Carbon dioxide levels in the atmosphere are rising more rapidly than expected because the ability of the land and oceans to soak up the greenhouse gas has been declining for decades, a new study shows.

CSIRO researcher Pep Canadell said his team's discovery highlighted the need for an urgent reduction in greenhouse gas emissions if CO₂ concentrations were to be stabalised at a safe level.

"The longer we wait to implement deep cuts in our emissions, the harder it will get," Dr Canadell said. "We will have less help from the natural sinks."

Dr Canadell said the study also found that the carbon intensity of the global economy had begun to rise in the past seven years, after a 30-year decline, which meant more carbon was emitted for every dollar of wealth generated.

In addition, the rate of increase in CO₂ emissions as a result of human activities has tripled since the 1990s, with emissions this year set to top 10 billion tonnes of carbon for the first time.

Dr Canadell, an executive director of the Global Carbon Project, said these three factors meant the world was on track to experience the worst-case scenarios forecast by the UN's Intergovernmental Panel on Climate Change.

"If we continue the growth rate of fossil fuel emissions we've seen over the last seven years we'll be following the higher end of the temperature and climate change projections of the IPCC," he said.

The IPCC predicted earlier this year that the global temperature could rise by up to 6.4 degrees by 2100.

The new study, by an international team that also includes Dr Mike Raupach of CSIRO, is published in the Proceeding of the National Academy of Sciences.

Dr Canadell said a decrease in the planet's ability to absorb carbon emissions had been expected to occur later this century. "We've found it has been happening over the past 50 years but we have not been able to detect it until now."

Fifty years ago, 600 kilograms out of every tonne of carbon dioxide emitted was removed by land and ocean sinks.

"In 2006, only 550 kilograms were removed per tonne and that amount is falling," he said.

Human activity has weakened one of the planet's most important carbon sinks, the Southern Ocean, by causing westerly winds to move closer to the poles and intensify, creating surface turbulence which increases the amount of carbon dioxide released from the water.

Recent droughts and warming, including in Australia, have weakened land sinks by reducing plant growth, the study concludes.

Between 1970 and 2000, the carbon intensity of the global economy steadily declined, as more efficient use was made of fossil fuels.

But in the last seven years this fall has halted, and a slight rise in carbon emissions for every dollar of global wealth produced has been detected, the study found.

Dr Canadell said the problem was that most climate models had assumed that the decline in carbon intensity would continue, meaning they would underestimate future emissions.

Total carbon emissions as a result of fossil fuel burning and cement production, and land use changes including deforestation, rose from 8.4 billion tonnes in 2000 to 9.9 billion tonnes in 2006.

Last year the annual rate of increase in emissions of 2.9 per cent was more than three times that of 0.7 per cent during the 1990s.