air coming from fossil fuel burning and that from natural sources.

It measures one type, or isotope, of carbon that decays over time - long since gone from fossil fuels.

As explained in the Journal of Geophysical Research , the method may prove useful in CO2 monitoring efforts.

However, experts say that the approach must be calibrated against existing carbon-measuring techniques.

The research was led by scientists from the National Oceanographic and Atmospheric Administration's Earth System Research Laboratory in the US, who studied six years' worth of atmospheric sampling data gathered by aircraft over two sites in the northeastern US.

The team focused on the rare isotope carbon-14, which is constantly produced in tiny amounts in the atmosphere when cosmic rays hit nitrogen atoms, and which decays away over thousands of years.

Buried away for millions of years underground, fossil fuels contain virtually no carbon-14; and neither does the CO2 emitted when the fuels burn.

But CO2 coming from plants does contain carbon-14.

That difference that showed up in the team's atmospheric samples as a ratio of natural, "biogenic" CO2 to fossil fuel CO2.

The more measurements we have from different sources, the more accurate we can make them all Jane Burston, Centre for Carbon Measurement

The trick could complement existing carbon accounting methods used to monitor how much CO2 countries and regions are producing, principal among them the self-reporting of fossil fuel usage.

"While the accounting-based approach is probably accurate at global scales, the uncertainties rise for smaller-scale regions," said the Earth System Research Laboratory's John Miller, lead author of the study.

"And as CO2 emissions targets become more widespread, there may be a greater temptation to under-report. But we'll be able to see through that."

However, the method may not stretch down to a level of geographic detail that is increasingly important for single CO2 sources such as power plants.

"Other types of physical measurement (of CO2 levels) are being driven by emissions trading, and I'm not sure how far this would be able to extend to application at the individual site and installation level," said Jane Burston, head of the Centre for Carbon Measurement at the UK's National Physical Laboratory.

The centre's head of emissions Rod Robinson explained that much work needs to be done to validate the assumptions of the method and ensure it gives similar quality results as existing physical and accounting measurements.

Nevertheless, Ms Burston said the method would be a valuable tool in the quest to understand fully how carbon is released and distributed.

"For things like CO2 concentration in the atmosphere, the best way of going about it is just getting as many measurements as you can from the ground, from the atmosphere, and from satellites," she told BBC News.

"The more measurements we have from different sources, the more accurate we can make them all."